如何在这段代码中实现while循环？答案

【问题标题】：How to implement the while loop in this piece of code?如何在这段代码中实现while循环？
【发布时间】：2012-03-18 10:31:00
【问题描述】：

我的程序在我输入无效输入后结束。我知道我需要使用while 循环来让用户输入数字。

代码如下：

import java.util.InputMismatchException;
import java.util.Scanner; 
public class AreaCircle { 
    public static void main(String[] args) {
        // TODO code application logic here
        Scanner sc = new Scanner(System.in); // read the keyboard
        System.out.println("This program will calculate the area of a circle");
        System.out.println("Enter radius:");//Print to screen

        try {
            double r = sc.nextDouble(); // Read in the double from the keyboard
            double area = (3.14 *r * r);

            String output = "Radius: " + r + "\n";
            output = output + "Area: " + area + "\n";
            System.out.println("The area of the circle  is " + area);

        }
        catch( NumberFormatException e ) {
            System.out.println("Invalid Input, please enter a number");
            //put a message or anything you want to tell the user that their input was weird.
        }
        catch( InputMismatchException e )
        {

            System.out.println("Input Mismatch, please enter a number");
            //put a message or anything you want to tell the user that there is an 
            //input mismatch.
        }
    }
}

【问题讨论】：

标识你想要重复的代码块；代码块应该在循环体内。确定您希望循环在什么条件下结束，这应该是被评估的 bool 表达式。可能有人会提出答案，但构建循环是非常基本的东西，我认为值得花时间自己学习。如果你在一个小时内仍然无法弄清楚，并且没有人回答你的问题，那么我很乐意自己提交一个例子=）
这与 Swing 没有任何关系。在将它们应用于问题之前，请仔细阅读标签。
你已经问过类似的问题here你自己尝试过什么吗？？

标签： java loops java.util.scanner

【解决方案1】：

只需围绕您想要继续执行的部分进行无限循环，直到您获得有效输入，如果您获得有效输入，则将其中断。

while (true){
  try {
    double r = sc.nextDouble(); // Read in the double from the keyboard
    double area = (3.14 *r * r);

    String output = "Radius: " + r + "\n";
    output = output + "Area: " + area + "\n";
    System.out.println("The area of the circle  is " + area);
    break;                              <---- New code that will break the loop

  }
  catch( NumberFormatException e ) {
    System.out.println("Invalid Input, please enter a number");
    //put a message or anything you want to tell the user that their input was weird.
  }
  catch( InputMismatchException e )
  {

    System.out.println("Input Mismatch, please enter a number");
    //put a message or anything you want to tell the user that there is an 
    //input mismatch.
  }
}
The execution will continue from here after break

【讨论】：

上述代码在输入无效时会陷入无限循环。请检查。
不，它不会因为从输入中读取是由 sc.nextDouble() 完成的。

【解决方案2】：

放双 r = sc.nextDouble();和 While 循环内的异常。

将自定义消息放入异常捕获块中，例如“输入不正确，请重新输入输入。如果在循环仍然执行时发生异常。

【讨论】：

【解决方案3】：

您宁愿在这里需要一个 do-while 循环。您使用 do-while 循环，其中循环中的代码块必须至少执行一次。以下是您将如何实现它：

import java.util.InputMismatchException;
import java.util.Scanner; 
public class Circle { 
    public static void main(String[] args) {

          boolean invalid_input = false;    

        do
        {
        // TODO code application logic here
        Scanner sc = new Scanner(System.in); // read the keyboard
        System.out.println("This program will calculate the area of a circle");
        System.out.println("Enter radius:");//Print to screen

        try {
            double r = sc.nextDouble(); // Read in the double from the keyboard
            double area = (3.14 *r * r);

            String output = "Radius: " + r + "\n";
            output = output + "Area: " + area + "\n";
            System.out.println("The area of the circle  is " + area);
                invalid_input = false;

        }
        catch( NumberFormatException e ) {
              invalid_input = true;
            System.out.println("Invalid Input, please enter a number");
            //put a message or anything you want to tell the user that their input was weird.
        }
        catch( InputMismatchException e )
        {
                invalid_input = true;
            System.out.println("Input Mismatch, please enter a number");
            //put a message or anything you want to tell the user that there is an 
            //input mismatch.
        }
        }while(invalid_input== true);



    }
}

【讨论】：

@user1190386 在 Java 中可能有其他处理异常的方法。我刚才给你看的，是一个例子。