定义

• while(循环条件）{
  循环操作 // 先判断，再执行
  }
• 程序调试
  -- 设置断点
  -- 单行运行
  -- 观察变量
• do{
  循环操作 //x先执行，后判断
  }while（循环条件）;
• for（参数初始化；条件判断；更新循环变量）
  {
  循环操作
  }
• break：改变程序控制流（中断，跳出整个循环）
• continue：跳出本次循环

例题

• 计算100以内（包括100）的偶数之和

int a=1;
int total=0;
while(i<=100)
{
if(a%2 == 0)
{
total=total+a;
}
a++;
}
System.out.priintln(total);

• 实现整数反转 用户输入任意一个数字比如12345，程序输出54321

Scanner scanner =  new Scanner(System.in);
        System.out.println("请输入一个数");
        int num = scanner.nextInt();
        while(num>0)
        {
            System.out.print(num % 10);
            num = num / 10;
        }

public class whileDemo2 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("输入学生姓名");
        String name = scanner.next();
        int score = 0;
        int total = 0;
        for(int i = 1; i <= 5; i++)
        {
            System.out.println("请输入"+i+"成绩");
            score = scanner.nextInt();
            total = total + score;
        }
        System.out.println(name+"的平均成绩是"+total/5);
    }
}

public class whileDemo2 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("输入学生姓名");
        String name = scanner.next();
        int score = 0;
        int total = 0;
        boolean error = false;
        for(int i = 1; i <= 5; i++)
        {
            System.out.println("请输入"+i+"成绩");
            score = scanner.nextInt();
            if(score<0 || score>100)
            {
                error = true;
                break;
            }
            total = total + score;
        }
        if(error == false)
        {
            System.out.println(name+"的平均成绩是"+total/5);
        }
        else
        {
            System.out.println("录入有误");
        }
    }
}

public static void main(String[] args) {
       int count = 0;
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入班级总人数");
        int personCount = scanner.nextInt();
        for(int i = 1; i <= personCount; i++)
        {
            System.out.println("请输入第" + i + "次成绩");     
            int score = scanner.nextInt();
            if(score < 80)
            {
                continue;
            }
            count++;//累加成绩大于等于80的次数
        }
        System.out.println("80分以上学生人数为" + count);
    }
}