Day001-2021-07-29 变量定义/数据类型/基础运算 判断/循环/数组
2024-04-08 08:13:18
第一天学习JAVA，也不知道能否坚持下去，做个笔记吧，记录自己学习过程，方便后期复习，也顺便监督一下自己
18周 JAVA从入门到入土
- 一、软件简介
- 1.1 什么是软件
- 1.2 为什么学软件
- 1.3 JAVA需要什么
- 1.4JAVAEE环境配置
- 二、JAVA入门
- 2.1 JAVA工程
- 2.2 HelloWorld
- 三、变量定义/数据类型/基础运算
- 3.1 数据类型
- 3.2 变量定义
- 3.3 基础运算
- 四、判断/循环/数组
- 4.1 判断
- 4.2 循环
- 4.3 数组
- 4.4 遍历
- 1° 遍历
- 2° 遍历+挑选
- 3° 遍历+计数
- 4° 遍历+累加/累乘
- 5° 遍历+挑最小/挑最大
一、软件简介
1.1 什么是软件
1· 向软件发送请求
2· 软件内部做运算
3· 软件输出其结果
1.2 为什么学软件
这个问题不做回答
1.3 JAVA需要什么
JAVA运算需要环境
1.4JAVAEE环境配置
1· 安装JDK
2· 配置环境变量
3· 运行Eclipse
关于环境变量配置：如果安装JDK时采用默认路径，则不需要配置环境变量，如果采用自定义安装路径，则需要配置JAVA_HOME和path路径。（以下划线处包含部分个人猜测） 至于环境变量配置，JAVA_HOME是一个常量，其值为JDK的安装路径，即JAVA_HOME是JDK安装路径的一个常量名，而path路径是cmd能直接调用一些文件的默认路径，在path路径里加入%JAVA_HOME%\bin，可以使bin文件夹下的java.exe和javac.exe可以直接在cmd中使用，不需要再先进入其文件夹，更加方便。
二、JAVA入门
2.1 JAVA工程
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工作空间Workspace
工程Project
SRC
包Package
代码文件.java
2.2 HelloWorld
public class HelloWorld {public static void main(String[] args) {System.out.println("HelloWorld");}
}
主函数main：
public static void main(String[] args) {}
输出语句：
System.out.println();
代码提示快捷键：Alt+/
三、变量定义/数据类型/基础运算
3.1 数据类型
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数据类型
基本数据类型
引用数据类型
整数型
浮点型
字符型
布尔型
String
字节型
短整型
整型
长整型
单精度浮点型
双精度浮点型
char
boolean
int
byte
short
long
float
double
3.2 变量定义
定义一个变量的格式：
数据类型 变量名 = 初始值 ;
public class Define {public static void main(String[] args) {byte a = 127;short b = 32767;int c = 2147483647;long d = 9223372036854775807L;float e = 0.12345678F;double f = 0.1234567890123456;char g = 'A';boolean h = true;String i = "你好,world";}
}
3.3 基础运算
基本数据类型的变量可以进行 加:+ 减:- 乘:* 除:/ 取余:% 运算，结果赋值给其对应数据类型的变量。
public class Calculate {public static void main(String[] args) {int a = 5;int b = 2;int add = a + b;int sub = a - b;int mul = a * b;int div = a / b;int rem = a % b;System.out.println("a b 和为：" + add);System.out.println("a b 差为：" + sub);System.out.println("a b 积为：" + mul);System.out.println("a b 商为：" + div);System.out.println("a b 余为：" + rem);}
}
运行结果：
String可以通过+来拼接其他的字符串或基本类型。
public class Calculate {public static void main(String[] args) {String a = "Hello";String b = "World";String c = a + b;System.out.println("c为：" + c);}
}
运行结果：
四、判断/循环/数组
4.1 判断
判断常用if语句，其格式：
if (boolean类型的判断) {true时执行这里;
} else {false时执行这里;
}
4.2 循环
循环常用for语句，其格式：
for (循环前的准备; boolean类型的循环执行条件; 单次循环结束时的动作) {每次循环的动作;
}
4.3 数组
数组是一个类型的数据的集合，其格式：
数据类型[] 数组名 = { 成员1 , 成员2 , 成员3 , ... }
数据类型可以使所有的基本数据类型或引用数据类型，但是同一数组只能包含其声明的类型的数据。数组的成员个数为数组的长度（length），每个成员的下标（index）从0开始依次增加，所以一个数组成员的全部下标为0 ~ length-1。单个成员可以通过 数组名[下标] 来赋值或者访问。
4.4 遍历
1° 遍历
遍历是指将一个数组的全部成员依次审查一遍，通常可以依靠for循环来实现。（这里的遍历是字面意思，并非通过迭代器实现的遍历方法）
public class iterate {public static void main(String[] args) {int[] arr = { 2, 4, 6, 8, 10, 12 };for (int i = 0; i < arr.length; i++) {System.out.println(arr[i]);}}
}
运行结果：
遍历数组时，还可以用for-each循环，其格式：
for (数组的数据类型 变量名 : 数组名){}
public class iterate {public static void main(String[] args) {int[] arr = { 2, 4, 6, 8, 10, 12 };for (int i : arr) {System.out.println(i);}}
}
运行结果：
for-each循环是依靠迭代器实现的。 普通for循环可以进行与下标有关的操作，其他情况下用加强for循环更加简洁明了。
2° 遍历+挑选
通过循环中增加一个判断来实现。
在一个String数组中，挑选出姓张的人和姓李的人：
public class Name {public static void main(String[] args) {String[] arr = { "张大炮", "李狗蛋", "赵铁柱", "王建国", "张二狗", "林待育", "李大壮", "张三疯" };for (int i = 0; i < arr.length; i++) {if (arr[i].startsWith("张")) {System.out.println(arr[i]);}}System.out.println("******");for (String name : arr) {if (name.startsWith("李")) {System.out.println(name);}}}
}
运行结果：
3° 遍历+计数
4° 遍历+累加/累乘
5° 遍历+挑最小/挑最大
在遍历过程中，想要进行计数、累加/累乘、挑最小/挑最大操作，需要在循环前先声明一个变量来储存这个数据，然后在循环过程中操作。
一个整型数组，计数偶数个数，累加，累乘，挑出最大最小值：
public class Count {public static void main(String[] args) {int[] arr = { 43, 24, 56, 77, 47, 34, 25, 59, 63, 58, 74, 13, 17, 32 };int count = 0;int addUp = 0;int mulUp = 1;int max = arr[0];int min = arr[0];for (int i = 0; i < arr.length; i++) {if (arr[i] % 2 == 0) {count++;}addUp += arr[i];mulUp *= arr[i];if (arr[i] > max) {max = arr[i];}if (arr[i] < min) {min = arr[i];}}System.out.println("偶数个数为：" + count);System.out.println("和为：" + addUp);System.out.println("积为：" + mulUp);System.out.println("最大值为：" + max);System.out.println("最小值为：" + min);}
}
运行结果：
Day001-2021-07-29 变量定义/数据类型/基础运算 判断/循环/数组相关推荐
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