Java核心类库（上）

参加拉勾教育大数据训练营课程笔记

学会查阅API文档，根据包索引，搜索等。

常用类

java.lang - 唯一一个Java虚拟机自动访问，所以System等不需要import就可以使用，例如：System, String等。
java.util - 提供大量工具类和集合类等，例如：Scanner , List, Random等。
java.io - 输入输出包，提供了文件读写，流读写等，例如：FileInputStream, FileOutputStream等。
java.net - 提供网络编程类，例如：ServerSocket类, Socket类等。
java.sql - 提供数据库访问类，数据操作的类和接口，例如：DriverManager类, Connection接口等。

需要记忆大量的核心类、接口。

`java.lang`

`Object`

Java中所有类的父类（根类），所有类都直接或间接继承自Object。没有显示使用extends关键字指定父类的类，其父类都是Object。而有明确父类的类也都间接继承自Object类。

常用方法：

Modifier and Type	Method	Description
`protected Object`	`clone()`	Creates and returns a copy of this object.
`boolean`	`equals(Object obj)`	Indicates whether some other object is “equal to” this one.
`protected void`	`finalize()`	Deprecated. The finalization mechanism is inherently problematic.
`Class<?>`	`getClass()`	Returns the runtime class of this `Object`.
`int`	`hashCode()`	Returns a hash code value for the object.
`void`	`notify()`	Wakes up a single thread that is waiting on this object’s monitor.
`void`	`notifyAll()`	Wakes up all threads that are waiting on this object’s monitor.
`String`	`toString()`	Returns a string representation of the object.
`void`	`wait()`	Causes the current thread to wait until it is awakened, typically by being notified or interrupted.
`void`	`wait(long timeoutMillis)`	Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`void`	`wait(long timeoutMillis, int nanos)`	Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.

重写equals，hashCode，toString方法

equals，hashCode必须保持一致性
toString方法默认返回对象地址

Notes

==操作符笔记地址，由于基本数据类型，内存空间本就是保存的实际数据值，所以可以直接使用==判断相等，而对象则是保存的指向堆区的地址，所以对象判断相等需要使用equals方法，而equals方法默认是比较地址，所以当需要自定义两个对象相等条件时，需要重写equals方法。

package module2.code.overridetest;import java.util.Objects;public class Student {private String name;private int age;public Student(String name, int age) {this.name = name;this.age = age;}@Overridepublic boolean equals(Object o) {if (this == o) return true;if (o == null || getClass() != o.getClass()) return false;Student student = (Student) o;return age == student.age && name.equals(student.name);}@Overridepublic int hashCode() {return Objects.hash(name, age);}@Overridepublic String toString() {return "Student{" +"name='" + name + '\'' +", age=" + age +'}';}
}

包装类

基本数据类型不是对象，为了使基本类型也是对象，Java提供对基本数据类型进行对象化的类，称作包装类。

包装类	对应的基本类型
`java.lang.Byte`	`byte`
`java.lang.Short`	`short`
`java.lang.Integer`	`int`
`java.lang.Long`	`long`
`java.lang.Float`	`float`
`java.lang.Double`	`double`
`java.lang.Boolean`	`boolean`
`java.lang.Character`	`char`

装箱(boxing)和拆箱(unboxing)

装箱：基本数据类型 → \to →对象

Integer it = Integer.valueOf(123);

拆箱：对象 → \to →基本数据类型

int ia = it.intValue();

从Java 1.5开始，增加了自动装箱和拆箱，无需再显示调用方法来实现装箱和拆箱

Integer it = 100;
int ia = it;

考点：

Integer it6 = 127; //128;
Integer it7 = 127; //128;
Integer it8 = new Integer(127); //new Integer(128);
Integer it9 = new Integer(127); //new Integer(128);
System.out.println(it6 == it7);      // 比较地址  false  true  地址一样
System.out.println(it6.equals(it7)); // 比较内容  true
System.out.println(it8 == it9);      // 比较地址  false
System.out.println(it8.equals(it9)); // 比较内容  true

`Integer`类

java.lang.Object
- java.lang.Number
  - java.lang.Integer

All Implemented Interfaces:

Serializable, Comparable<Integer>

public final class Integer
extends Number
implements Comparable<Integer>

常用常量

Modifier and Type	Field	Description
`static int`	`BYTES`	The number of bytes used to represent an `int` value in two’s complement binary form.
`static int`	`MAX_VALUE`	A constant holding the maximum value an `int` can have, 231-1.
`static int`	`MIN_VALUE`	A constant holding the minimum value an `int` can have, -231.
`static int`	`SIZE`	The number of bits used to represent an `int` value in two’s complement binary form.
`static Class<Integer>`	`TYPE`	The `Class` instance representing the primitive type `int`.

常用方法

Modifier and Type	Method	Description
`int`	`intValue()`	Returns the value of this `Integer` as an `int`.
`long`	`longValue()`	Returns the value of this `Integer` as a `long` after a widening primitive conversion.
`boolean`	`equals(Object obj)`	Compares this object to the specified object.
`String`	`toString()`	Returns a `String` object representing this `Integer`'s value.
`static Integer`	`valueOf(int i)`	Returns an `Integer` instance representing the specified `int` value.
`static int`	`bitCount(int i)`	Returns the number of one-bits in the two’s complement binary representation of the specified `int` value.
`static Integer`	`valueOf(String s)`	Returns an `Integer` object holding the value of the specified `String`.
`static Integer`	`valueOf(String s, int radix)`	Returns an `Integer` object holding the value extracted from the specified `String` when parsed with the radix given by the second argument.
`static int`	`parseInt(String s)`	Parses the string argument as a signed decimal integer.
`static String`	`toBinaryString(int i)`	Returns a string representation of the integer argument as an unsigned integer in base 2.
`static String`	`toHexString(int i)`	Returns a string representation of the integer argument as an unsigned integer in base 16.
`static String`	`toOctalString(int i)`	Returns a string representation of the integer argument as an unsigned integer in base 8.

`Double`类

java.lang.Object
- java.lang.Number
  - java.lang.Double

All Implemented Interfaces:

Serializable, Comparable<Integer>

public final class Double
extends Number
implements Comparable<Double>

常用常量

Modifier and Type	Field	Description
`static int`	`BYTES`	The number of bytes used to represent a `double` value.
`static int`	`MAX_EXPONENT`	Maximum exponent a finite `double` variable may have.
`static double`	`MAX_VALUE`	A constant holding the largest positive finite value of type `double`, (2-2-52)·21023.
`static int`	`MIN_EXPONENT`	Minimum exponent a normalized `double` variable may have.
`static double`	`MIN_NORMAL`	A constant holding the smallest positive normal value of type `double`, 2-1022.
`static double`	`MIN_VALUE`	A constant holding the smallest positive nonzero value of type `double`, 2-1074.
`static double`	`NaN`	A constant holding a Not-a-Number (NaN) value of type `double`.
`static double`	`NEGATIVE_INFINITY`	A constant holding the negative infinity of type `double`.
`static double`	`POSITIVE_INFINITY`	A constant holding the positive infinity of type `double`.
`static int`	`SIZE`	The number of bits used to represent a `double` value.
`static Class<Double>`	`TYPE`	The `Class` instance representing the primitive type `double`.

常用方法

Modifier and Type	Method	Description
`byte`	`byteValue()`	Returns the value of this `Double` as a `byte` after a narrowing primitive conversion.
`double`	`doubleValue()`	Returns the `double` value of this `Double` object.
`boolean`	`equals(Object obj)`	Compares this object against the specified object.
`String`	`toString()`	Returns a string representation of this `Double` object.
`boolean`	`isNaN()`	Returns `true` if this `Double` value is a Not-a-Number (NaN), `false` otherwise.
`static Double`	`valueOf(double d)`	Returns a `Double` instance representing the specified `double` value.
`static Double`	`valueOf(String s)`	Returns a `Double` object holding the `double` value represented by the argument string `s`.
`static double`	`parseDouble(String s)`	Returns a new `double` initialized to the value represented by the specified `String`, as performed by the `valueOf` method of class `Double`.

Notes

java.lang.Number是一个抽象类，是数字类型所有类（java.lang.Byte, java.lang.Short, java.lang.Integer, java.lang.Long, java.lang.Float, java.lang.Double）的父类，用来描述他们的共有成员

`Boolean`类

java.lang.Object
- java.lang.Boolean

All Implemented Interfaces:

Serializable, Comparable<Boolean>

public final class Boolean
extends Object
implements Serializable, Comparable<Boolean>

常用常量

Modifier and Type	Field	Description
`static Boolean`	`FALSE`	The `Boolean` object corresponding to the primitive value `false`.
`static Boolean`	`TRUE`	The `Boolean` object corresponding to the primitive value `true`.
`static Class<Boolean>`	`TYPE`	The Class object representing the primitive type boolean.

常用方法

Modifier and Type	Method	Description
`boolean`	`booleanValue()`	Returns the value of this `Boolean` object as a boolean primitive.
`boolean`	`equals(Object obj)`	Returns `true` if and only if the argument is not `null` and is a `Boolean` object that represents the same `boolean` value as this object.
`String`	`toString()`	Returns a `String` object representing this Boolean’s value.
`static Boolean`	`valueOf(boolean b)`	Returns a `Boolean` instance representing the specified `boolean` value.
`static Boolean`	`valueOf(String s)`	Returns a `Boolean` with a value represented by the specified string.
`static boolean`	`parseBoolean(String s)`	Parses the string argument as a boolean (return true only when s = “true”, ignore case).

public static boolean parseBoolean(String s) {return "true".equalsIgnoreCase(s);
}

`Character`类

java.lang.Object
- java.lang.Character

All Implemented Interfaces:

Serializable, Comparable<Boolean>

public final class Character
extends Object
implements Serializable, Comparable<Character>

常用常量

Modifier and Type	Field	Description
`static int`	`BYTES`	The number of bytes used to represent a `char` value in unsigned binary form.
`static int`	`SIZE`	The number of bits used to represent a `char` value in unsigned binary form, constant `16`.
`static Class<Character>`	`TYPE`	The `Class` instance representing the primitive type `char`.

常用方法

Modifier and Type	Method	Description
`char`	`charValue()`	Returns the value of this `Character` object.
`boolean`	`equals(Object obj)`	Compares this object against the specified object.
`String`	`toString()`	Returns a `String` object representing this `Character`'s value.
`static Character`	`valueOf(char c)`	Returns a `Character` instance representing the specified `char` value.
`static boolean`	`isUpperCase(char ch)`	Determines if the specified character is an uppercase character.
`static boolean`	`isLowerCase(char ch)`	Determines if the specified character is a lowercase character.
`static boolean`	`isWhitespace(char ch)`	Determines if the specified character is white space according to Java.
`static boolean`	`isDigit(char ch)`	Determines if the specified character is a digit.
`static boolean`	`isAlphabetic(int codePoint)`	Determines if the specified character (Unicode code point) is an alphabet.
`static char`	`toUpperCase(char ch)`	Converts the character argument to uppercase using case mapping information from the UnicodeData file.
`static char`	`toLowerCase(char ch)`	Converts the character argument to lowercase using case mapping information from the UnicodeData file.

数学处理类

`Math`类

java.lang.Object
- java.lang.Math

public final class Math
extends Object

常用方法

Modifier and Type	Method	Description
`static double`	`abs(double a)`	Returns the absolute value of a `double` value.
`static float`	`abs(float a)`	Returns the absolute value of a `float` value.
`static int`	`abs(int a)`	Returns the absolute value of an `int` value.
`static long`	`abs(long a)`	Returns the absolute value of a `long` value.
`static double`	`max(double a, double b)`	Returns the greater of two `double` values.
`static float`	`max(float a, float b)`	Returns the greater of two `float` values.
`static int`	`max(int a, int b)`	Returns the greater of two `int` values.
`static long`	`max(long a, long b)`	Returns the greater of two `long` values.
`static double`	`min(double a, double b)`	Returns the smaller of two `double` values.
`static float`	`min(float a, float b)`	Returns the smaller of two `float` values.
`static int`	`min(int a, int b)`	Returns the smaller of two `int` values.
`static long`	`min(long a, long b)`	Returns the smaller of two `long` values.
`static double`	`pow(double a, double b)`	Returns the value of the first argument raised to the power of the second argument.
`static double`	`random()`	Returns a `double` value with a positive sign, greater than or equal to `0.0` and less than `1.0`.
`static double`	`sqrt(double a)`	Returns the correctly rounded positive square root of a `double` value.
`static long`	`round(double a)`	Returns the closest `long` to the argument, with ties rounding to positive infinity.
`static int`	`round(float a)`	Returns the closest `int` to the argument, with ties rounding to positive infinity.

