JAVA学习笔记之（三）流程控制

桃扇骨 2022-06-10 08:41 264阅读 0赞

## 1 读入数据 ##

需要用到java.util.Scanner类

package com.mlj.coreJava.day3;
    
    import java.util.Scanner;
    
    public class TestScanner {
        public static void main(String args[]) {
            Scanner scanner = new Scanner(System.in);
            System.out.println("请输入一个整数：");
            int a = scanner.nextInt();
            System.out.println(a + "收到了！");
        }
    }
        运行结果：

![这里写图片描述][SouthEast]

读入整数、字符串、小数分别用Scanner类中的nextInt()、nextLine()、nextDouble()方法

## 2 分支结构 ##

### 2.1 if语句 ###

语法：if（布尔表达式）{
             代码块1;
         }else{
             代码块2;
         }

package com.mlj.coreJava.day3;
    
    import java.util.Scanner;
    
    public class TestBigger {
        public static void main(String args[]){
            Scanner scanner = new Scanner(System.in);
            System.out.println("请输入一个整数：");
            int num1 = scanner.nextInt();
            System.out.println("请输入第二个整数：");
            int num2 = scanner.nextInt();
            if (num1>num2){
                System.out.println("num1最大");
            }else {
                System.out.println("num2最大");
            }
        }
    }

运行截图：  
![这里写图片描述][SouthEast 1]

### 2.2 if语句的多重分支 ###

语法：if(条件1){
             条件1代码块；
         }else if(条件2){
             条件2代码块；
         }else if(条件3){
             条件3代码块；
         }

package com.mlj.coreJava.day3;
    
    import java.util.Scanner;
    
    public class TestScop {
        public static void main(String args[]){
            Scanner scanner = new Scanner(System.in);
            System.out.println("请输入你的成绩");
            int scop = scanner.nextInt();
            if (scop>0 && scop<60){
                System.out.println("很遗憾！你不及格");
            }else if (scop>=60 && scop<80){
                System.out.println("恭喜你！你及格了");
            }else if (scop>=80 && scop<=100){
                System.out.println("成绩优秀");
            }else {
                System.out.println("成绩输入不合法");
            }
        }
    }

运行截图：  
![这里写图片描述][SouthEast 2]

### 2.3 switch语句 ###

语法：switch(value){
             case value1:
                 语句块1；
                 break;
            case value2:
                 语句块2；
                 break;
            case value2:
                 语句块2；
                 break;
            default:语句块;         
         }
    注意：*value只能是int byte char short*

package com.mlj.coreJava.day3;
    
    import java.util.Scanner;
    
    public class TestSwitch {
        public static void main(String args[]){
            Scanner sc = new Scanner(System.in);
            System.out.println("1：手机充值\n2:人工服务\n3:业务办理\n...");
            System.out.println("请输入您需要哪种服务:");
            int num = sc.nextInt();
            switch (num){
                case 1:
                    System.out.println("进入手机充值");
                    break;
                case 2:
                    System.out.println("进入人工服务");
                    break;
                case 3:
                    System.out.println("进入业务办理");
                    break;
                default:System.out.println("输入有误！");
            }
        }
    }

运行截图：  
![这里写图片描述][SouthEast 3]  
![这里写图片描述][SouthEast 4]

## 3 循环控制 ##

### 3.1 for循环 ###

语法：for(初始化;循环条件；迭代操作){
             循环体；
         }

package com.mlj.coreJava.day3;
    
    public class TestFor {
        public static void main(String args[]){
            for (int i = 10;i>0;i--){
                System.out.println(i+"\n");//输出10-1的整数
            }
        }
    }

运行截图：  
![这里写图片描述][SouthEast 5]

### 3.2 while与do…while循环 ###

#### 3.3.1 while循环 ####

语法：while(循环条件){
             循环体；
         }

package com.mlj.coreJava.day3;
    
    import java.util.Scanner;
    
    public class TestWhile {
        public static void main(String args[]) {
            Scanner scanner = new Scanner(System.in);
            System.out.println("请输入一个整数:");
            int num = scanner.nextInt();
            int count = 0;//运行次数
            while (num > 5) {
                System.out.println(num);
                num--;
                count++;
            }
            System.out.println("运行次数："+count);
        }
    }

