用于运行/停止程序的for循环

Question

im正在编写一个比较两个int的程序，我希望该程序一直运行到用户感到厌倦并想退出为止，为此用户将输入q退出。我想写一个while循环，比较两个字符，一个是用户输入的，另一个是分配给它的"q"。然而，当我输入q时，我的程序崩溃了，为什么是这样，我如何修复

import java.io.IOException;
import java.util.Scanner;

public class numComp {
public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    int a, b; 
    char key  = 'q';
    System.out.println("enter 'q' to quit program and any other key to start it");
    char btn = 0;
    try {
        btn = (char) System.in.read();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    while (btn != key){
    System.out.println("enter two numbers to compare them");
    System.out.println("please enter first number:");
    a = in.nextInt();
    System.out.println("please enter second number:");
    b = in.nextInt();
    if (a==b)
        System.out.println("the numbers you entered are the same");
    else if (a>b)
        System.out.println("your first number is greater then your second number by "+ (a-b));
    else
        System.out.println("your first number is smaller then your second number by "+ (b-a));
    }
}
}

Answer 1

这是因为在while循环中，您扫描的是整数，但"q"是一个字符。您需要让用户有机会在while循环中读取btn。

Answer 2

您的循环应该是这样的：

try{  
    btn = (char) System.in.read();
    while (btn != key){
    System.out.println("enter two numbers to compare them");
    System.out.println("please enter first number:");
    a = in.nextInt();
    System.out.println("please enter second number:");
    b = in.nextInt();
   if (a==b)
      System.out.println("the numbers you entered are the same");
  else if (a>b)
      System.out.println("your first number is greater then your second number by "+ (a-b));
  else
     System.out.println("your first number is smaller then your second number by "+ (b-a));
   System.out.println("enter 'q' to quit program and any other key to start it again");
   btn = (char) System.in.read();
  } // END OF WHILE LOOP
} catch (IOException e) {
  // TODO Auto-generated catch block
  e.printStackTrace();
}

还要注意，您需要在while循环结束之前再次从用户那里获得输入。

Answer 3

检查时需要更改btn。在循环之前只设置一次，并且永远不会在中更新

例外情况是因为

a = in.nextInt();

当您输入char时，您尝试读取int。

您需要在循环的末尾添加以下内容：

btn = (char) System.in.read();

Answer 4

这是一个简化的程序。

public static void main(String[] args){
    // Scanner
    Scanner s = new Scanner(System.in);
    // Enter the program
    System.out.println("enter 'q' to quit program and any other key to start it");
    int a, b;
    try {
        char key;
        while ((key = s.next().charAt(0))!='q'){
            System.out.println("enter two numbers to compare them");
            System.out.println("please enter first number:");
            a = s.nextInt();
            System.out.println("please enter second number:");
            b = s.nextInt();
            if (a==b)
                System.out.println("the numbers you entered are the same");
            else if (a>b)
                System.out.println("your first number is greater then your second number by "+ (a-b));
            else
                System.out.println("your first number is smaller then your second number by "+ (b-a));
        }
    } catch (Exception e){ e.printStackTrace(); }
}