`BigDecimal`类

实现浮点数的精确计算

java.lang.Object
- java.lang.Number
  - java.math.BigDecimal

All Implemented Interfaces:

Serializable, Comparable<Integer>

public class BigDecimal
extends Number
implements Comparable<BigDecimal>

常用变量

Modifier and Type	Field	Description
`static BigDecimal`	`ONE`	The value 1, with a scale of 0.
`static int`	`ROUND_CEILING`	Deprecated.Use `RoundingMode.CEILING` instead.
`static int`	`ROUND_DOWN`	Deprecated.Use `RoundingMode.DOWN` instead.
`static int`	`ROUND_FLOOR`	Deprecated.Use `RoundingMode.FLOOR` instead.
`static int`	`ROUND_HALF_DOWN`	Deprecated.Use `RoundingMode.HALF_DOWN` instead.
`static int`	`ROUND_HALF_EVEN`	Deprecated.Use `RoundingMode.HALF_EVEN` instead.
`static int`	`ROUND_HALF_UP`	Deprecated.Use `RoundingMode.HALF_UP` instead.
`static int`	`ROUND_UNNECESSARY`	Deprecated.Use `RoundingMode.UNNECESSARY` instead.
`static int`	`ROUND_UP`	Deprecated.Use `RoundingMode.UP` instead.
`static BigDecimal`	`TEN`	The value 10, with a scale of 0.
`static BigDecimal`	`ZERO`	The value 0, with a scale of 0.

构造方法

Constructor	Description
`BigDecimal(String val)`	Translates the string representation of a `BigDecimal` into a `BigDecimal`.

常用方法

Modifier and Type	Method	Description
`BigDecimal`	`abs()`	Returns a `BigDecimal` whose value is the absolute value of this `BigDecimal`, and whose scale is `this.scale()`.
`BigDecimal`	`add(BigDecimal augend)`	Returns a `BigDecimal` whose value is `(this + augend)`, and whose scale is `max(this.scale(), augend.scale())`.
`BigDecimal`	`subtract(BigDecimal subtrahend)`	Returns a `BigDecimal` whose value is `(this - subtrahend)`, and whose scale is `max(this.scale(), subtrahend.scale())`.
`BigDecimal`	`multiply(BigDecimal multiplicand)`	Returns a `BigDecimal` whose value is `(this × multiplicand)`, and whose scale is `(this.scale() + multiplicand.scale())`.
`BigDecimal`	`divide(BigDecimal divisor)`	Returns a `BigDecimal` whose value is `(this / divisor)`, and whose preferred scale is `(this.scale() - divisor.scale())`; if the exact quotient cannot be represented (because it has a non-terminating decimal expansion) an `ArithmeticException` is thrown.
`BigDecimal`	`pow(int n)`	Returns a `BigDecimal` whose value is `(thisn)`, The power is computed exactly, to unlimited precision.

`BigInteger`类

表示比long更大的整数范围

java.lang.Object
- java.lang.Number
  - java.math.BigInteger

All Implemented Interfaces:

Serializable, Comparable<Integer>

public class BigInteger
extends Number
implements Comparable<BigInteger>

常量

Modifier and Type	Field	Description
`static BigInteger`	`ONE`	The BigInteger constant one.
`static BigInteger`	`TEN`	The BigInteger constant ten.
`static BigInteger`	`TWO`	The BigInteger constant two.
`static BigInteger`	`ZERO`	The BigInteger constant zero.

构造方法

Constructor	Description
`BigInteger(String val)`	Translates the decimal String representation of a BigInteger into a BigInteger.
`BigInteger(String val, int radix)`	Translates the String representation of a BigInteger in the specified radix into a BigInteger.

常用方法

Modifier and Type	Method	Description
`BigInteger`	`abs()`	Returns a BigInteger whose value is the absolute value of this BigInteger.
`BigInteger`	`add(BigInteger val)`	Returns a BigInteger whose value is `(this + val)`.
`BigInteger`	`subtract(BigInteger val)`	Returns a BigInteger whose value is `(this - val)`.
`BigInteger`	`multiply(BigInteger val)`	Returns a BigInteger whose value is `(this * val)`.
`BigInteger`	`divide(BigInteger val)`	Returns a BigInteger whose value is `(this / val)`.
`BigInteger`	`remainder(BigInteger val)`	Returns a BigInteger whose value is `(this % val)`.
`BigInteger[]`	`divideAndRemainder(BigInteger val)`	Returns an array of two BigIntegers containing `(this / val)` followed by `(this % val)`.

`String`类

java.lang.Object
- java.math.String

All Implemented Interfaces:

Serializable, CharSequence, Comparable<Integer>

public final class String
extends Object
implements Serializable, Comparable<String>, CharSequence

final修饰，String类不能被继承。从Java 1.9开始底层不再使用char[]来存储，而是使用byte[]加上编码，从而节约了一些空间。String是常量，不可更改，因此可以被共享使用。

常量池

String描述的字符串内容不可改变，因此Java虚拟机将首次出现的字符串放入常量池中，若后续代码中出现了相同字符串内容则直接使用常量池中已有的字符串对象，而无需申请内存和创建对象，从而提高性能。

String str1 = "abc";
String str2 = "abc";
System.out.println(str1 == str2); // 比较地址  true

构造方法

Constructor	Description
`String()`	Initializes a newly created `String` object so that it represents an empty character sequence.
`String(byte[] bytes)`	Constructs a new `String` by decoding the specified array of bytes using the platform’s default charset.
`String(byte[] bytes, int offset, int length)`	Constructs a new `String` by decoding the specified subarray of bytes using the platform’s default charset.
`String(char[] value)`	Allocates a new `String` so that it represents the sequence of characters currently contained in the character array argument.
`String(char[] value, int offset, int count)`	Allocates a new `String` that contains characters from a subarray of the character array argument.
`String(byte[] bytes, String charsetName)`	Constructs a new `String` by decoding the specified array of bytes using the specified charset.
`String(byte[] bytes, Charset charset)`	Constructs a new `String` by decoding the specified array of bytes using the specified charset.
`String(String original)`	Initializes a newly created `String` object so that it represents the same sequence of characters as the argument; in other words, the newly created string is a copy of the argument string.

常用方法

Modifier and Type	Method	Description
`char`	`charAt(int index)`	Returns the `char` value at the specified index.
`int`	`length()`	Returns the length of this string.
`boolean`	`isBlank()`	Returns `true` if the string is empty or contains only `white space` codepoints, otherwise `false`.
`boolean`	`isEmpty()`	Returns `true` if, and only if, `length()` is `0`.
`int`	`compareTo(String anotherString)`	Compares two strings lexicographically.
`int`	`compareToIgnoreCase(String str)`	Compares two strings lexicographically, ignoring case differences.
`String`	`concat(String str)`	Concatenates the specified string to the end of this string.
`boolean`	`contains(CharSequence s)`	Returns true if and only if this string contains the specified sequence of char values.
`String`	`toLowerCase()`	Converts all of the characters in this `String` to lower case using the rules of the default locale.
`String`	`toUpperCase()`	Converts all of the characters in this `String` to upper case using the rules of the default locale.
`String`	`trim()`	Returns a string whose value is this string, with all leading and trailing space removed, where space is defined as any character whose codepoint is less than or equal to `'U+0020'` (the space character).
`boolean`	`startsWith(String prefix)`	Tests if this string starts with the specified prefix.
`boolean`	`startsWith(String prefix, int toffset)`	Tests if the substring of this string beginning at the specified index starts with the specified prefix.
`boolean`	`endsWith(String suffix)`	Tests if this string ends with the specified suffix.
`boolean`	`equals(Object anObject)`	Compares this string to the specified object.
`boolean`	`equalsIgnoreCase(String anotherString)`	Compares this `String` to another `String`, ignoring case considerations.
`int`	`hashCode()`	Returns a hash code for this string.
`int`	`indexOf(int ch)`	Returns the index within this string of the first occurrence of the specified character.
`int`	`indexOf(int ch, int fromIndex)`	Returns the index within this string of the first occurrence of the specified character, starting the search at the specified index.
`int`	`indexOf(String str)`	Returns the index within this string of the first occurrence of the specified substring.
`int`	`indexOf(String str, int fromIndex)`	Returns the index within this string of the first occurrence of the specified substring, starting at the specified index.
`int`	`lastIndexOf(int ch)`	Returns the index within this string of the last occurrence of the specified character.
`int`	`lastIndexOf(int ch, int fromIndex)`	Returns the index within this string of the last occurrence of the specified character, searching backward starting at the specified index.
`int`	`lastIndexOf(String str)`	Returns the index within this string of the last occurrence of the specified substring.
`int`	`lastIndexOf(String str, int fromIndex)`	Returns the index within this string of the last occurrence of the specified substring, searching backward starting at the specified index.
`String`	`substring(int beginIndex)`	Returns a string that is a substring of this string.
`String`	`substring(int beginIndex, int endIndex)`	Returns a string that is a substring of this string.
`String`	`repeat(int count)`	Returns a string whose value is the concatenation of this string repeated `count` times.

回文判断：

String str1 = new String("上海自来水来自海上");
System.out.println("str1 = " + str1); // 上海自来水来自海上
bool flag = true;
for (int i = 0; i < str1.length()/2; i++) {if (str1.charAt(i) != str1.charAt(str1.length()-i-1)) {break;}
}
System.out.println(flag ? "是回文！":"不是回文！");

正则表达式

String中与正则表达式相关的方法，正则内容太过复杂，将单独写一篇博客。

Modifier and Type	Method	Description
`boolean`	`matches(String regex)`	Tells whether or not this string matches the given regular expression.
`String[]`	`split(String regex)`	Splits this string around matches of the given regular expression.
`String[]`	`split(String regex, int limit)`	Splits this string around matches of the given regular expression.
`String`	`replace(char oldChar, char newChar)`	Returns a string resulting from replacing all occurrences of `oldChar` in this string with `newChar`.
`String`	`replace(CharSequence target, CharSequence replacement)`	Replaces each substring of this string that matches the literal target sequence with the specified literal replacement sequence.
`String`	`replaceAll(String regex, String replacement)`	Replaces each substring of this string that matches the given regular expression with the given replacement.
`String`	`replaceFirst(String regex, String replacement)`	Replaces the first substring of this string that matches the given regular expression with the given replacement.

String笔试考点

使用两种方式实现字符串"12345"转换为整数12345并打印

方式一

String str2 = new String("12345");
int ia = Integer.parseInt(str2);

方式二

String str2 = new String("12345");
int ib = 0;
for (int i = 0; i < str2.length(); i++) {ib = ib*10 + (str2.charAt(i) - '0');
}

常量池

// 1.请问下面的代码会创建几个对象？分别存放在什么地方？
String str1 = "hello";  // 1个对象  存放在常量池中
String str1 = new String("helo"); // 2个对象  1个在常量池中，1个在堆区// 2.常量池和堆区对象的比较
String str1 = "hello";  // 常量池
String str2 = "hello";  // 常量池
String str3 = new String("hello"); // 堆区
String str4 = new String("hello"); // 堆区
System.out.println(str1 == str2);       // 比较地址  true
System.out.println(str1.equals(str2));  // 比较内容  true
System.out.println(str3 == str4);       // 比较地址  false
System.out.println(str3.equals(str4));  // 比较内容  true
System.out.println(str2 == str4);       // 比较地址  false
System.out.println(str2.equals(str4));  // 比较内容 true// 3.常量有优化机制，变量没有
String str5 = "abcd";
String str6 = "ab" + "cd";  // 常量优化机制  "abcd"
System.out.println(str5 == str6); // 比较地址  trueString str7 = "ab";
String str8 = str7 + "cd"; // 没有常量优化
System.out.println(str5 == str8); // 比较地址 false

正则表达式的使用

`StringBuilder`、`StringBuffer` - 可变字符串

`StringBuilder` - not thread-safe, not synchronized

public final class StringBuilder
extends Object
implements Serializable, Comparable<StringBuilder>, CharSequence

A mutable sequence of characters. This class provides an API compatible with StringBuffer, but with no guarantee of synchronization.

Constructors

Constructor	Description
`StringBuilder()`	Constructs a string builder with no characters in it and an initial capacity of 16 characters.
`StringBuilder(int capacity)`	Constructs a string builder with no characters in it and an initial capacity specified by the `capacity` argument.
`StringBuilder(CharSequence seq)`	Constructs a string builder that contains the same characters as the specified `CharSequence`.
`StringBuilder(String str)`	Constructs a string builder initialized to the contents of the specified string.

