public class EqualsDemo {public static void main(String[] args) {Student stu1 = new Student("lucy",18,001);Student stu2 = new Student("lucy",18,001);System.out.println(stu1.equals(stu2));//true  对象内容System.out.println(stu1 == stu2);//false 地址值}}
import java.util.Objects;public class Student {int age;String name;int id;public Student(){}public Student(String name, int age,int id) {this.name = name;this.age = age;this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}@Override//重写后 就是对对象内容比较public boolean equals(Object o) {if (this == o) return true;if (o == null || getClass() != o.getClass()) return false;Student  student  = (Student ) o;return id == student .id && Objects.equals(name, student.name);}
}

import java.util.Date;//当前时间进行推算
public class DateDemo {public static void main(String[] args) {long time1 = System.currentTimeMillis();System.out.println(time1);//1627557698806Date date = new Date();System.out.println(date);//Wed Jul 28 19:21:38 CST 2021Date dateNew = new Date((date.getTime()+1l*24*60*60*1000));date = dateNew;System.out.println(date);//Thu Jul 29 19:21:38 CST 2021System.out.println(new Date());//Wed Jul 28 19:21:38 CST 2021// 创建日期对象，把当前的时间System.out.println(new Date(0L));//Thu Jan 01 08:00:00 CST 1970// 创建日期对象，把当前的毫秒值转成日期对象（Date(0)格林威治时间）/** long 和 Date 之间的转换long -> Datea.  new Date(long)b.  date.setTime(long)Date -> longdate.getTime()* *//*public long getTime()返回自 1970 年 1 月 1 日 00:00:00 GMT以来此 Date 对象表示的毫秒数。 */System.out.println(date.getTime());//1627557698806/** public void setTime(long time)设置此 Date 对象，* 以表示 1970 年 1 月 1 日 00:00:00 GMT 以后 time 毫秒的时间点。* 参数：毫秒数。*/long time2 = date.getTime();time2 = time2 + 24l * 60 * 60 * 1000;//将推算好的时间设置回date中date.setTime(time2);System.out.println(date);//Fri Jul 30 19:21:38 CST 2021/*boolean after(Date when)测试此日期是否在指定日期之后。boolean before(Date when)测试此日期是否在指定日期之前。 */Date date1 = new Date(time2);boolean before = date1.before(date);System.out.println(before);//falseboolean after = date1.after(date);System.out.println(after);//false}
}

3、Calendar类

java.util.Calendar 是抽象类，在Date后出现，替换掉了许多Date的⽅法。该类将所有可能⽤到的时间信息封装为静态成员变量，⽅便获取。⽇历类就是⽅便获取各个时间属性的。

常用API

Calendar 和 Date的转换
          Date getTime()  -> Calendar 转换成 Date
           void setTime(Date) -> Date 转换成 Calendar

calendar 和 long的转换
             getTimeInMillis() 
             setTimeInMillis()

get() - 获取
           set(字段,int value) - 设置,时间的跳转
           add(字段,int amount) - 时间的推算

import java.util.Calendar;
import java.util.Date;//用来做时间的跳转
public class CalendarDemo {public static void main(String[] args) {/*get()set()add()根据日历的规则，为给定的日历字段添加或减去指定的时间量。Date getTime() ->  Calendar 和 Date之间的转换Calendar 转换成 Datevoid setTime(Date) ->  Calendar 和 Date之间的转换Date -> Calendar*/Calendar calendar = Calendar.getInstance();//里面存的都是静态常量System.out.println(calendar.get(Calendar.DAY_OF_MONTH));System.out.println(calendar.get(Calendar.DATE));//setTime(Date date)calendar.setTime(new Date(0));System.out.println(calendar.get(Calendar.YEAR));//1970//add()calendar.add(Calendar.DAY_OF_WEEK,-5);}
}

