如何重新启动一个java程序

Question

我对Java很陌生，只是随处可用基础知识。我做了某种计算器。

这里（有可能更容易计算器，只需用什么来给我的脑海里工作）是我的代码：

import java.util.Scanner; 

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

     Scanner input = new Scanner(System.in); 


     double second; 

     System.out.println("Please tell me a number"); 

     double first = input.nextDouble(); 
     input.nextLine(); // Consume newline left-over 

     System.out.println("You chose the number " + first); 

     System.out 
       .println("Now please choose what you want to do with the number. (divide, add, multiply or subtract)"); 
     String text = input.nextLine(); 

     switch (text) { 
     case "divide": 
      System.out.println("What do you want to divide your number " + first + " with?"); 
      second = input.nextDouble(); 
      double resultDivide = first/second; 
      System.out.println(first + "/" + second + " = " + resultDivide); 
      break; 
     case "add": 
      System.out.println("What do you want to add to your number " + first + "?"); 
      second = input.nextDouble(); 
      double resultAdd = first + second; 
      System.out.println(first + " + " + second + " = " + resultAdd); 
      break; 
     case "multiply": 
      System.out.println("What do you want to multiply your number " + first + " with?"); 
      second = input.nextDouble(); 
      double resultMultiply = first * second; 
      System.out.println(first + " * " + second + " = " + resultMultiply); 
      break; 
     case "subtract": 
      System.out.println("What do you want to subtract your number " + first + " with?"); 
      second = input.nextDouble(); 
      double resultSubtract = first - second; 
      System.out.println(first + " - " + second + " = " + resultSubtract); 
      break; 
     default: 
      System.out.println("Sorry, I did not quite understand that. Please try again."); 

     } 

    } 

所以我的问题是：当它在开关case语句达到默认情况下，我可以做该程序从重新启动？

在此先感谢。

Answer 1

A while循环是你需要的。循环的输入条件将是default的情况。在循环中，你必须把这段代码你使用之前：

System.out.println("Now please choose what you want to do with the number. (divide, add, multiply or subtract)"); 
     String text = input.nextLine(); 

因此，将继续要求输入，直到你没有得到一个default情况。

Answer 2

似乎所有你想要做的是循环直到用户指定了一个有效的操作。在这种情况下，你可以使用while循环，看一个变量：

import java.util.Scanner; 

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

     Scanner input = new Scanner(System.in); 

     double second; 
     System.out.println("Please tell me a number"); 
     double first = input.nextDouble(); 
     input.nextLine(); // Consume newline left-over 

     System.out.println("You chose the number " + first); 

     System.out 
       .println("Now please choose what you want to do with the number. (divide, add, multiply or subtract)"); 
     String text = input.nextLine(); 

     boolean correctOperation = true; 

     do { 
      correctOperation = true; 

      switch (text) { 
       case "divide": 
        System.out.println("What do you want to divide your number " + first + " with?"); 
        second = input.nextDouble(); 
        double resultDivide = first/second; 
        System.out.println(first + "/" + second + " = " + resultDivide); 
        break; 
       case "add": 
        System.out.println("What do you want to add to your number " + first + "?"); 
        second = input.nextDouble(); 
        double resultAdd = first + second; 
        System.out.println(first + " + " + second + " = " + resultAdd); 
        break; 
       case "multiply": 
        System.out.println("What do you want to multiply your number " + first + " with?"); 
        second = input.nextDouble(); 
        double resultMultiply = first * second; 
        System.out.println(first + " * " + second + " = " + resultMultiply); 
        break; 
       case "subtract": 
        System.out.println("What do you want to subtract your number " + first + " with?"); 
        second = input.nextDouble(); 
        double resultSubtract = first - second; 
        System.out.println(first + " - " + second + " = " + resultSubtract); 
        break; 
       default: 
        System.out.println("Sorry, I did not quite understand that. Please try again."); 
        correctOperation = false; 
        text = input.nextLine(); 
      } 
     } while(!correctOperation) 
    } 
} 

Answer 3

我认为你需要一个while循环，例如：

String text = input.readLine(); 
while(text != null) { 
    switch(...) 
} 

多了，我想给你一些建议：

• 你应该检查输入的“text”字符串是否为null，如果“text”字符串为null，则会遇到NullPointerException。
• 如果你会使用这么多的case语句，你必须考虑设计patten，链接是Replacing if else statement with pattern。