Methods

Modifier and Type	Method	Description
`StringBuffer`	`append(String str)`	Appends the specified string to this character sequence.
`StringBuffer`	`append(StringBuffer sb)`	Appends the specified `StringBuffer` to this sequence.
`int`	`capacity()`	Returns the current capacity.
`char`	`charAt(int index)`	Returns the `char` value in this sequence at the specified index.
`StringBuffer`	`delete(int start, int end)`	Removes the characters in a substring of this sequence.
`StringBuffer`	`deleteCharAt(int index)`	Removes the `char` at the specified position in this sequence.
`StringBuffer`	`insert(int offset, String str)`	Inserts the string into this character sequence.
`int`	`lastIndexOf(String str)`	Returns the index within this string of the last occurrence of the specified substring.
`int`	`lastIndexOf(String str, int fromIndex)`	Returns the index within this string of the last occurrence of the specified substring, searching backward starting at the specified index.
`int`	`length()`	Returns the length (character count).
`StringBuffer`	`replace(int start, int end, String str)`	Replaces the characters in a substring of this sequence with characters in the specified `String`.
`StringBuffer`	`reverse()`	Causes this character sequence to be replaced by the reverse of the sequence.
`void`	`setCharAt(int index, char ch)`	The character at the specified index is set to `ch`.
`void`	`setLength(int newLength)`	Sets the length of the character sequence.
`String`	`substring(int start)`	Returns a new `String` that contains a subsequence of characters currently contained in this character sequence.
`String`	`substring(int start, int end)`	Returns a new `String` that contains a subsequence of characters currently contained in this sequence.

`StringBuffer` - thread-safe, synchronized

public final class StringBuffer
extends Object
implements Serializable, Comparable<StringBuffer>, CharSequence

A thread-safe, mutable sequence of characters. A string buffer is like a String, but can be modified. At any point in time it contains some particular sequence of characters, but the length and content of the sequence can be changed through certain method calls.

Constructors

Constructor	Description
`StringBuffer()`	Constructs a string buffer with no characters in it and an initial capacity of 16 characters.
`StringBuffer(int capacity)`	Constructs a string buffer with no characters in it and the specified initial capacity.
`StringBuffer(CharSequence seq)`	Constructs a string buffer that contains the same characters as the specified `CharSequence`.
`StringBuffer(String str)`	Constructs a string buffer initialized to the contents of the specified string.

Methods

Modifier and Type	Method	Description
`StringBuffer`	`append(String str)`	Appends the specified string to this character sequence.
`StringBuffer`	`append(StringBuffer sb)`	Appends the specified `StringBuffer` to this sequence.
`int`	`capacity()`	Returns the current capacity.
`char`	`charAt(int index)`	Returns the `char` value in this sequence at the specified index.
`StringBuffer`	`delete(int start, int end)`	Removes the characters in a substring of this sequence.
`StringBuffer`	`deleteCharAt(int index)`	Removes the `char` at the specified position in this sequence.
`StringBuffer`	`insert(int offset, String str)`	Inserts the string into this character sequence.
`int`	`lastIndexOf(String str)`	Returns the index within this string of the last occurrence of the specified substring.
`int`	`lastIndexOf(String str, int fromIndex)`	Returns the index within this string of the last occurrence of the specified substring, searching backward starting at the specified index.
`int`	`length()`	Returns the length (character count).
`StringBuffer`	`replace(int start, int end, String str)`	Replaces the characters in a substring of this sequence with characters in the specified `String`.
`StringBuffer`	`reverse()`	Causes this character sequence to be replaced by the reverse of the sequence.
`void`	`setCharAt(int index, char ch)`	The character at the specified index is set to `ch`.
`void`	`setLength(int newLength)`	Sets the length of the character sequence.
`String`	`substring(int start)`	Returns a new `String` that contains a subsequence of characters currently contained in this character sequence.
`String`	`substring(int start, int end)`	Returns a new `String` that contains a subsequence of characters currently contained in this sequence.

`java.util.Date`

获取当前时间

java.lang.System.currentTimeMillis()

Modifier and Type	Method	Description
`static long`	`currentTimeMillis()`	Returns the current time in milliseconds.

@Test
public void getCurrentTime() {final long l = System.currentTimeMillis();System.out.println(l);
}

Output:

1627390821407

Process finished with exit code 0

public class Date
extends Object
implements Serializable, Cloneable, Comparable<Date>

The class Date represents a specific instant in time, with millisecond precision.

Constructors

Constructor	Description
`Date()`	Allocates a `Date` object and initializes it so that it represents the time at which it was allocated, measured to the nearest millisecond.
`Date(int year, int month, int date)`	Deprecated. As of JDK version 1.1, replaced by `Calendar.set(year + 1900, month, date)` or `GregorianCalendar(year + 1900, month, date)`.
`Date(int year, int month, int date, int hrs, int min)`	Deprecated. As of JDK version 1.1, replaced by `Calendar.set(year + 1900, month, date, hrs, min)` or `GregorianCalendar(year + 1900, month, date, hrs, min)`.
`Date(int year, int month, int date, int hrs, int min, int sec)`	Deprecated. As of JDK version 1.1, replaced by `Calendar.set(year + 1900, month, date, hrs, min, sec)` or `GregorianCalendar(year + 1900, month, date, hrs, min, sec)`.
`Date(long date)`	Allocates a `Date` object and initializes it to represent the specified number of milliseconds since the standard base time known as “the epoch”, namely January 1, 1970, 00:00:00 GMT.
`Date(String s)`	Deprecated. As of JDK version 1.1, replaced by `DateFormat.parse(String s)`.

Methods

Modifier and Type	Method	Description
`boolean`	`after(Date when)`	Tests if this date is after the specified date.
`boolean`	`before(Date when)`	Tests if this date is before the specified date.
`int`	`compareTo(Date anotherDate)`	Compares two Dates for ordering.
`boolean`	`equals(Object obj)`	Compares two dates for equality.
`static Date`	`from(Instant instant)`	Obtains an instance of `Date` from an `Instant` object.
`int`	`getDate()`	Deprecated.As of JDK version 1.1, replaced by `Calendar.get(Calendar.DAY_OF_MONTH)`.
`int`	`getDay()`	Deprecated.As of JDK version 1.1, replaced by `Calendar.get(Calendar.DAY_OF_WEEK)`.
`int`	`getHours()`	Deprecated.As of JDK version 1.1, replaced by `Calendar.get(Calendar.HOUR_OF_DAY)`.
`int`	`getMinutes()`	Deprecated.As of JDK version 1.1, replaced by `Calendar.get(Calendar.MINUTE)`.
`int`	`getMonth()`	Deprecated.As of JDK version 1.1, replaced by `Calendar.get(Calendar.MONTH)`.
`int`	`getSeconds()`	Deprecated.As of JDK version 1.1, replaced by `Calendar.get(Calendar.SECOND)`.
`long`	`getTime()`	Returns the number of milliseconds since January 1, 1970, 00:00:00 GMT represented by this `Date` object.
`int`	`getTimezoneOffset()`	Deprecated.As of JDK version 1.1, replaced by `-(Calendar.get(Calendar.ZONE_OFFSET) + Calendar.get(Calendar.DST_OFFSET)) / (60 * 1000)`.
`int`	`getYear()`	Deprecated.As of JDK version 1.1, replaced by `Calendar.get(Calendar.YEAR) - 1900`.
`int`	`hashCode()`	Returns a hash code value for this object.
`static long`	`parse(String s)`	Deprecated.As of JDK version 1.1, replaced by `DateFormat.parse(String s)`.
`void`	`setDate(int date)`	Deprecated.As of JDK version 1.1, replaced by `Calendar.set(Calendar.DAY_OF_MONTH, int date)`.
`void`	`setHours(int hours)`	Deprecated.As of JDK version 1.1, replaced by `Calendar.set(Calendar.HOUR_OF_DAY, int hours)`.
`void`	`setMinutes(int minutes)`	Deprecated.As of JDK version 1.1, replaced by `Calendar.set(Calendar.MINUTE, int minutes)`.
`void`	`setMonth(int month)`	Deprecated.As of JDK version 1.1, replaced by `Calendar.set(Calendar.MONTH, int month)`.
`void`	`setSeconds(int seconds)`	Deprecated.As of JDK version 1.1, replaced by `Calendar.set(Calendar.SECOND, int seconds)`.
`void`	`setTime(long time)`	Sets this `Date` object to represent a point in time that is `time` milliseconds after January 1, 1970 00:00:00 GMT.
`void`	`setYear(int year)`	Deprecated.As of JDK version 1.1, replaced by `Calendar.set(Calendar.YEAR, year + 1900)`.
`String`	`toGMTString()`	Deprecated.As of JDK version 1.1, replaced by `DateFormat.format(Date date)`, using a GMT `TimeZone`.
`Instant`	`toInstant()`	Converts this `Date` object to an `Instant`.
`String`	`toLocaleString()`	Deprecated.As of JDK version 1.1, replaced by `DateFormat.format(Date date)`.
`String`	`toString()`	Converts this `Date` object to a `String` of the form:
`static long`	`UTC(int year, int month, int date, int hrs, int min, int sec)`	Deprecated.As of JDK version 1.1, replaced by `Calendar.set(year + 1900, month, date, hrs, min, sec)` or `GregorianCalendar(year + 1900, month, date, hrs, min, sec)`, using a UTC `TimeZone`, followed by `Calendar.getTime().getTime()`.

@Test
public void newDate() {Date d = new Date();System.out.println(d);final long l = System.currentTimeMillis();Date d1 = new Date(l);System.out.println(d1);
}

Output

Tue Jul 27 23:16:17 CST 2021
Tue Jul 27 23:16:17 CST 2021

Process finished with exit code 0

SimpleDateFormat

public class SimpleDateFormat
extends DateFormat

SimpleDateFormat is a concrete class for formatting and parsing dates in a locale-sensitive manner. It allows for formatting (date → text), parsing (text → date), and normalization.

Constructors

Constructor	Description
`SimpleDateFormat()`	Constructs a `SimpleDateFormat` using the default pattern and date format symbols for the default `FORMAT` locale.
`SimpleDateFormat(String pattern)`	Constructs a `SimpleDateFormat` using the given pattern and the default date format symbols for the default `FORMAT` locale.
`SimpleDateFormat(String pattern, DateFormatSymbols formatSymbols)`	Constructs a `SimpleDateFormat` using the given pattern and date format symbols.
`SimpleDateFormat(String pattern, Locale locale)`	Constructs a `SimpleDateFormat` using the given pattern and the default date format symbols for the given locale.

Methods

Modifier and Type	Method	Description
`String`	`format(Date date)`	Formats a `Date` into a date-time string.
`Date`	`parse(String source)`	Parses text from the beginning of the given string to produce a date.

@Testpublic void simpleDateFormat() {final long l = System.currentTimeMillis();Date d1 = new Date(l);SimpleDateFormat simpleDateFormat = new SimpleDateFormat("y/MM/dd HH:mm:ss.S");System.out.println(simpleDateFormat.format(d1));}

Output:

2021/7/27 23:38:1.723

Process finished with exit code 0

Letter	Date or Time Component	Presentation	Examples
`G`	Era designator	Text	`AD`
`y`	Year	Year	`1996`; `96`
`Y`	Week year	Year	`2009`; `09`
`M`	Month in year (context sensitive)	Month	`July`; `Jul`; `07`
`L`	Month in year (standalone form)	Month	`July`; `Jul`; `07`
`w`	Week in year	Number	`27`
`W`	Week in month	Number	`2`
`D`	Day in year	Number	`189`
`d`	Day in month	Number	`10`
`F`	Day of week in month	Number	`2`
`E`	Day name in week	Text	`Tuesday`; `Tue`
`u`	Day number of week (1 = Monday, …, 7 = Sunday)	Number	`1`
`a`	Am/pm marker	Text	`PM`
`H`	Hour in day (0-23)	Number	`0`
`k`	Hour in day (1-24)	Number	`24`
`K`	Hour in am/pm (0-11)	Number	`0`
`h`	Hour in am/pm (1-12)	Number	`12`
`m`	Minute in hour	Number	`30`
`s`	Second in minute	Number	`55`
`S`	Millisecond	Number	`978`
`z`	Time zone	General time zone	`Pacific Standard Time`; `PST`; `GMT-08:00`
`Z`	Time zone	RFC 822 time zone	`-0800`
`X`	Time zone	ISO 8601 time zone	`-08`; `-0800`; `-08:00`

Examples:

Date and Time Pattern	Result
`"yyyy.MM.dd G 'at' HH:mm:ss z"`	`2001.07.04 AD at 12:08:56 PDT`
`"EEE, MMM d, ''yy"`	`Wed, Jul 4, '01`
`"h:mm a"`	`12:08 PM`
`"hh 'o''clock' a, zzzz"`	`12 o'clock PM, Pacific Daylight Time`
`"K:mm a, z"`	`0:08 PM, PDT`
`"yyyyy.MMMMM.dd GGG hh:mm aaa"`	`02001.July.04 AD 12:08 PM`
`"EEE, d MMM yyyy HH:mm:ss Z"`	`Wed, 4 Jul 2001 12:08:56 -0700`
`"yyMMddHHmmssZ"`	`010704120856-0700`
`"yyyy-MM-dd'T'HH:mm:ss.SSSZ"`	`2001-07-04T12:08:56.235-0700`
`"yyyy-MM-dd'T'HH:mm:ss.SSSXXX"`	`2001-07-04T12:08:56.235-07:00`
`"YYYY-'W'ww-u"`	`2001-W27-3`

Calendar

public abstract class Calendar
extends Object
implements Serializable, Cloneable, Comparable<Calendar>

The Calendar class is an abstract class that provides methods for converting between a specific instant in time and a set of calendar fieldssuch as YEAR, MONTH, DAY_OF_MONTH, HOUR, and so on, and for manipulating the calendar fields, such as getting the date of the next week. An instant in time can be represented by a millisecond value that is an offset from the Epoch, January 1, 1970 00:00:00.000 GMT (Gregorian).

