Java（课后笔记）

switch多选择结构

• switch case语句判断一个变量与一系列值中某个值是否相等，每个值称为一个分支

```plaintext
package com.struct;

public class Swicth01 {
    public static void main(String[] args) {
        //case穿透//switch匹配一个具体的值
        char grade = 'B';

        switch (grade){
            case 'A':
                System.out.println("优秀");
                break;//可选
            case 'B' :
                System.out.println("良好");
                break;//可选
            case 'C' :
                System.out.println("及格");
                break;//可选
            case 'D' :
                System.out.println("再接再厉");
                break;//可选
            case 'E' :
                System.out.println("挂科");
                break;//可选
            default:
                System.out.println("未知等级");
        }
    }
}

package com.struct;

public class Swicth02 {
    public static void main(String[] args) {
        String name = "琴江";
        //JDK7新特性，表达式结果可以是字符串
        //字符的本质是数字
        //反编译   java---class(字节码文件)----反编译（IDEA）
        switch (name){
            case "琴江":
                System.out.println("琴江");
                break;
            case "狂神":
                System.out.println("狂神");
                break;
            default:
                System.out.println("弄啥嘞！");
        }
    }
}

循环结构

1.while循环

输出0~100

package com.struct;

public class While01 {
    public static void main(String[] args) {
        //输出0~100
        int i = 0;
        while (i<100){
            i++;
            System.out.println(i);
        }
    }
}

死循环

package com.struct;

public class While02 {
    public static void main(String[] args) {
        //死循环
        while (true){
            //等待客户端连接
            //定时检查
            //。。。。。。。
        }
    }
}

计算1+2+3+...+100=?

package com.struct;

public class While03 {
    public static void main(String[] args) {
        //计算1+2+3+...+100=?
        int i=0;
        int sum = 0;
        while (i<=100){
            sum = sum + i;
            i++;
        }
        System.out.println(sum);
    }
}

2.do...while循环

package com.struct;

public class DoWhile01 {
    public static void main(String[] args) {
        int i =0;
        int  sum = 0;
        do {
            sum = sum + i;
            i++;
        }while (i<=100);
        System.out.println(sum);
    }
}

while和do...while的区别

package com.struct;

public class DoWhile02 {
    public static void main(String[] args) {
        int a = 0;
        while (a<0){
            System.out.println(a);
            a++;
        }
        System.out.println("==========");
        do {
            System.out.println(a);
            a++;
        }while (a<0);
    }
}

3.for循环

package com.struct;

public class For01 {
    public static void main(String[] args) {
        int a = 1;//初始化条件
        while (a<=100) {//条件判断
            System.out.println(a);
            a+=2;//迭代
        }
        System.out.println("while循环结束！");
        //初始化+布尔表达式+更新
        for (int i=1;i<=100;i++){
            System.out.println(i);
        }
        System.out.println("for循环结束");
        //死循环
        for (; ; ) {
        }
    }
}

练习1：

package com.struct;

public class For02 {
    public static void main(String[] args) {
       int oddSum = 0;
       int evenSum = 0;
        for (int i = 0; i < 100; i++) {
            if (i%2!=0){
                oddSum+=i;
            }else {
                evenSum+=i;
            }
        }
        System.out.println("奇数的和"+oddSum);
        System.out.println("偶数的和"+evenSum);
        
    }
}

练习2：

package com.struct;

public class For03 {
    public static void main(String[] args) {
        
        for (int i = 0; i <= 1000; i++) {
            if (i%5==0){
                System.out.println(i+"\t");
            }
            if (1%(5*3)==0){
                System.out.println();//换行
                //System.out.print("\n");换行
            }
        }
        //println 输出完会换行
        //print 输出完不会换行
    }
}

练习3：

package com.struct;

public class For04 {
    public static void main(String[] args) {
        //1.先打印第一列
        //2.把固定的1再用一个循环包起来
        //3.去掉重复项 i <= j
        //4.调整样式
        for (int j = 1; j <= 9; j++) {
            for (int i = 1; i <= j; i++) {
                System.out.print(j+"*"+i+"="+(j*i) + "\t");
            }
            System.out.println();
        }
    }
}

4.增强for循环

package com.struct;

public class For05 {
    public static void main(String[] args) {
        int[] numbers = {10,20,30,40,50,};//定义了一个数组

        for (int i = 0; i<5;i++){
            System.out.println(numbers[i]);
        }
        System.out.println("========");
        //遍历数组的元素
        for (int x:numbers){
            System.out.println(x);
        }
    }
}

5.break continue goto

break

package com.struct;

public class Break01 {
    public static void main(String[] args) {
        int i = 0;
        while (i<100){
            i++;
            System.out.println(i);
            if (i==30){
                break;
            }
        }
        System.out.println("123");
    }
}

continue

package com.struct;

public class Continue01 {
    public static void main(String[] args) {
        int i = 0;
        while (i<100){
            i++;
            if(i%10==0){
                System.out.println();
                continue;
            }
            System.out.print(i);
        }
    }
}

goto (lable标签）

package com.struct;

public class Label01 {
    public static void main(String[] args) {
        //打印101~150之间所有的质数
        //质数是指再大于1的自然数中，出了1和它本身意外不再有其他因数的自然数。
        int count = 0;
        //不建议使用
        outer:for (int i=101;i<150;i++){
            for (int j=2;j<i/2;j++){
                if (i%j==0){
                    continue outer;
                }
            }
            System.out.print(i+" ");
        }
    }
}

流程控制练习

• 打印三角形

package com.struct;

public class Test01 {
    public static void main(String[] args) {
        //打印三角形 5行
        for (int i = 1; i <= 5; i++) {
            for (int j = 5; j >= i; j--) {
                System.out.print(" ");
            }
            for (int j = 1; j <= i; j++) {
                System.out.print("*");
            }
            for (int j = 1; j < i; j++) {
                System.out.print("*");
            }
            System.out.println();
        }
    }
}