4、long，Date，Calendar三个之间的转换

import java.io.UnsupportedEncodingException;import java.util.Arrays;//字符串的创建
public class StringDemo01 {public static void main(String[] args) throws UnsupportedEncodingException {//普通赋值方式String s = "hi";System.out.println(s);//hi//数组int[] i = {1,5,2,2};System.out.println(i);//[I@50cbc42f 地址值System.out.println(Arrays.toString(i));//[1, 5, 2, 2]char[] c = {'h','i'};System.out.println(c);//hi 这样直接输出，跟上面普通赋值输出一样，说明，字符串本质上跟字符数组（字符序列）一样System.out.println(Arrays.toString(c));//[h, i]//String构造方法String s1 = new String(s);//传入一个参数sSystem.out.println(s1);//hi//public String()String s2 = new String(c);System.out.println(s2);//hi//String(byte[] bytes)byte[] b = {97,98,99,100};String b1 = new String(b);System.out.println(b1);//abcd  -->因为是以字符串类型输出的，而//String(byte[] bytes, Charset charset)byte[] b2 = {'o'};String b3 = new String(b2,"UTF-8");//throws UnsupportedEncodingException 会抛出不支持异常System.out.println(b3);}
}

//编码，解码，乱码问题import java.io.UnsupportedEncodingException;import java.util.Arrays;/*编码问题解析:编码: 字符 -> 字节解码: 字节 -> 字符UTF-8 -> 1字符 = 3字节GBK -> 1字符 = 2字节*/
public class StringDemo02 {public static void main(String[] args) throws UnsupportedEncodingException {String str = "哈哈哈";// 把看得懂的方式变成看不懂的方式 -> 编码//以默认方式进行解析byte[] bsUTF = str.getBytes();1字符 = 3字节 UTF-8//[-27, -109, -120, -27, -109, -120]System.out.println(Arrays.toString(bsUTF));//以指定方式进行解析byte[] bsGBK = str.getBytes("GBK");/*[-71, -2, -71, -2]1字符 = 2字节 -> GBK*/System.out.println(Arrays.toString(bsGBK));// 解码: 把看不懂的变成看得懂//乱码的产生String gbk = new String(bsUTF, "GBK");System.out.println(gbk);//鍝堝搱String utf = new String(bsGBK, "UTF-8");System.out.println(utf);//����//解决乱码://1.按照错误的方式进行编码:byte[] gbks = gbk.getBytes("GBK");//[-27, -109, -120, -27, -109, -120]System.out.println(Arrays.toString(gbks));//2.重新解码:String s = new String(gbks);System.out.println(s);}
}

/*校验密码是否合法。必须至少9个字符。必须至少2个大写字符。必须包含小写字母和数字。例如: asd12345QWER条件: a - zA - Z0 - 9*/
public class StringDemo03 {public static void main(String[] args) {String str = "asd12345QWER";int lowerCount = 0;//小写字母int numCount = 0;//数字int upCount = 0;//大写字母//将str转换成字符数组char[] chars = str.toCharArray();for (int i = 0; i < chars.length; i++) {if(chars[i] >= 'a' && chars[i] <= 'z'){//如果符合lowerCount++;}else if(chars[i] >= '0' && chars[i] <= '9'){//如果符合numCount++;}else if(chars[i] >= 'A' && chars[i] <= 'Z'){//如果符合upCount++;}}if(str.length() > 9 && upCount > 2&& numCount > 0 && lowerCount > 0){System.out.println("符合条件");}else{System.out.println("不符合条件");}}
}

/*常用API*/
public class StringDemo04 {public static void main(String[] args) {//int length()String str = "asdfc";int length = str.length();System.out.println(length);//和字符相关的方法://char charAt(int index)char c = str.charAt(1);System.out.println(c);//char[] toCharArray()char[] chars = str.toCharArray();System.out.println(chars);//java中大小写非常敏感String s1 = "gbk";String s2 = "GBK";String s3 = "gbK";System.out.println(s1.equals(s2));//转换成全大写String s = s1.toUpperCase();System.out.println(s.equals(s2));//转换成全小写String s4 = s2.toLowerCase();System.out.println(s4.equals(s1));System.out.println("--------------");//忽略大小写boolean b = s3.equalsIgnoreCase(s2);System.out.println(b);}
}

2、可变长字符串

存在于java.lang包下的StringBuilder 和 StringBuffer类。

StringBuilder类属于一个没有加锁不安全的可变字符序列，StringBuffer类属于线程安全的可变字符序列。

区别:
      a.版本:
          StringBuilder -> JDK1.5
          StringBuffer  -> JDK1.0

b.线程安全
          StringBuilder: 线程不安全，效率高。
          StringBuffer:  线程安全，效率低。

c.使用环境:
          操作少量的数据使用 String
          单线程环境下大量数据使用 StringBuilder
          多线程环境下大量数据使用 StringBuffer