The class also provides additional fields and methods for implementing a concrete calendar system outside the package. Those fields and methods are defined as protected.

Fields

Modifier and Type	Field	Description
`static int`	`ALL_STYLES`	A style specifier for `getDisplayNames` indicating names in all styles, such as “January” and “Jan”.
`static int`	`AM`	Value of the `AM_PM` field indicating the period of the day from midnight to just before noon.
`static int`	`AM_PM`	Field number for `get` and `set` indicating whether the `HOUR` is before or after noon.
`static int`	`APRIL`	Value of the `MONTH` field indicating the fourth month of the year in the Gregorian and Julian calendars.
`protected boolean`	`areFieldsSet`	True if `fields[]` are in sync with the currently set time.
`static int`	`AUGUST`	Value of the `MONTH` field indicating the eighth month of the year in the Gregorian and Julian calendars.
`static int`	`DATE`	Field number for `get` and `set` indicating the day of the month.
`static int`	`DAY_OF_MONTH`	Field number for `get` and `set` indicating the day of the month.
`static int`	`DAY_OF_WEEK`	Field number for `get` and `set` indicating the day of the week.
`static int`	`DAY_OF_WEEK_IN_MONTH`	Field number for `get` and `set` indicating the ordinal number of the day of the week within the current month.
`static int`	`DAY_OF_YEAR`	Field number for `get` and `set` indicating the day number within the current year.
`static int`	`DECEMBER`	Value of the `MONTH` field indicating the twelfth month of the year in the Gregorian and Julian calendars.
`static int`	`DST_OFFSET`	Field number for `get` and `set` indicating the daylight saving offset in milliseconds.
`static int`	`ERA`	Field number for `get` and `set` indicating the era, e.g., AD or BC in the Julian calendar.
`static int`	`FEBRUARY`	Value of the `MONTH` field indicating the second month of the year in the Gregorian and Julian calendars.
`static int`	`FIELD_COUNT`	The number of distinct fields recognized by `get` and `set`.
`protected int[]`	`fields`	The calendar field values for the currently set time for this calendar.
`static int`	`FRIDAY`	Value of the `DAY_OF_WEEK` field indicating Friday.
`static int`	`HOUR`	Field number for `get` and `set` indicating the hour of the morning or afternoon.
`static int`	`HOUR_OF_DAY`	Field number for `get` and `set` indicating the hour of the day.
`protected boolean[]`	`isSet`	The flags which tell if a specified calendar field for the calendar is set.
`protected boolean`	`isTimeSet`	True if then the value of `time` is valid.
`static int`	`JANUARY`	Value of the `MONTH` field indicating the first month of the year in the Gregorian and Julian calendars.
`static int`	`JULY`	Value of the `MONTH` field indicating the seventh month of the year in the Gregorian and Julian calendars.
`static int`	`JUNE`	Value of the `MONTH` field indicating the sixth month of the year in the Gregorian and Julian calendars.
`static int`	`LONG`	A style specifier for `getDisplayName` and `getDisplayNames` equivalent to `LONG_FORMAT`.
`static int`	`LONG_FORMAT`	A style specifier for `getDisplayName` and `getDisplayNames` indicating a long name used for format.
`static int`	`LONG_STANDALONE`	A style specifier for `getDisplayName` and `getDisplayNames` indicating a long name used independently, such as a month name as calendar headers.
`static int`	`MARCH`	Value of the `MONTH` field indicating the third month of the year in the Gregorian and Julian calendars.
`static int`	`MAY`	Value of the `MONTH` field indicating the fifth month of the year in the Gregorian and Julian calendars.
`static int`	`MILLISECOND`	Field number for `get` and `set` indicating the millisecond within the second.
`static int`	`MINUTE`	Field number for `get` and `set` indicating the minute within the hour.
`static int`	`MONDAY`	Value of the `DAY_OF_WEEK` field indicating Monday.
`static int`	`MONTH`	Field number for `get` and `set` indicating the month.
`static int`	`NARROW_FORMAT`	A style specifier for `getDisplayName` and `getDisplayNames` indicating a narrow name used for format.
`static int`	`NARROW_STANDALONE`	A style specifier for `getDisplayName` and `getDisplayNames` indicating a narrow name independently.
`static int`	`NOVEMBER`	Value of the `MONTH` field indicating the eleventh month of the year in the Gregorian and Julian calendars.
`static int`	`OCTOBER`	Value of the `MONTH` field indicating the tenth month of the year in the Gregorian and Julian calendars.
`static int`	`PM`	Value of the `AM_PM` field indicating the period of the day from noon to just before midnight.
`static int`	`SATURDAY`	Value of the `DAY_OF_WEEK` field indicating Saturday.
`static int`	`SECOND`	Field number for `get` and `set` indicating the second within the minute.
`static int`	`SEPTEMBER`	Value of the `MONTH` field indicating the ninth month of the year in the Gregorian and Julian calendars.
`static int`	`SHORT`	A style specifier for `getDisplayName` and `getDisplayNames` equivalent to `SHORT_FORMAT`.
`static int`	`SHORT_FORMAT`	A style specifier for `getDisplayName` and `getDisplayNames` indicating a short name used for format.
`static int`	`SHORT_STANDALONE`	A style specifier for `getDisplayName` and `getDisplayNames` indicating a short name used independently, such as a month abbreviation as calendar headers.
`static int`	`SUNDAY`	Value of the `DAY_OF_WEEK` field indicating Sunday.
`static int`	`THURSDAY`	Value of the `DAY_OF_WEEK` field indicating Thursday.
`protected long`	`time`	The currently set time for this calendar, expressed in milliseconds after January 1, 1970, 0:00:00 GMT.
`static int`	`TUESDAY`	Value of the `DAY_OF_WEEK` field indicating Tuesday.
`static int`	`UNDECIMBER`	Value of the `MONTH` field indicating the thirteenth month of the year.
`static int`	`WEDNESDAY`	Value of the `DAY_OF_WEEK` field indicating Wednesday.
`static int`	`WEEK_OF_MONTH`	Field number for `get` and `set` indicating the week number within the current month.
`static int`	`WEEK_OF_YEAR`	Field number for `get` and `set` indicating the week number within the current year.
`static int`	`YEAR`	Field number for `get` and `set` indicating the year.
`static int`	`ZONE_OFFSET`	Field number for `get` and `set` indicating the raw offset from GMT in milliseconds.

Constructors

Modifier	Constructor	Description
`protected`	`Calendar()`	Constructs a Calendar with the default time zone and the default `FORMAT` locale.
`protected`	`Calendar(TimeZone zone, Locale aLocale)`	Constructs a calendar with the specified time zone and locale.

Methods

Modifier and Type	Method	Description
`abstract void`	`add(int field, int amount)`	Adds or subtracts the specified amount of time to the given calendar field, based on the calendar’s rules.
`boolean`	`after(Object when)`	Returns whether this `Calendar` represents a time after the time represented by the specified `Object`.
`boolean`	`before(Object when)`	Returns whether this `Calendar` represents a time before the time represented by the specified `Object`.
`void`	`clear()`	Sets all the calendar field values and the time value (millisecond offset from the Epoch) of this `Calendar` undefined.
`void`	`clear(int field)`	Sets the given calendar field value and the time value (millisecond offset from the Epoch) of this `Calendar` undefined.
`Object`	`clone()`	Creates and returns a copy of this object.
`int`	`compareTo(Calendar anotherCalendar)`	Compares the time values (millisecond offsets from the Epoch) represented by two `Calendar` objects.
`protected void`	`complete()`	Fills in any unset fields in the calendar fields.
`protected abstract void`	`computeFields()`	Converts the current millisecond time value `time` to calendar field values in `fields[\]`.
`protected abstract void`	`computeTime()`	Converts the current calendar field values in `fields[\]` to the millisecond time value `time`.
`boolean`	`equals(Object obj)`	Compares this `Calendar` to the specified `Object`.
`int`	`get(int field)`	Returns the value of the given calendar field.
`int`	`getActualMaximum(int field)`	Returns the maximum value that the specified calendar field could have, given the time value of this `Calendar`.
`int`	`getActualMinimum(int field)`	Returns the minimum value that the specified calendar field could have, given the time value of this `Calendar`.
`static Set<String>`	`getAvailableCalendarTypes()`	Returns an unmodifiable `Set` containing all calendar types supported by `Calendar` in the runtime environment.
`static Locale[]`	`getAvailableLocales()`	Returns an array of all locales for which the `getInstance` methods of this class can return localized instances.
`String`	`getCalendarType()`	Returns the calendar type of this `Calendar`.
`String`	`getDisplayName(int field, int style, Locale locale)`	Returns the string representation of the calendar `field` value in the given `style` and `locale`.
`Map<String,Integer>`	`getDisplayNames(int field, int style, Locale locale)`	Returns a `Map` containing all names of the calendar `field` in the given `style` and `locale` and their corresponding field values.
`int`	`getFirstDayOfWeek()`	Gets what the first day of the week is; e.g., `SUNDAY` in the U.S., `MONDAY` in France.
`abstract int`	`getGreatestMinimum(int field)`	Returns the highest minimum value for the given calendar field of this `Calendar` instance.
`static Calendar`	`getInstance()`	Gets a calendar using the default time zone and locale.
`static Calendar`	`getInstance(Locale aLocale)`	Gets a calendar using the default time zone and specified locale.
`static Calendar`	`getInstance(TimeZone zone)`	Gets a calendar using the specified time zone and default locale.
`static Calendar`	`getInstance(TimeZone zone, Locale aLocale)`	Gets a calendar with the specified time zone and locale.
`abstract int`	`getLeastMaximum(int field)`	Returns the lowest maximum value for the given calendar field of this `Calendar` instance.
`abstract int`	`getMaximum(int field)`	Returns the maximum value for the given calendar field of this `Calendar` instance.
`int`	`getMinimalDaysInFirstWeek()`	Gets what the minimal days required in the first week of the year are; e.g., if the first week is defined as one that contains the first day of the first month of a year, this method returns 1.
`abstract int`	`getMinimum(int field)`	Returns the minimum value for the given calendar field of this `Calendar` instance.
`Date`	`getTime()`	Returns a `Date` object representing this `Calendar`'s time value (millisecond offset from the Epoch").
`long`	`getTimeInMillis()`	Returns this Calendar’s time value in milliseconds.
`TimeZone`	`getTimeZone()`	Gets the time zone.
`int`	`getWeeksInWeekYear()`	Returns the number of weeks in the week year represented by this `Calendar`.
`int`	`getWeekYear()`	Returns the week year represented by this `Calendar`.
`int`	`hashCode()`	Returns a hash code for this calendar.
`protected int`	`internalGet(int field)`	Returns the value of the given calendar field.
`boolean`	`isLenient()`	Tells whether date/time interpretation is to be lenient.
`boolean`	`isSet(int field)`	Determines if the given calendar field has a value set, including cases that the value has been set by internal fields calculations triggered by a `get` method call.
`boolean`	`isWeekDateSupported()`	Returns whether this `Calendar` supports week dates.
`abstract void`	`roll(int field, boolean up)`	Adds or subtracts (up/down) a single unit of time on the given time field without changing larger fields.
`void`	`roll(int field, int amount)`	Adds the specified (signed) amount to the specified calendar field without changing larger fields.
`void`	`set(int field, int value)`	Sets the given calendar field to the given value.
`void`	`set(int year, int month, int date)`	Sets the values for the calendar fields `YEAR`, `MONTH`(starts from 0), and `DAY_OF_MONTH`.
`void`	`set(int year, int month, int date, int hourOfDay, int minute)`	Sets the values for the calendar fields `YEAR`, `MONTH`, `DAY_OF_MONTH`, `HOUR_OF_DAY`, and `MINUTE`.
`void`	`set(int year, int month, int date, int hourOfDay, int minute, int second)`	Sets the values for the fields `YEAR`, `MONTH`, `DAY_OF_MONTH`, `HOUR_OF_DAY`, `MINUTE`, and `SECOND`.
`void`	`setFirstDayOfWeek(int value)`	Sets what the first day of the week is; e.g., `SUNDAY` in the U.S., `MONDAY` in France.
`void`	`setLenient(boolean lenient)`	Specifies whether or not date/time interpretation is to be lenient.
`void`	`setMinimalDaysInFirstWeek(int value)`	Sets what the minimal days required in the first week of the year are; For example, if the first week is defined as one that contains the first day of the first month of a year, call this method with value 1.
`void`	`setTime(Date date)`	Sets this Calendar’s time with the given `Date`.
`void`	`setTimeInMillis(long millis)`	Sets this Calendar’s current time from the given long value.
`void`	`setTimeZone(TimeZone value)`	Sets the time zone with the given time zone value.
`void`	`setWeekDate(int weekYear, int weekOfYear, int dayOfWeek)`	Sets the date of this `Calendar` with the given date specifiers - week year, week of year, and day of week.
`Instant`	`toInstant()`	Converts this object to an `Instant`.
`String`	`toString()`	Return a string representation of this calendar.

