JavaSE基础篇：for ，while，do while，switch 等简单循环结构详解

顺序结构

package com.cl.struct;

//顺序结构

public class Demo01 {
    public static void main(String[] args) {
        System.out.println("hello world1");
        System.out.println("hello world2");
        System.out.println("hello world3");
        System.out.println("hello world4");
        //除非特别指明，否则从上至下依次执行
    }
}

while循环

 

  //输出1~100；
package com.cl.struct;

public class WhileDemo01 {

    public static void main(String[] args) {

        int i=0;

        while(i<100){
            i++;
            System.out.println(i);

        }
    }
}

 

do while

 

    //输出1~100；
package com.cl.struct;

public class DoWhileDemo {
    public static void main(String[] args) {
        int i=0;
        int sum=0;

        do{
            sum+=i;
            i++;

        }while(i<100);
        System.out.println(sum);

    }
}

 

For循环，重点！！

//打印1~100
package com.cl.struct;

public class ForDemo01 {
    public static void main(String[] args) {
        for(int i=1;i<=100;i++){
            System.out.println(i);
        }
        System.out.println("结束");
    }
}

//练习1；计算0~100之间的奇数和偶数的和

package com.cl.struct;

public class ForDemo02 {
    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：用while或for循环输出1~1000之间能被5整除的数，并且每行输出3个
package com.cl.struct;

public class ForDemo03 {
    public static void main(String[] args) {
  

        for (int i = 0; i < 1000; i++) {
            if(i%5==0){
                System.out.print(i+"\t");
            }
            if (i%(5*3)==0){//每行
                System.out.println();
                //System.out.println("\n");
            }
        }
    }
}

 //遍历数组元素
package com.cl.struct;

public class ForDemo04 {
    public static void main(String[] args) {
        int[] numbers= {10,20,30,40,50};//定义一个数组

        for(int x:numbers){
            System.out.println(x);

        }
    }
}

//打印九九乘法表

package com.cl.struct;

public class ForDemo05 {
    public static void main(String[] args) {

        for (int j = 0; j <=9; j++) {
            for (int i = 0; i <=j; i++) {
                System.out.print(j+"*"+i+"="+(j*i)+"\t");

            }
            System.out.println();
            //println换行输出
            //print换行输出
        }

    }
}

if
//判断字符串是否相等
package com.cl.struct;

import java.util.Scanner;

public class ifDemo01 {
    public static void main(String[] args) {
        Scanner scanner=new Scanner(System.in);

        System.out.println("请输入内容：");
        String s=scanner.nextLine();

        //equals:判断字符串是否相等
        if(s.equals("Hello")){
            System.out.println(s);
        }

        System.out.println("End");
        scanner.close();



    }
}

//考试分数大于60及格，小于不及格

package com.cl.struct;

import java.util.Scanner;

public class ifDemo02 {
    public static void main(String[] args) {
 
        Scanner scanner=new Scanner(System.in);

        System.out.println("请输入成绩");
        int score=scanner.nextInt();

        if(score>60){
            System.out.println("及格");
        }else{
            System.out.println("不及格");
        }



    }
}

swich

 

package com.cl.struct;

public class SwitchDemo01 {
    public static void main(String[] args) {
        //自java7后，String类型也可以
        char grade='C';

        switch(grade){
            case 'A':
                System.out.println("优秀");
                break;
            case 'B':
                System.out.println("良好");
                break;
            case 'C':
                System.out.println("及格");
                break;
            case 'D':
                System.out.println("挂科");
                break;
            default:
                System.out.println("输入错误");

        }
    }
}

 

For，打印三角形

 

package com.cl.struct;

public class TestDemo {

    public static void main(String[] args) {
        //打印三角形 5行
        //主要是内嵌的3个for作用比较大

        for (int i = 1; i <=5; i++) {
            for(int j=5;j>=i;j--){
                System.out.print(" ");
            }//倒三角用空格先占位置
            for (int j=1;j<=i;j++){
                System.out.print("*");
            }//开始输出左半面星星，规律：第几行就有几个星星
             //    *
             //   **
             //  ***
             // ****
            for (int j=1;j<i;j++){
                System.out.print("*");
            }
            //开始画右半面，上面的for理解了这个也就理解了

            System.out.println();
        }
    }
}