/*字符串一旦创建就不可变字符串的频繁拼接会产生新的对象,所以String不建议做频繁的拼接*/
/*StringBuilder的初始化StringBuilder append(Object obj)  - 追加StringBuilder delete(int start, int end)  - 删除StringBuilder insert(int offset, boolean b)   - 插入StringBuilder replace(int start, int end, String str)  - 替换StringBuilder reverse()  - 反转面试题:String 和 StringBuilder  和 StringBuffer的区别(效率)*/
public class StringBuilderDemo {public static void main(String[] args) {//无参构造方法StringBuilder sb = new StringBuilder();//追加字符串sb.append("哈哈");sb.append("嘻嘻");//将StringBuilder 转换成 StringString str = sb.toString();System.out.println(str);String str1 = "大乔";//有参构造方法StringBuilder sb1 = new StringBuilder(str1);//在原有的字符串的基础上进行追加内容sb1.append("貂蝉");sb1.append("西施");sb1.append("妲己");sb1.append("杨玉环");System.out.println(sb1);//StringBuilder delete(int start, int end)  - 删除子串//字符子串//前包含后不包含StringBuilder delete = sb1.delete(2,4);System.out.println(delete);//StringBuilder insert(int offset, Object obj)   - 插入sb1.insert(2,22);System.out.println(sb1);//StringBuilder replace(int start, int end, String str)  - 替换//前包含后不包含sb1.replace(0,2,"小乔");System.out.println(sb1);//StringBuilder reverse()  - 反转(回文数)StringBuilder reverse = sb1.reverse();System.out.println(reverse);}
}

/*效率对比String < StringBuffer < StringBuilder*/
public class StringBufferDemo {public static void main(String[] args) {//String的效率String str = "a";long time1 = System.currentTimeMillis();for (int i = 0; i < 1000000; i++) {str += i;}long time2 = System.currentTimeMillis();System.out.println(time2 - time1);//StringBuilder的效率StringBuilder sb1 = new StringBuilder();for (int i = 0; i < 10000000; i++) {sb1.append(i);}long time3 = System.currentTimeMillis();System.out.println(time3 - time2);//StringBuffer的效率StringBuffer sb2 = new StringBuffer();for (int i = 0; i < 10000000; i++) {sb2.append(i);}long time4 = System.currentTimeMillis();System.out.println(time4 - time3);}
}

3、正则表达式

正则表达式常用API:
     String replaceAll(String regex, String replacement)
     boolean matches(String regex)
     String[] split(String regex)

import java.util.Arrays;/*2.电话号码 11位 第一位1  后10位随便的数字3.验证163邮箱 4.将所有的数字替换成*，连贯的数字只需要替换一个*将字符串按照字母切割(分离),提取出所有的数字，并且连续数字作为一个整体例如:s2 ="abc123der52ersdf45sdf34sewr76"正则表达式常用API:String replaceAll(String regex, String replacement)boolean matches(String regex)String[] split(String regex)
*/
public class RegularDemo {public static void main(String[] args) {//2.电话号码 11位 第一位1  后10位随便的数字String phone = "15356297421";//regex = [1]\d{10}  / 1[0-9]{10}String regex = "[1]\\d{10}";boolean matches = phone.matches(regex);System.out.println(matches);//3.验证163邮箱 -> Qss_7733@163.comString email = "Qss_7733@163.com";// [a-zA-Z]\w{5,17}@163\.comString regex1 = "[a-zA-Z]\\w{5,17}@163\\.com";boolean matches1 = email.matches(regex1);System.out.println(matches1);//4.将所有的数字替换成*,连贯的数字只需要替换一个*String s1 = "我去你大爷,xxx,你个sb";String regex2 = "去你大爷|xxx|sb";String s = s1.replaceAll(regex2, "*");System.out.println(s);/*将字符串按照字母切割(分离),提取出所有的数字，并且连续数字作为一个整体例如:s2 ="abc123der52ersdf45sdf34sewr76"*/String s2 ="abc123der52ersdf45sdf34sewr76";//[a-zA-Z]+String regex3 = "[a-zA-Z]+";String[] split = s2.split(regex3);//[, 123, 52, 45, 34, 76]System.out.println(Arrays.toString(split));}
}

5.包装类