Parameter month in set method starts from 0 (Calendar.JULY=6).

Calendar calendar = Calendar.getInstance();  // return Calendar's extended class instance
calendar.set(2021, Calendar.JULY, 29, 22, 9);
calendar.set(Calendar.YEAR, 2019);
calendar.set(Calendar.MONTH, Calendar.DECEMBER);
calendar.set(Calendar.HOUR, 8);
calendar.add(Calendar.HOUR, 2);

GregorianCalendar

public class GregorianCalendar
extends Calendar

Constructors

Constructor	Description
`GregorianCalendar()`	Constructs a default `GregorianCalendar` using the current time in the default time zone with the default `FORMAT` locale.
`GregorianCalendar(int year, int month, int dayOfMonth)`	Constructs a `GregorianCalendar` with the given date set in the default time zone with the default locale.
`GregorianCalendar(int year, int month, int dayOfMonth, int hourOfDay, int minute)`	Constructs a `GregorianCalendar` with the given date and time set for the default time zone with the default locale.
`GregorianCalendar(int year, int month, int dayOfMonth, int hourOfDay, int minute, int second)`	Constructs a GregorianCalendar with the given date and time set for the default time zone with the default locale.
`GregorianCalendar(Locale aLocale)`	Constructs a `GregorianCalendar` based on the current time in the default time zone with the given locale.
`GregorianCalendar(TimeZone zone)`	Constructs a `GregorianCalendar` based on the current time in the given time zone with the default `FORMAT` locale.
`GregorianCalendar(TimeZone zone, Locale aLocale)`	Constructs a `GregorianCalendar` based on the current time in the given time zone with the given locale.

Methods

Modifier and Type	Method	Description
`void`	`add(int field, int amount)`	Adds the specified (signed) amount of time to the given calendar field, based on the calendar’s rules.
`protected void`	`computeFields()`	Converts the time value (millisecond offset from the Epoch) to calendar field values.
`protected void`	`computeTime()`	Converts calendar field values to the time value (millisecond offset from the Epoch).
`boolean`	`equals(Object obj)`	Compares this `GregorianCalendar` to the specified `Object`.
`static GregorianCalendar`	`from(ZonedDateTime zdt)`	Obtains an instance of `GregorianCalendar` with the default locale from a `ZonedDateTime` object.
`int`	`getActualMaximum(int field)`	Returns the maximum value that this calendar field could have, taking into consideration the given time value and the current values of the `getFirstDayOfWeek`, `getMinimalDaysInFirstWeek`, `getGregorianChange` and `getTimeZone` methods.
`int`	`getActualMinimum(int field)`	Returns the minimum value that this calendar field could have, taking into consideration the given time value and the current values of the `getFirstDayOfWeek`, `getMinimalDaysInFirstWeek`, `getGregorianChange` and `getTimeZone` methods.
`String`	`getCalendarType()`	Returns `"gregory"` as the calendar type.
`int`	`getGreatestMinimum(int field)`	Returns the highest minimum value for the given calendar field of this `GregorianCalendar` instance.
`Date`	`getGregorianChange()`	Gets the Gregorian Calendar change date.
`int`	`getLeastMaximum(int field)`	Returns the lowest maximum value for the given calendar field of this `GregorianCalendar` instance.
`int`	`getMaximum(int field)`	Returns the maximum value for the given calendar field of this `GregorianCalendar` instance.
`int`	`getMinimum(int field)`	Returns the minimum value for the given calendar field of this `GregorianCalendar` instance.
`int`	`getWeeksInWeekYear()`	Returns the number of weeks in the week year represented by this `GregorianCalendar`.
`int`	`getWeekYear()`	Returns the week year represented by this `GregorianCalendar`.
`int`	`hashCode()`	Generates the hash code for this `GregorianCalendar` object.
`boolean`	`isLeapYear(int year)`	Determines if the given year is a leap year.
`boolean`	`isWeekDateSupported()`	Returns `true` indicating this `GregorianCalendar` supports week dates.
`void`	`roll(int field, boolean up)`	Adds or subtracts (up/down) a single unit of time on the given time field without changing larger fields.
`void`	`roll(int field, int amount)`	Adds a signed amount to the specified calendar field without changing larger fields.
`void`	`setGregorianChange(Date date)`	Sets the `GregorianCalendar` change date.
`void`	`setWeekDate(int weekYear, int weekOfYear, int dayOfWeek)`	Sets this `GregorianCalendar` to the date given by the date specifiers - `weekYear`, `weekOfYear`, and `dayOfWeek`.
`ZonedDateTime`	`toZonedDateTime()`	Converts this object to a `ZonedDateTime` that represents the same point on the time-line as this `GregorianCalendar`.

多态的使用场合

参数个数不同形成多态
父类指向不同子类形成多态
返回值不同形成多态

Java 8日期相关类

Java 8发布的新的处理日期时间类数据的方案：

package java.time：The main API for dates, times, instants, and durations.

The classes defined here represent the principle date-time concepts, including instants, durations, dates, times, time-zones and periods. They are based on the ISO calendar system, which is the de facto world calendar following the proleptic Gregorian rules. All the classes are immutable and thread-safe.
package java.time.chrono：Generic API for calendar systems other than the default ISO.

The main API is based around the calendar system defined in ISO-8601. However, there are other calendar systems, and this package provides basic support for them.
package java.time.format：Provides classes to print and parse dates and times.

Printing and parsing is based around the DateTimeFormatter class. Instances are generally obtained from DateTimeFormatter, however DateTimeFormatterBuilder can be used if more power is needed.

Localization occurs by calling withLocale(Locale) on the formatter. Further customization is possible using DecimalStyle.
package java.time.temporal：Access to date and time using fields and units, and date time adjusters.

This package expands on the base package to provide additional functionality for more powerful use cases. Support is included for:
- Units of date-time, such as years, months, days and hours
- Fields of date-time, such as month-of-year, day-of-week or hour-of-day
- Date-time adjustment functions
- Different definitions of weeks
package java.time.zone：Support for time-zones and their rules.

Daylight Saving Time and Time-Zones are concepts used by Governments to alter local time. This package provides support for time-zones, their rules and the resulting gaps and overlaps in the local time-line typically caused by Daylight Saving Time.

class `LocalDate`

public final class LocalDate
extends Object
implements Temporal, TemporalAdjuster, ChronoLocalDate, Serializable

A date without a time-zone in the ISO-8601 calendar system, such as 2007-12-03.

This class is immutable and thread-safe.

Fields

Modifier and Type	Field	Description
`static LocalDate`	`EPOCH`	The epoch year `LocalDate`, ‘1970-01-01’.
`static LocalDate`	`MAX`	The maximum supported `LocalDate`, ‘+999999999-12-31’.
`static LocalDate`	`MIN`	The minimum supported `LocalDate`, ‘-999999999-01-01’.

Methods

Modifier and Type	Method	Description
`static LocalDate`	`now()`	Obtains the current date from the system clock in the default time-zone.
`static LocalDate`	`now(Clock clock)`	Obtains the current date from the specified clock.
`static LocalDate`	`now(ZoneId zone)`	Obtains the current date from the system clock in the specified time-zone.
`Temporal`	`adjustInto(Temporal temporal)`	Adjusts the specified temporal object to have the same date as this object.
`LocalDateTime`	`atStartOfDay()`	Combines this date with the time of midnight to create a `LocalDateTime` at the start of this date.
`ZonedDateTime`	`atStartOfDay(ZoneId zone)`	Returns a zoned date-time from this date at the earliest valid time according to the rules in the time-zone.
`LocalDateTime`	`atTime(int hour, int minute)`	Combines this date with a time to create a `LocalDateTime`.
`LocalDateTime`	`atTime(int hour, int minute, int second)`	Combines this date with a time to create a `LocalDateTime`.
`LocalDateTime`	`atTime(int hour, int minute, int second, int nanoOfSecond)`	Combines this date with a time to create a `LocalDateTime`.
`LocalDateTime`	`atTime(LocalTime time)`	Combines this date with a time to create a `LocalDateTime`.
`OffsetDateTime`	`atTime(OffsetTime time)`	Combines this date with an offset time to create an `OffsetDateTime`.
`int`	`compareTo(ChronoLocalDate other)`	Compares this date to another date.
`Stream<LocalDate>`	`datesUntil(LocalDate endExclusive)`	Returns a sequential ordered stream of dates.
`Stream<LocalDate>`	`datesUntil(LocalDate endExclusive, Period step)`	Returns a sequential ordered stream of dates by given incremental step.
`boolean`	`equals(Object obj)`	Checks if this date is equal to another date.
`String`	`format(DateTimeFormatter formatter)`	Formats this date using the specified formatter.
`static LocalDate`	`from(TemporalAccessor temporal)`	Obtains an instance of `LocalDate` from a temporal object.
`int`	`get(TemporalField field)`	Gets the value of the specified field from this date as an `int`.
`IsoChronology`	`getChronology()`	Gets the chronology of this date, which is the ISO calendar system.
`int`	`getDayOfMonth()`	Gets the day-of-month field.
`DayOfWeek`	`getDayOfWeek()`	Gets the day-of-week field, which is an enum `DayOfWeek`.
`int`	`getDayOfYear()`	Gets the day-of-year field.
`IsoEra`	`getEra()`	Gets the era applicable at this date.
`long`	`getLong(TemporalField field)`	Gets the value of the specified field from this date as a `long`.
`Month`	`getMonth()`	Gets the month-of-year field using the `Month` enum.
`int`	`getMonthValue()`	Gets the month-of-year field from 1 to 12.
`int`	`getYear()`	Gets the year field.
`int`	`hashCode()`	A hash code for this date.
`boolean`	`isAfter(ChronoLocalDate other)`	Checks if this date is after the specified date.
`boolean`	`isBefore(ChronoLocalDate other)`	Checks if this date is before the specified date.
`boolean`	`isEqual(ChronoLocalDate other)`	Checks if this date is equal to the specified date.
`boolean`	`isLeapYear()`	Checks if the year is a leap year, according to the ISO proleptic calendar system rules.
`boolean`	`isSupported(TemporalField field)`	Checks if the specified field is supported.
`boolean`	`isSupported(TemporalUnit unit)`	Checks if the specified unit is supported.
`int`	`lengthOfMonth()`	Returns the length of the month represented by this date.
`int`	`lengthOfYear()`	Returns the length of the year represented by this date.
`LocalDate`	`minus(long amountToSubtract, TemporalUnit unit)`	Returns a copy of this date with the specified amount subtracted.
`LocalDate`	`minus(TemporalAmount amountToSubtract)`	Returns a copy of this date with the specified amount subtracted.
`LocalDate`	`minusDays(long daysToSubtract)`	Returns a copy of this `LocalDate` with the specified number of days subtracted.
`LocalDate`	`minusMonths(long monthsToSubtract)`	Returns a copy of this `LocalDate` with the specified number of months subtracted.
`LocalDate`	`minusWeeks(long weeksToSubtract)`	Returns a copy of this `LocalDate` with the specified number of weeks subtracted.
`LocalDate`	`minusYears(long yearsToSubtract)`	Returns a copy of this `LocalDate` with the specified number of years subtracted.
`static LocalDate`	`of(int year, int month, int dayOfMonth)`	Obtains an instance of `LocalDate` from a year, month and day.
`static LocalDate`	`of(int year, Month month, int dayOfMonth)`	Obtains an instance of `LocalDate` from a year, month and day.
`static LocalDate`	`ofEpochDay(long epochDay)`	Obtains an instance of `LocalDate` from the epoch day count.
`static LocalDate`	`ofInstant(Instant instant, ZoneId zone)`	Obtains an instance of `LocalDate` from an `Instant` and zone ID.
`static LocalDate`	`ofYearDay(int year, int dayOfYear)`	Obtains an instance of `LocalDate` from a year and day-of-year.
`static LocalDate`	`parse(CharSequence text)`	Obtains an instance of `LocalDate` from a text string such as `2007-12-03`.
`static LocalDate`	`parse(CharSequence text, DateTimeFormatter formatter)`	Obtains an instance of `LocalDate` from a text string using a specific formatter.
`LocalDate`	`plus(long amountToAdd, TemporalUnit unit)`	Returns a copy of this date with the specified amount added.
`LocalDate`	`plus(TemporalAmount amountToAdd)`	Returns a copy of this date with the specified amount added.
`LocalDate`	`plusDays(long daysToAdd)`	Returns a copy of this `LocalDate` with the specified number of days added.
`LocalDate`	`plusMonths(long monthsToAdd)`	Returns a copy of this `LocalDate` with the specified number of months added.
`LocalDate`	`plusWeeks(long weeksToAdd)`	Returns a copy of this `LocalDate` with the specified number of weeks added.
`LocalDate`	`plusYears(long yearsToAdd)`	Returns a copy of this `LocalDate` with the specified number of years added.
`<R> R`	`query(TemporalQuery<R> query)`	Queries this date using the specified query.
`ValueRange`	`range(TemporalField field)`	Gets the range of valid values for the specified field.
`long`	`toEpochSecond(LocalTime time, ZoneOffset offset)`	Converts this `LocalDate` to the number of seconds since the epoch of 1970-01-01T00:00:00Z.
`String`	`toString()`	Outputs this date as a `String`, such as `2007-12-03`.
`Period`	`until(ChronoLocalDate endDateExclusive)`	Calculates the period between this date and another date as a `Period`.
`long`	`until(Temporal endExclusive, TemporalUnit unit)`	Calculates the amount of time until another date in terms of the specified unit.
`LocalDate`	`with(TemporalAdjuster adjuster)`	Returns an adjusted copy of this date.
`LocalDate`	`with(TemporalField field, long newValue)`	Returns a copy of this date with the specified field set to a new value.
`LocalDate`	`withDayOfMonth(int dayOfMonth)`	Returns a copy of this `LocalDate` with the day-of-month altered.
`LocalDate`	`withDayOfYear(int dayOfYear)`	Returns a copy of this `LocalDate` with the day-of-year altered.
`LocalDate`	`withMonth(int month)`	Returns a copy of this `LocalDate` with the month-of-year altered.
`LocalDate`	`withYear(int year)`	Returns a copy of this `LocalDate` with the year altered.