运行截图:  
![这里写图片描述][SouthEast 6]

#### 3.3.2 do…while循环 ####

语法：do{
             循环体；
         }while(循环条件);

package com.mlj.coreJava.day3;
    
    import java.util.Scanner;
    
    public class TestWhile {
        public static void main(String args[]) {
            Scanner scanner = new Scanner(System.in);
            System.out.println("请输入一个整数:");
            int num = scanner.nextInt();
            int count = 0;//运行次数
            do {
                System.out.println(num);
                num--;
                count++;
            } while (num > 5);
    
            System.out.println("运行次数：" + count);
        }
    }

运行截图：  
![这里写图片描述][SouthEast 7]

***两者区别：do…while至少执行一次***

## 3.3 break和continue ##

break:跳出当前的循环
    continue：跳出本次循环

package com.mlj.coreJava.day3;
    
    public class Testbc {
        public static void main(String args[]){
            for (int i = 10;i>0;i--){
                if (i==5) continue;
                System.out.println(i);
            }
        }
    }

当i=5时跳出本次循环，所以没有打印5  
![这里写图片描述][SouthEast 8]

package com.mlj.coreJava.day3;
    
    public class Testbc {
        public static void main(String args[]) {
            for (int i = 10; i > 0; i--) {
                if (i == 5) break;
                System.out.println(i);
            }
        }
    }

当i=5时，跳出当前循环，所以5及之后的数字没有打印  
![这里写图片描述][SouthEast 9]

### 3.4 多重循环 ###

#### 3.4.1 ####

打印     *
          * * *
        * * * * *

package com.mlj.coreJava.day3;
    
    import java.util.Scanner;
    
    public class TestPrint {
        public static void main(String args[]) {
            Scanner scanner = new Scanner(System.in);
            System.out.println("请输入一个整数：");
            int num = scanner.nextInt();
    
            for (int i = 1; i <= num; i++) {
       //控制行数
                for (int j = 1; j <= num - i; j++) {
       //控制空格数
                    System.out.print(" ");
                }
                for (int m = 1; m <= 2 * i - 1; m++) {
       //控制*数
                    System.out.print("*");
                }
                System.out.println();
            }
        }
    }

运行截图：  
![这里写图片描述][SouthEast 10]

#### 3.4.2 打印乘法表 ####

package com.mlj.coreJava.day3;
    
    public class TestMultiplication {
        public static void main(String args[]) {
            for (int i = 1; i <= 9; i++) {
                for (int n = 1; n <= i; n++) {
                    System.out.print(n + " x " + i + " = " + i * n + "\t");
                }
                System.out.println();
            }
        }
    }

运行结果:  
![这里写图片描述][SouthEast 11]

[SouthEast]: /images/20220610/c9a2b8ba601a4b1ca42be8201ac0b11e.png
[SouthEast 1]: /images/20220610/69277c99979d48ae85d1e5af1cad6c37.png
[SouthEast 2]: /images/20220610/530ea431e367468794d3d2bdddc7db9c.png
[SouthEast 3]: /images/20220610/ec625bd8d8b34e53a79134aa513d1b57.png
[SouthEast 4]: /images/20220610/f4a0434fa8564a3893a7d4508f305b36.png
[SouthEast 5]: /images/20220610/bb591e66dba94f5988b74fe370cbbacc.png
[SouthEast 6]: /images/20220610/e907ef83db9c4b599541cca5096d1931.png
[SouthEast 7]: /images/20220610/c57bcc0a5bc04b0a9fa3a586bc4151c8.png
[SouthEast 8]: /images/20220610/dafb0969a7ff4dfba62b30dc6302bd34.png
[SouthEast 9]: /images/20220610/6a39f3f4fc6045a5a50fddc1e4f4c657.png
[SouthEast 10]: /images/20220610/f5cb46dce5ef462e9be096fbf2daf9dd.png
[SouthEast 11]: /images/20220610/08d605aeb0f84617afad97b39224e2e6.png