java新手笔记3 运算符&循环

1.包

2.运算符

public class Operator {

   public static void main(String[] args) {
       int a = 5;
       System.out.println("a = " + a);
       //a = -a; //+   -
       System.out.println("a = " + a);
       //+ 字符串链接
       System.out.println("影分身" + "软件开发");
       System.out.println(3 + 5 + "软件开发" + 6 + 8);

       double b = a / 2;
       System.out.println("a / 2 = " + (a / 2));
       System.out.println("b = " + b);
       int weith = 10;
       int height = 6;
       double area =  1.0 / 2 * weith * height;
       System.out.println("area = " + area);

       int c = a % 3;//取余数
       System.out.println("c = " + c);

       //int d = c++;//  c++  c = c + 1; 先赋值 后自增
       int d = ++c;//先自增 后赋值
       System.out.println("d = " + d);
       System.out.println("c = " + c);

       //赋值
       int e = 10; //-=  *= /= %=
       e += 1;// e = e + 1;
       System.out.println("e = " + e);

       //比较 > >=  5 >= 5  5 > 5  < <=  ==
       boolean isEquals = (1 == 1);

       System.out.println("isEquals = " + isEquals);
       int year = 2012;
       boolean isLeap = (year % 4 == 0 && year % 100 != 0 || year % 400 == 0);
       System.out.println(year +" 是否是闰年 " + isLeap);

       System.out.println("1 != 2 ? " + (1 != 2));
       //逻辑
       boolean isTrue = false;

       System.out.println("isTrue ? " + isTrue);
       System.out.println("!isTrue ? " + !isTrue);

       System.out.println("true || false ? " + (true || false));//或运算

       System.out.println("true && false ? " + (true && false));//与运算

       int score = 85;

       //score > 60  及格   > 70  良   > 85 优秀
       // javase > 60 && oracle > 70  && xml > 70 

       // javase > 60 || oracle > 60  || xml > 60 

       int move = 16;
       System.out.println("move = " + (move >> 3));//100   10
       System.out.println("move = " + (move << 1));

    }

}

3.选择语句

import java.util.Random;//导入类
public class IfDemo {

   public static void main(String[] args) {
      Random ran = new Random();//new 创建对象
      int score = ran.nextInt(101);//ran.nextInt(41) + 60;//随机数 0 ~ 100
      System.out.println("score = " +  score);
      /*
      if(score > 60) {//满足条件执行
         System.out.println("============");
         System.out.println("   及格了..");
         System.out.println("============");
      }
      */
      //if的第二种形式
      /*
      if(score > 60) {//满足条件执行
         System.out.println("============");
         System.out.println("   及格了..");
         System.out.println("============");
      } else {  //不满足条件执行
         System.out.println("============");
         System.out.println("   很遗憾..");
         System.out.println("============");
      }
      */
      //多分支  匹配其中一个条件
      /*
      if( score >= 90) {
         System.out.println("成绩： A");
      } else if ( score >= 80 /*&& score < 90) {
         System.out.println("成绩： B");
      } else if ( score >= 60 /*&& score < 80) {
         System.out.println("成绩： C");
      } else {
         System.out.println("成绩： D");
      }
      */

      switch(score / 10) {//如果匹配 下面代码执行
        case 10: //System.out.println("成绩： A");break;
        case 9: System.out.println("成绩： A");break;
        case 8: System.out.println("成绩： B");break;
        case 7: System.out.println("成绩： C");break;
        case 6: System.out.println("成绩： D");break;
        default : System.out.println("成绩： 不及格");

      }
      //三目运算符
      boolean isPass = ( score > 60 ? true : false);

      System.out.println("成绩 === " + isPass);

      String passed = score > 60 ? "及格" : "挂了";

      System.out.println("成绩 ==== " + passed);

    }

}

4.while循环

import java.util.Random;
public class LoopDemo {

   public static void main(String[] args) {

        int i = 0;//计数器
       //循环  不满足条件不执行   可能执行0次
       while( i < 10 ){
          System.out.println( i + " Hello World!...");//执行代码
          i++;//运行时 i值每次发生改变 i = 10
       }

       System.out.println( "i = " + i );

       i = 0; // 0  9

       do{//先执行 后判断  至少执行一次
          System.out.println( i + " 快乐!...");
          i++;
       }
       while (i < 10);   

       int a = ran.nextInt(101);
       System.out.println( "a = " + a );
       int i = 2;
       while( i < a) {//67
          if( a % i == 0) {
             break;//跳出循环
          }
          i++;
       }

       System.out.println( "i = " + i );

       if(i >= a)//正常退出循环
          System.out.println( a +"是素数");
       else
          System.out.println( a +"不是素数");

    }

}

5.for循环

import java.util.Random;
public class LoopDemo2 {

   public static void main(String[] args) {
      int k = 10; //方法中的变量 作用域 在方法中使用
      //i 局部变量

      for(int i = 0;i < 10 ; i++ ) {//三条语句
         System.out.println( i + " Hello World!...");

      }

      //循环嵌套
      /*
      for(int i = 0; i < 5; i++) {
         //k = k + i;
         for(int j = 0; j < 3; j++) {

            System.out.print(" * ");
         }
         System.out.print("\n");//输出换行
      }
      */

      for(int i = 1; i < 6; i++) {

         for(int j = 1; j <= i; j++) {
            System.out.print(j + " * " + i + " = "+ i * j + "  ");
         }
         System.out.print("\n");
      }

      int sum = 0;
      int i;
      for(i = 1; i < 200; i++) {

         sum += i;
         if(sum >= 200){
            break;
         }

      }
      System.out.println("sum = " + sum);
      System.out.println("i = " + i);
    }

}

6.标记循环

import java.util.Random;
public class LoopDemo3 {

   public static void main(String[] args) {

      int sum = 0;
      int i,k = -1;
      loopi: for(i = 1; i < 200; i++) {
         for(int j = 1; j < 200; j++) {
            sum += j;
            if(sum >= 200){
               k = j;
               break loopi;//指定跳出位置
            }

         }
         System.out.println("k = " + k);
         System.out.println("i = " + i);
      }
      System.out.println("sum = " + sum);
      System.out.println("k = " + k);

      for(int a = 1; a < 101; a++ ) {

         if( a % 3 != 0){//不能被3整除
             continue;//结束本次后面代码执行
         }
         System.out.print( a + "\t");
      }

    }

}

7.双循环

public class LoopDemo4 {

   public static void main(String[] args) {

      int sum = 0;

      for(int i = 1; i < 11; i++) {//控制的是行

         for(int j = 1; j <= i; j++) {//控制的是列
            System.out.print(" * ");
            /**/
            if(j > 5){ //1 2 3 4 5 6   7 8 9 10
               break;//跳出内层循环
            }  

         }

         System.out.println();
      }

    }

}