final LocalDate now = LocalDate.now();
System.out.println(now);

Output:

2021-07-29

Process finished with exit code 0

class `LocalDateTime`

public final class LocalDateTime
extends Object
implements Temporal, TemporalAdjuster, ChronoLocalDateTime<LocalDate>, Serializable

A date-time without a time-zone in the ISO-8601 calendar system, such as 2007-12-03T10:15:30.

LocalDateTime is an immutable date-time object that represents a date-time, often viewed as year-month-day-hour-minute-second. Other date and time fields, such as day-of-year, day-of-week and week-of-year, can also be accessed. Time is represented to nanosecond precision. For example, the value “2nd October 2007 at 13:45.30.123456789” can be stored in a LocalDateTime.

This class is immutable and thread-safe.

Fields

Modifier and Type	Field	Description
`static LocalDateTime`	`MAX`	The maximum supported `LocalDateTime`, ‘+999999999-12-31T23:59:59.999999999’.
`static LocalDateTime`	`MIN`	The minimum supported `LocalDateTime`, ‘-999999999-01-01T00:00:00’.

Methods

Modifier and Type	Method	Description
`Temporal`	`adjustInto(Temporal temporal)`	Adjusts the specified temporal object to have the same date and time as this object.
`OffsetDateTime`	`atOffset(ZoneOffset offset)`	Combines this date-time with an offset to create an `OffsetDateTime`.
`ZonedDateTime`	`atZone(ZoneId zone)`	Combines this date-time with a time-zone to create a `ZonedDateTime`.
`int`	`compareTo(ChronoLocalDateTime<?> other)`	Compares this date-time to another date-time.
`boolean`	`equals(Object obj)`	Checks if this date-time is equal to another date-time.
`String`	`format(DateTimeFormatter formatter)`	Formats this date-time using the specified formatter.
`static LocalDateTime`	`from(TemporalAccessor temporal)`	Obtains an instance of `LocalDateTime` from a temporal object.
`int`	`get(TemporalField field)`	Gets the value of the specified field from this date-time as an `int`.
`int`	`getDayOfMonth()`	Gets the day-of-month field.
`DayOfWeek`	`getDayOfWeek()`	Gets the day-of-week field, which is an enum `DayOfWeek`.
`int`	`getDayOfYear()`	Gets the day-of-year field.
`int`	`getHour()`	Gets the hour-of-day field.
`long`	`getLong(TemporalField field)`	Gets the value of the specified field from this date-time as a `long`.
`int`	`getMinute()`	Gets the minute-of-hour field.
`Month`	`getMonth()`	Gets the month-of-year field using the `Month` enum.
`int`	`getMonthValue()`	Gets the month-of-year field from 1 to 12.
`int`	`getNano()`	Gets the nano-of-second field.
`int`	`getSecond()`	Gets the second-of-minute field.
`int`	`getYear()`	Gets the year field.
`int`	`hashCode()`	A hash code for this date-time.
`boolean`	`isAfter(ChronoLocalDateTime<?> other)`	Checks if this date-time is after the specified date-time.
`boolean`	`isBefore(ChronoLocalDateTime<?> other)`	Checks if this date-time is before the specified date-time.
`boolean`	`isEqual(ChronoLocalDateTime<?> other)`	Checks if this date-time is equal to the specified date-time.
`boolean`	`isSupported(TemporalField field)`	Checks if the specified field is supported.
`boolean`	`isSupported(TemporalUnit unit)`	Checks if the specified unit is supported.
`LocalDateTime`	`minus(long amountToSubtract, TemporalUnit unit)`	Returns a copy of this date-time with the specified amount subtracted.
`LocalDateTime`	`minus(TemporalAmount amountToSubtract)`	Returns a copy of this date-time with the specified amount subtracted.
`LocalDateTime`	`minusDays(long days)`	Returns a copy of this `LocalDateTime` with the specified number of days subtracted.
`LocalDateTime`	`minusHours(long hours)`	Returns a copy of this `LocalDateTime` with the specified number of hours subtracted.
`LocalDateTime`	`minusMinutes(long minutes)`	Returns a copy of this `LocalDateTime` with the specified number of minutes subtracted.
`LocalDateTime`	`minusMonths(long months)`	Returns a copy of this `LocalDateTime` with the specified number of months subtracted.
`LocalDateTime`	`minusNanos(long nanos)`	Returns a copy of this `LocalDateTime` with the specified number of nanoseconds subtracted.
`LocalDateTime`	`minusSeconds(long seconds)`	Returns a copy of this `LocalDateTime` with the specified number of seconds subtracted.
`LocalDateTime`	`minusWeeks(long weeks)`	Returns a copy of this `LocalDateTime` with the specified number of weeks subtracted.
`LocalDateTime`	`minusYears(long years)`	Returns a copy of this `LocalDateTime` with the specified number of years subtracted.
`static LocalDateTime`	`now()`	Obtains the current date-time from the system clock in the default time-zone.
`static LocalDateTime`	`now(Clock clock)`	Obtains the current date-time from the specified clock.
`static LocalDateTime`	`now(ZoneId zone)`	Obtains the current date-time from the system clock in the specified time-zone.
`static LocalDateTime`	`of(int year, int month, int dayOfMonth, int hour, int minute)`	Obtains an instance of `LocalDateTime` from year, month, day, hour and minute, setting the second and nanosecond to zero.
`static LocalDateTime`	`of(int year, int month, int dayOfMonth, int hour, int minute, int second)`	Obtains an instance of `LocalDateTime` from year, month, day, hour, minute and second, setting the nanosecond to zero.
`static LocalDateTime`	`of(int year, int month, int dayOfMonth, int hour, int minute, int second, int nanoOfSecond)`	Obtains an instance of `LocalDateTime` from year, month, day, hour, minute, second and nanosecond.
`static LocalDateTime`	`of(int year, Month month, int dayOfMonth, int hour, int minute)`	Obtains an instance of `LocalDateTime` from year, month, day, hour and minute, setting the second and nanosecond to zero.
`static LocalDateTime`	`of(int year, Month month, int dayOfMonth, int hour, int minute, int second)`	Obtains an instance of `LocalDateTime` from year, month, day, hour, minute and second, setting the nanosecond to zero.
`static LocalDateTime`	`of(int year, Month month, int dayOfMonth, int hour, int minute, int second, int nanoOfSecond)`	Obtains an instance of `LocalDateTime` from year, month, day, hour, minute, second and nanosecond.
`static LocalDateTime`	`of(LocalDate date, LocalTime time)`	Obtains an instance of `LocalDateTime` from a date and time.
`static LocalDateTime`	`ofEpochSecond(long epochSecond, int nanoOfSecond, ZoneOffset offset)`	Obtains an instance of `LocalDateTime` using seconds from the epoch of 1970-01-01T00:00:00Z.
`static LocalDateTime`	`ofInstant(Instant instant, ZoneId zone)`	Obtains an instance of `LocalDateTime` from an `Instant` and zone ID.
`static LocalDateTime`	`parse(CharSequence text)`	Obtains an instance of `LocalDateTime` from a text string such as `2007-12-03T10:15:30`.
`static LocalDateTime`	`parse(CharSequence text, DateTimeFormatter formatter)`	Obtains an instance of `LocalDateTime` from a text string using a specific formatter.
`LocalDateTime`	`plus(long amountToAdd, TemporalUnit unit)`	Returns a copy of this date-time with the specified amount added.
`LocalDateTime`	`plus(TemporalAmount amountToAdd)`	Returns a copy of this date-time with the specified amount added.
`LocalDateTime`	`plusDays(long days)`	Returns a copy of this `LocalDateTime` with the specified number of days added.
`LocalDateTime`	`plusHours(long hours)`	Returns a copy of this `LocalDateTime` with the specified number of hours added.
`LocalDateTime`	`plusMinutes(long minutes)`	Returns a copy of this `LocalDateTime` with the specified number of minutes added.
`LocalDateTime`	`plusMonths(long months)`	Returns a copy of this `LocalDateTime` with the specified number of months added.
`LocalDateTime`	`plusNanos(long nanos)`	Returns a copy of this `LocalDateTime` with the specified number of nanoseconds added.
`LocalDateTime`	`plusSeconds(long seconds)`	Returns a copy of this `LocalDateTime` with the specified number of seconds added.
`LocalDateTime`	`plusWeeks(long weeks)`	Returns a copy of this `LocalDateTime` with the specified number of weeks added.
`LocalDateTime`	`plusYears(long years)`	Returns a copy of this `LocalDateTime` with the specified number of years added.
`<R> R`	`query(TemporalQuery<R> query)`	Queries this date-time using the specified query.
`ValueRange`	`range(TemporalField field)`	Gets the range of valid values for the specified field.
`LocalDate`	`toLocalDate()`	Gets the `LocalDate` part of this date-time.
`LocalTime`	`toLocalTime()`	Gets the `LocalTime` part of this date-time.
`String`	`toString()`	Outputs this date-time as a `String`, such as `2007-12-03T10:15:30`.
`LocalDateTime`	`truncatedTo(TemporalUnit unit)`	Returns a copy of this `LocalDateTime` with the time truncated.
`long`	`until(Temporal endExclusive, TemporalUnit unit)`	Calculates the amount of time until another date-time in terms of the specified unit.
`LocalDateTime`	`with(TemporalAdjuster adjuster)`	Returns an adjusted copy of this date-time.
`LocalDateTime`	`with(TemporalField field, long newValue)`	Returns a copy of this date-time with the specified field set to a new value.
`LocalDateTime`	`withDayOfMonth(int dayOfMonth)`	Returns a copy of this `LocalDateTime` with the day-of-month altered.
`LocalDateTime`	`withDayOfYear(int dayOfYear)`	Returns a copy of this `LocalDateTime` with the day-of-year altered.
`LocalDateTime`	`withHour(int hour)`	Returns a copy of this `LocalDateTime` with the hour-of-day altered.
`LocalDateTime`	`withMinute(int minute)`	Returns a copy of this `LocalDateTime` with the minute-of-hour altered.
`LocalDateTime`	`withMonth(int month)`	Returns a copy of this `LocalDateTime` with the month-of-year altered.
`LocalDateTime`	`withNano(int nanoOfSecond)`	Returns a copy of this `LocalDateTime` with the nano-of-second altered.
`LocalDateTime`	`withSecond(int second)`	Returns a copy of this `LocalDateTime` with the second-of-minute altered.
`LocalDateTime`	`withYear(int year)`	Returns a copy of this `LocalDateTime` with the year altered.

final LocalDateTime now = LocalDateTime.now();
System.out.println(now);
final LocalDateTime dt = LocalDateTime.of(2021, 7, 29, 10, 53);
System.out.println(dt);

Output:

2021-07-29T22:47:08.724731600

2021-07-29T10:53

Process finished with exit code 0

class `Instant`

public final class Instant
extends Object
implements Temporal, TemporalAdjuster, Comparable<Instant>, Serializable

An instantaneous point on the time-line. This class models a single instantaneous point on the time-line. This might be used to record event time-stamps in the application.

