在执行代码块后循环回到 Java 程序的开头-解网

问：

我写了一个Java程序，更具体地说是一个基本的计算器。我想回到“主菜单”，要求用户在解决数学问题后输入“0”。我想出的解决方案似乎没有按照我想要的方式工作。返回的概念是有一个变量“pick”，它位于程序的开头，并且尚未分配任何内容。程序询问用户他要执行什么操作，每个操作都被分配一个数字，整个事情是在 while/do 循环的帮助下执行的。循环询问用户在“pick”为 0 时要执行什么操作。例如，如果用户选择正在求解二次方程的操作编号 2，则变量“pick”的值将变为 2，并且如果变量 pick 等于 2，则激活一个 if，并且求解二次方程的代码块开始执行。求解二次方程后，程序会询问用户是否要按 0 返回“主菜单”，我以为如果变量“pick”的值再次为 0，程序将重新开始，但似乎不是，这就是我写这篇文章的原因。

这是代码：

import java.util.Objects;
import java.util.Scanner;
import javax.swing.JOptionPane;

public class Main {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);

        String pick_String;
        int pick;
        do {
            pick_String = JOptionPane.showInputDialog("Which tool would you like to use? (Type just the number) " +
                    "\n1.Basic Calculator" +
                    "\n2.Solve a quadratic equation" +
                    "\n3.Find a side in a right triangle" +
                    "\n4.Find the distance between two points in a coordinate system" +
                    "\n5.Exit"
            );
            pick = Integer.parseInt(pick_String);
        } while (pick == 0);


        if (pick == 1) {
            //code for the basic calculator
                System.out.println("If you want to get back to the menu type 0");
                pick = scanner.nextInt();
        } else if (pick == 2) {
            //code for solving a the quadratic equation
                System.out.println("If you want to get back to the menu type 0");
                pick = scanner.nextInt();
        } else if (pick == 3) {
            //code for finding a side in a right triangle
                System.out.println("If you want to get back to the menu type 0");
                pick = scanner.nextInt();
        } else if (pick == 4) {
            //code for finding the distance between two points in a coordinate system
                System.out.println("If you want to get back to the menu type 0");
                pick = scanner.nextInt();
        } else if (pick == 5) {
            System.exit(0);
        }
    }

}

我认为它可能不起作用，因为程序是从顶部执行的，然后一直到底部，并且在执行“主菜单”后无法再次执行，但我不确定。

Java while-loop do-while

你有一个循环，只要输入是 0，它就会不断要求输入，但你没有任何 if 条件会在你稍后键入 0 时再次运行循环。像这样，你不能通过键入 0 再次运行循环。在执行其中一项计算后键入 0 的效果将与键入大于 5 或小于 1 的任何其他数字的效果相同。如果要在键入 0 时运行主菜单循环，则应为此添加 if 条件，例如：

if (pick == 0) {
    //the main menu loop
}

此外，由于 if-else-block 之后没有任何内容，因此程序将在您执行下一个输入后结束。如果你想让它运行多次，你必须循环程序。我建议将 do-while-loop 变成 while-loop，然后在 while （true） - 循环中运行整个代码：

public class Main {

public static void main(String[] args) {

    Scanner scanner = new Scanner(System.in);

    String pick_String;
    int pick = 0;

    while (true) {
 
        if (pick == 0) {
            while (pick==0) {
                pick_String = JOptionPane.showInputDialog("Which tool would you like to use? (Type just the number) " +
                "\n1.Basic Calculator" +
                "\n2.Solve a quadratic equation" +
                "\n3.Find a side in a right triangle" +
                "\n4.Find the distance between two points in a coordinate system" +
                "\n5.Exit");
                pick = Integer.parseInt(pick_String);
            }
        }

        if (pick == 1) {
            //code for the basic calculator
            System.out.println("If you want to get back to the menu type 0");
            pick = scanner.nextInt();
        } else if (pick == 2) {
            //code for solving a the quadratic equation
            System.out.println("If you want to get back to the menu type 0");
            pick = scanner.nextInt();
        } else if (pick == 3) {
            //code for finding a side in a right triangle
            System.out.println("If you want to get back to the menu type 0");
            pick = scanner.nextInt();
        } else if (pick == 4) {
            //code for finding the distance between two points in a coordinate system
            System.out.println("If you want to get back to the menu type 0");
            pick = scanner.nextInt();
        } else if (pick == 5) {
            System.exit(0); // this will exit while(true) along with the program
        }
    
    
    }

}

在执行代码块后循环回到 Java 程序的开头

Looping back to the start of a java program after executing a block of code

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