如何重新啓動一個java程序

Question

我對Java很陌生，只是隨處可用基礎知識。我做了某種計算器。

這裏（有可能更容易計算器，只需用什麼來給我的腦海裏工作）是我的代碼：

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."); 

     } 

    } 

所以我的問題是：當它在開關case語句達到默認情況下，我可以做該程序從重新啓動？

在此先感謝。

Answer 1

A while循環是你需要的。循環的輸入條件將是default的情況。在循環中，你必須把這段代碼你使用之前：

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，將繼續要求輸入，直到你沒有得到一個default情況。

Answer 2

似乎所有你想要做的是循環直到用戶指定了一個有效的操作。在這種情況下，你可以使用while循環，看一個變量：

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

我認爲你需要一個while循環，例如：

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

多了，我想給你一些建議：

• 你應該檢查輸入的「text」字符串是否爲null，如果「text」字符串爲null，則會遇到NullPointerException。
• 如果你會使用這麼多的case語句，你必須考慮設計patten，鏈接是Replacing if else statement with pattern。