This class is immutable and thread-safe.

Fields

Modifier and Type	Field	Description
`static Instant`	`EPOCH`	Constant for the 1970-01-01T00:00:00Z epoch instant.
`static Instant`	`MAX`	The maximum supported `Instant`, ‘1000000000-12-31T23:59:59.999999999Z’.
`static Instant`	`MIN`	The minimum supported `Instant`, ‘-1000000000-01-01T00:00Z’.

Methods

Modifier and Type	Method	Description
`Temporal`	`adjustInto(Temporal temporal)`	Adjusts the specified temporal object to have this instant.
`OffsetDateTime`	`atOffset(ZoneOffset offset)`	Combines this instant with an offset to create an `OffsetDateTime`.
`ZonedDateTime`	`atZone(ZoneId zone)`	Combines this instant with a time-zone to create a `ZonedDateTime`.
`int`	`compareTo(Instant otherInstant)`	Compares this instant to the specified instant.
`boolean`	`equals(Object otherInstant)`	Checks if this instant is equal to the specified instant.
`static Instant`	`from(TemporalAccessor temporal)`	Obtains an instance of `Instant` from a temporal object.
`int`	`get(TemporalField field)`	Gets the value of the specified field from this instant as an `int`.
`long`	`getEpochSecond()`	Gets the number of seconds from the Java epoch of 1970-01-01T00:00:00Z.
`long`	`getLong(TemporalField field)`	Gets the value of the specified field from this instant as a `long`.
`int`	`getNano()`	Gets the number of nanoseconds, later along the time-line, from the start of the second.
`int`	`hashCode()`	Returns a hash code for this instant.
`boolean`	`isAfter(Instant otherInstant)`	Checks if this instant is after the specified instant.
`boolean`	`isBefore(Instant otherInstant)`	Checks if this instant is before the specified instant.
`boolean`	`isSupported(TemporalField field)`	Checks if the specified field is supported.
`boolean`	`isSupported(TemporalUnit unit)`	Checks if the specified unit is supported.
`Instant`	`minus(long amountToSubtract, TemporalUnit unit)`	Returns a copy of this instant with the specified amount subtracted.
`Instant`	`minus(TemporalAmount amountToSubtract)`	Returns a copy of this instant with the specified amount subtracted.
`Instant`	`minusMillis(long millisToSubtract)`	Returns a copy of this instant with the specified duration in milliseconds subtracted.
`Instant`	`minusNanos(long nanosToSubtract)`	Returns a copy of this instant with the specified duration in nanoseconds subtracted.
`Instant`	`minusSeconds(long secondsToSubtract)`	Returns a copy of this instant with the specified duration in seconds subtracted.
`static Instant`	`now()`	Obtains the current instant from the system clock (UTC 0).
`static Instant`	`now(Clock clock)`	Obtains the current instant from the specified clock.
`static Instant`	`ofEpochMilli(long epochMilli)`	Obtains an instance of `Instant` using milliseconds from the epoch of 1970-01-01T00:00:00Z.
`static Instant`	`ofEpochSecond(long epochSecond)`	Obtains an instance of `Instant` using seconds from the epoch of 1970-01-01T00:00:00Z.
`static Instant`	`ofEpochSecond(long epochSecond, long nanoAdjustment)`	Obtains an instance of `Instant` using seconds from the epoch of 1970-01-01T00:00:00Z and nanosecond fraction of second.
`static Instant`	`parse(CharSequence text)`	Obtains an instance of `Instant` from a text string such as `2007-12-03T10:15:30.00Z`.
`Instant`	`plus(long amountToAdd, TemporalUnit unit)`	Returns a copy of this instant with the specified amount added.
`Instant`	`plus(TemporalAmount amountToAdd)`	Returns a copy of this instant with the specified amount added.
`Instant`	`plusMillis(long millisToAdd)`	Returns a copy of this instant with the specified duration in milliseconds added.
`Instant`	`plusNanos(long nanosToAdd)`	Returns a copy of this instant with the specified duration in nanoseconds added.
`Instant`	`plusSeconds(long secondsToAdd)`	Returns a copy of this instant with the specified duration in seconds added.
`<R> R`	`query(TemporalQuery<R> query)`	Queries this instant using the specified query.
`ValueRange`	`range(TemporalField field)`	Gets the range of valid values for the specified field.
`long`	`toEpochMilli()`	Converts this instant to the number of milliseconds from the epoch of 1970-01-01T00:00:00Z.
`String`	`toString()`	A string representation of this instant using ISO-8601 representation.
`Instant`	`truncatedTo(TemporalUnit unit)`	Returns a copy of this `Instant` truncated to the specified unit.
`long`	`until(Temporal endExclusive, TemporalUnit unit)`	Calculates the amount of time until another instant in terms of the specified unit.
`Instant`	`with(TemporalAdjuster adjuster)`	Returns an adjusted copy of this instant.
`Instant`	`with(TemporalField field, long newValue)`	Returns a copy of this instant with the specified field set to a new value.

final OffsetDateTime offsetDateTime = Instant.now().atOffset(ZoneOffset.ofHours(8));
System.out.println(offsetDateTime);

Output:

2021-07-29T23:13:17.997944800+08:00

Process finished with exit code 0

class `DateTimeFormatter`

public final class DateTimeFormatter
extends Object

Formatter for printing and parsing date-time objects.

This class provides the main application entry point for printing and parsing and provides common implementations of DateTimeFormatter:

Using predefined constants, such as ISO_LOCAL_DATE
Using pattern letters, such as uuuu-MMM-dd
Using localized styles, such as long or medium

More complex formatters are provided by DateTimeFormatterBuilder.

Predefined Formatters

Formatter	Description	Example
`ofLocalizedDate(dateStyle)`	Formatter with date style from the locale	‘2011-12-03’
`ofLocalizedTime(timeStyle)`	Formatter with time style from the locale	‘10:15:30’
`ofLocalizedDateTime(dateTimeStyle)`	Formatter with a style for date and time from the locale	‘3 Jun 2008 11:05:30’
`ofLocalizedDateTime(dateStyle,timeStyle)`	Formatter with date and time styles from the locale	‘3 Jun 2008 11:05’
`BASIC_ISO_DATE`	Basic ISO date	‘20111203’
`ISO_LOCAL_DATE`	ISO Local Date	‘2011-12-03’
`ISO_OFFSET_DATE`	ISO Date with offset	‘2011-12-03+01:00’
`ISO_DATE`	ISO Date with or without offset	‘2011-12-03+01:00’; ‘2011-12-03’
`ISO_LOCAL_TIME`	Time without offset	‘10:15:30’
`ISO_OFFSET_TIME`	Time with offset	‘10:15:30+01:00’
`ISO_TIME`	Time with or without offset	‘10:15:30+01:00’; ‘10:15:30’
`ISO_LOCAL_DATE_TIME`	ISO Local Date and Time	‘2011-12-03T10:15:30’
`ISO_OFFSET_DATE_TIME`	Date Time with Offset	‘2011-12-03T10:15:30+01:00’
`ISO_ZONED_DATE_TIME`	Zoned Date Time	‘2011-12-03T10:15:30+01:00[Europe/Paris]’
`ISO_DATE_TIME`	Date and time with ZoneId	‘2011-12-03T10:15:30+01:00[Europe/Paris]’
`ISO_ORDINAL_DATE`	Year and day of year	‘2012-337’
`ISO_WEEK_DATE`	Year and Week	‘2012-W48-6’
`ISO_INSTANT`	Date and Time of an Instant	‘2011-12-03T10:15:30Z’
`RFC_1123_DATE_TIME`	RFC 1123 / RFC 822	‘Tue, 3 Jun 2008 11:05:30 GMT’

Pattern Letters and Symbols

Symbol	Meaning	Presentation	Examples
G	era	text	AD; Anno Domini; A
u	year	year	2004; 04
y	year-of-era	year	2004; 04
D	day-of-year	number	189
M/L	month-of-year	number/text	7; 07; Jul; July; J
d	day-of-month	number	10
g	modified-julian-day	number	2451334
Q/q	quarter-of-year	number/text	3; 03; Q3; 3rd quarter
Y	week-based-year	year	1996; 96
w	week-of-week-based-year	number	27
W	week-of-month	number	4
E	day-of-week	text	Tue; Tuesday; T
e/c	localized day-of-week	number/text	2; 02; Tue; Tuesday; T
F	day-of-week-in-month	number	3
a	am-pm-of-day	text	PM
h	clock-hour-of-am-pm (1-12)	number	12
K	hour-of-am-pm (0-11)	number	0
k	clock-hour-of-day (1-24)	number	24
H	hour-of-day (0-23)	number	0
m	minute-of-hour	number	30
s	second-of-minute	number	55
S	fraction-of-second	fraction	978
A	milli-of-day	number	1234
n	nano-of-second	number	987654321
N	nano-of-day	number	1234000000
V	time-zone ID	zone-id	America/Los_Angeles; Z; -08:30
v	generic time-zone name	zone-name	Pacific Time; PT
z	time-zone name	zone-name	Pacific Standard Time; PST
O	localized zone-offset	offset-O	GMT+8; GMT+08:00; UTC-08:00
X	zone-offset ‘Z’ for zero	offset-X	Z; -08; -0830; -08:30; -083015; -08:30:15
x	zone-offset	offset-x	+0000; -08; -0830; -08:30; -083015; -08:30:15
Z	zone-offset	offset-Z	+0000; -0800; -08:00
p	pad next	pad modifier	1
’	escape for text	delimiter
‘’	single quote	literal	’
[	optional section start
]	optional section end
#	reserved for future use
{	reserved for future use
}	reserved for future use

The count of pattern letters determines the format.

Fields

Modifier and Type	Field	Description
`static DateTimeFormatter`	`BASIC_ISO_DATE`	The ISO date formatter that formats or parses a date without an offset, such as ‘20111203’.
`static DateTimeFormatter`	`ISO_DATE`	The ISO date formatter that formats or parses a date with the offset if available, such as ‘2011-12-03’ or ‘2011-12-03+01:00’.
`static DateTimeFormatter`	`ISO_DATE_TIME`	The ISO-like date-time formatter that formats or parses a date-time with the offset and zone if available, such as ‘2011-12-03T10:15:30’, ‘2011-12-03T10:15:30+01:00’ or ‘2011-12-03T10:15:30+01:00[Europe/Paris]’.
`static DateTimeFormatter`	`ISO_INSTANT`	The ISO instant formatter that formats or parses an instant in UTC, such as ‘2011-12-03T10:15:30Z’.
`static DateTimeFormatter`	`ISO_LOCAL_DATE`	The ISO date formatter that formats or parses a date without an offset, such as ‘2011-12-03’.
`static DateTimeFormatter`	`ISO_LOCAL_DATE_TIME`	The ISO date-time formatter that formats or parses a date-time without an offset, such as ‘2011-12-03T10:15:30’.
`static DateTimeFormatter`	`ISO_LOCAL_TIME`	The ISO time formatter that formats or parses a time without an offset, such as ‘10:15’ or ‘10:15:30’.
`static DateTimeFormatter`	`ISO_OFFSET_DATE`	The ISO date formatter that formats or parses a date with an offset, such as ‘2011-12-03+01:00’.
`static DateTimeFormatter`	`ISO_OFFSET_DATE_TIME`	The ISO date-time formatter that formats or parses a date-time with an offset, such as ‘2011-12-03T10:15:30+01:00’.
`static DateTimeFormatter`	`ISO_OFFSET_TIME`	The ISO time formatter that formats or parses a time with an offset, such as ‘10:15+01:00’ or ‘10:15:30+01:00’.
`static DateTimeFormatter`	`ISO_ORDINAL_DATE`	The ISO date formatter that formats or parses the ordinal date without an offset, such as ‘2012-337’.
`static DateTimeFormatter`	`ISO_TIME`	The ISO time formatter that formats or parses a time, with the offset if available, such as ‘10:15’, ‘10:15:30’ or ‘10:15:30+01:00’.
`static DateTimeFormatter`	`ISO_WEEK_DATE`	The ISO date formatter that formats or parses the week-based date without an offset, such as ‘2012-W48-6’.
`static DateTimeFormatter`	`ISO_ZONED_DATE_TIME`	The ISO-like date-time formatter that formats or parses a date-time with offset and zone, such as ‘2011-12-03T10:15:30+01:00[Europe/Paris]’.
`static DateTimeFormatter`	`RFC_1123_DATE_TIME`	The RFC-1123 date-time formatter, such as ‘Tue, 3 Jun 2008 11:05:30 GMT’.

Methods

Modifier and Type	Method	Description
`String`	`format(TemporalAccessor temporal)`	Formats a date-time object using this formatter.
`void`	`formatTo(TemporalAccessor temporal, Appendable appendable)`	Formats a date-time object to an `Appendable` using this formatter.
`Chronology`	`getChronology()`	Gets the overriding chronology to be used during formatting.
`DecimalStyle`	`getDecimalStyle()`	Gets the DecimalStyle to be used during formatting.
`Locale`	`getLocale()`	Gets the locale to be used during formatting.
`Set<TemporalField>`	`getResolverFields()`	Gets the resolver fields to use during parsing.
`ResolverStyle`	`getResolverStyle()`	Gets the resolver style to use during parsing.
`ZoneId`	`getZone()`	Gets the overriding zone to be used during formatting.
`DateTimeFormatter`	`localizedBy(Locale locale)`	Returns a copy of this formatter with localized values of the locale, calendar, region, decimal style and/or timezone, that supercede values in this formatter.
`static DateTimeFormatter`	`ofLocalizedDate(FormatStyle dateStyle)`	Returns a locale specific date format for the ISO chronology.
`static DateTimeFormatter`	`ofLocalizedDateTime(FormatStyle dateTimeStyle)`	Returns a locale specific date-time formatter for the ISO chronology.
`static DateTimeFormatter`	`ofLocalizedDateTime(FormatStyle dateStyle, FormatStyle timeStyle)`	Returns a locale specific date and time format for the ISO chronology.
`static DateTimeFormatter`	`ofLocalizedTime(FormatStyle timeStyle)`	Returns a locale specific time format for the ISO chronology.
`static DateTimeFormatter`	`ofPattern(String pattern)`	Creates a formatter using the specified pattern.
`static DateTimeFormatter`	`ofPattern(String pattern, Locale locale)`	Creates a formatter using the specified pattern and locale.
`TemporalAccessor`	`parse(CharSequence text)`	Fully parses the text producing a temporal object.
`TemporalAccessor`	`parse(CharSequence text, ParsePosition position)`	Parses the text using this formatter, providing control over the text position.
`<T> T`	`parse(CharSequence text, TemporalQuery<T> query)`	Fully parses the text producing an object of the specified type.
`TemporalAccessor`	`parseBest(CharSequence text, TemporalQuery<?>... queries)`	Fully parses the text producing an object of one of the specified types.
`static TemporalQuery<Period>`	`parsedExcessDays()`	A query that provides access to the excess days that were parsed.
`static TemporalQuery<Boolean>`	`parsedLeapSecond()`	A query that provides access to whether a leap-second was parsed.
`TemporalAccessor`	`parseUnresolved(CharSequence text, ParsePosition position)`	Parses the text using this formatter, without resolving the result, intended for advanced use cases.
`Format`	`toFormat()`	Returns this formatter as a `java.text.Format` instance.
`Format`	`toFormat(TemporalQuery<?> parseQuery)`	Returns this formatter as a `java.text.Format` instance that will parse using the specified query.
`String`	`toString()`	Returns a description of the underlying formatters.
`DateTimeFormatter`	`withChronology(Chronology chrono)`	Returns a copy of this formatter with a new override chronology.
`DateTimeFormatter`	`withDecimalStyle(DecimalStyle decimalStyle)`	Returns a copy of this formatter with a new DecimalStyle.
`DateTimeFormatter`	`withLocale(Locale locale)`	Returns a copy of this formatter with a new locale.
`DateTimeFormatter`	`withResolverFields(TemporalField... resolverFields)`	Returns a copy of this formatter with a new set of resolver fields.
`DateTimeFormatter`	`withResolverFields(Set<TemporalField> resolverFields)`	Returns a copy of this formatter with a new set of resolver fields.
`DateTimeFormatter`	`withResolverStyle(ResolverStyle resolverStyle)`	Returns a copy of this formatter with a new resolver style.
`DateTimeFormatter`	`withZone(ZoneId zone)`	Returns a copy of this formatter with a new override zone.

enum FormatStyle

public enum FormatStyle
extends Enum<FormatStyle>

Enumeration of the style of a localized date, time or date-time formatter.

This is an immutable and thread-safe enum.

Enum Constants

Enum Constant	Description
`FULL`	Full text style, with the most detail.
`LONG`	Long text style, with lots of detail.
`MEDIUM`	Medium text style, with some detail.
`SHORT`	Short text style, typically numeric.

class `Locale`

public final class Locale
extends Object
implements Cloneable, Serializable

A Locale object represents a specific geographical, political, or cultural region. An operation that requires a Locale to perform its task is called locale-sensitive and uses the Locale to tailor information for the user. For example, displaying a number is a locale-sensitive operation— the number should be formatted according to the customs and conventions of the user’s native country, region, or culture.

Nested classes

Modifier and Type	Class	Description
`static class`	`Locale.Builder`	`Builder` is used to build instances of `Locale` from values configured by the setters.
`static class`	`Locale.Category`	Enum for locale categories.
`static class`	`Locale.FilteringMode`	This enum provides constants to select a filtering mode for locale matching.
`static class`	`Locale.IsoCountryCode`	Enum for specifying the type defined in ISO 3166.
`static class`	`Locale.LanguageRange`	This class expresses a Language Range defined in RFC 4647 Matching of Language Tags.

Fields

Modifier and Type	Field	Description
`static Locale`	`CANADA`	Useful constant for country.
`static Locale`	`CANADA_FRENCH`	Useful constant for country.
`static Locale`	`CHINA`	Useful constant for country.
`static Locale`	`CHINESE`	Useful constant for language.
`static Locale`	`ENGLISH`	Useful constant for language.
`static Locale`	`FRANCE`	Useful constant for country.
`static Locale`	`FRENCH`	Useful constant for language.
`static Locale`	`GERMAN`	Useful constant for language.
`static Locale`	`GERMANY`	Useful constant for country.
`static Locale`	`ITALIAN`	Useful constant for language.
`static Locale`	`ITALY`	Useful constant for country.
`static Locale`	`JAPAN`	Useful constant for country.
`static Locale`	`JAPANESE`	Useful constant for language.
`static Locale`	`KOREA`	Useful constant for country.
`static Locale`	`KOREAN`	Useful constant for language.
`static Locale`	`PRC`	Useful constant for country.
`static char`	`PRIVATE_USE_EXTENSION`	The key for the private use extension (‘x’).
`static Locale`	`ROOT`	Useful constant for the root locale.
`static Locale`	`SIMPLIFIED_CHINESE`	Useful constant for language.
`static Locale`	`TAIWAN`	Useful constant for country.
`static Locale`	`TRADITIONAL_CHINESE`	Useful constant for language.
`static Locale`	`UK`	Useful constant for country.
`static char`	`UNICODE_LOCALE_EXTENSION`	The key for Unicode locale extension (‘u’).
`static Locale`	`US`	Useful constant for country.

Constructors

Constructor	Description
`Locale(String language)`	Construct a locale from a language code.
`Locale(String language, String country)`	Construct a locale from language and country.
`Locale(String language, String country, String variant)`	Construct a locale from language, country and variant.

Methods

Modifier and Type	Method	Description
`Object`	`clone()`	Overrides Cloneable.
`boolean`	`equals(Object obj)`	Returns true if this Locale is equal to another object.
`static List<Locale>`	`filter(List<Locale.LanguageRange> priorityList, Collection<Locale> locales)`	Returns a list of matching `Locale` instances using the filtering mechanism defined in RFC 4647.
`static List<Locale>`	`filter(List<Locale.LanguageRange> priorityList, Collection<Locale> locales, Locale.FilteringMode mode)`	Returns a list of matching `Locale` instances using the filtering mechanism defined in RFC 4647.
`static List<String>`	`filterTags(List<Locale.LanguageRange> priorityList, Collection<String> tags)`	Returns a list of matching languages tags using the basic filtering mechanism defined in RFC 4647.
`static List<String>`	`filterTags(List<Locale.LanguageRange> priorityList, Collection<String> tags, Locale.FilteringMode mode)`	Returns a list of matching languages tags using the basic filtering mechanism defined in RFC 4647.
`static Locale`	`forLanguageTag(String languageTag)`	Returns a locale for the specified IETF BCP 47 language tag string.
`static Locale[]`	`getAvailableLocales()`	Returns an array of all installed locales.
`String`	`getCountry()`	Returns the country/region code for this locale, which should either be the empty string, an uppercase ISO 3166 2-letter code, or a UN M.49 3-digit code.
`static Locale`	`getDefault()`	Gets the current value of the default locale for this instance of the Java Virtual Machine.
`static Locale`	`getDefault(Locale.Category category)`	Gets the current value of the default locale for the specified Category for this instance of the Java Virtual Machine.
`String`	`getDisplayCountry()`	Returns a name for the locale’s country that is appropriate for display to the user.
`String`	`getDisplayCountry(Locale inLocale)`	Returns a name for the locale’s country that is appropriate for display to the user.
`String`	`getDisplayLanguage()`	Returns a name for the locale’s language that is appropriate for display to the user.
`String`	`getDisplayLanguage(Locale inLocale)`	Returns a name for the locale’s language that is appropriate for display to the user.
`String`	`getDisplayName()`	Returns a name for the locale that is appropriate for display to the user.
`String`	`getDisplayName(Locale inLocale)`	Returns a name for the locale that is appropriate for display to the user.
`String`	`getDisplayScript()`	Returns a name for the locale’s script that is appropriate for display to the user.
`String`	`getDisplayScript(Locale inLocale)`	Returns a name for the locale’s script that is appropriate for display to the user.
`String`	`getDisplayVariant()`	Returns a name for the locale’s variant code that is appropriate for display to the user.
`String`	`getDisplayVariant(Locale inLocale)`	Returns a name for the locale’s variant code that is appropriate for display to the user.
`String`	`getExtension(char key)`	Returns the extension (or private use) value associated with the specified key, or null if there is no extension associated with the key.
`Set<Character>`	`getExtensionKeys()`	Returns the set of extension keys associated with this locale, or the empty set if it has no extensions.
`String`	`getISO3Country()`	Returns a three-letter abbreviation for this locale’s country.
`String`	`getISO3Language()`	Returns a three-letter abbreviation of this locale’s language.
`static String[]`	`getISOCountries()`	Returns a list of all 2-letter country codes defined in ISO 3166.
`static Set<String>`	`getISOCountries(Locale.IsoCountryCode type)`	Returns a `Set` of ISO3166 country codes for the specified type.
`static String[]`	`getISOLanguages()`	Returns a list of all 2-letter language codes defined in ISO 639.
`String`	`getLanguage()`	Returns the language code of this Locale.
`String`	`getScript()`	Returns the script for this locale, which should either be the empty string or an ISO 15924 4-letter script code.
`Set<String>`	`getUnicodeLocaleAttributes()`	Returns the set of unicode locale attributes associated with this locale, or the empty set if it has no attributes.
`Set<String>`	`getUnicodeLocaleKeys()`	Returns the set of Unicode locale keys defined by this locale, or the empty set if this locale has none.
`String`	`getUnicodeLocaleType(String key)`	Returns the Unicode locale type associated with the specified Unicode locale key for this locale.
`String`	`getVariant()`	Returns the variant code for this locale.
`boolean`	`hasExtensions()`	Returns `true` if this `Locale` has any extensions.
`int`	`hashCode()`	Override hashCode.
`static Locale`	`lookup(List<Locale.LanguageRange> priorityList, Collection<Locale> locales)`	Returns a `Locale` instance for the best-matching language tag using the lookup mechanism defined in RFC 4647.
`static String`	`lookupTag(List<Locale.LanguageRange> priorityList, Collection<String> tags)`	Returns the best-matching language tag using the lookup mechanism defined in RFC 4647.
`static void`	`setDefault(Locale newLocale)`	Sets the default locale for this instance of the Java Virtual Machine.
`static void`	`setDefault(Locale.Category category, Locale newLocale)`	Sets the default locale for the specified Category for this instance of the Java Virtual Machine.
`Locale`	`stripExtensions()`	Returns a copy of this `Locale` with no extensions.
`String`	`toLanguageTag()`	Returns a well-formed IETF BCP 47 language tag representing this locale.
`String`	`toString()`	Returns a string representation of this `Locale` object, consisting of language, country, variant, script, and extensions as below: language + “" + country + "” + (variant + “#" \| “#”) + script + "” + extensions Language is always lower case, country is always upper case, script is always title case, and extensions are always lower case.