我很难跳到一个循环的正确部分

Question

我要做的是让用户输入一个分子和一个分母，并显示商。我正在使用一个方法，使用Integer.Parseint和try catch来检查并查看该数字是否有效。问题是，如果try catch捕获异常并显示错误，它将继续到代码的下一部分，而不是跳回到循环的开始。

public static void main(String[] args) {
        String numerator = "";
        String denominator = "";
        String message = "";
        int num = 0;
        int den = 0;
        int quo = 0;
        int yesNo = 0;
        do
        {
            // Displays a JOptionPane asking the user to input a numerator.
            numerator = JOptionPane.showInputDialog(null, "Please enter a numerator.", "Enter a Numerator",JOptionPane.QUESTION_MESSAGE);
            if(numerator == null)// This breaks the loop if  the user clicks the "cancel" button.
                break;
            num = IsValid(numerator); // Calls the IsValid method, passing it the String entered by the user to validate if it is a proper integer.
            denominator = JOptionPane.showInputDialog(null, "Please enter a denominator.", "Enter a Denominator",JOptionPane.QUESTION_MESSAGE);
            if(denominator == null) // same as above but for the denominator
                break;
            den = IsValid(denominator);
            /*if(den != 0 && num<den) 
            { // This section is unrelated to my problem
                quo = num/den;
                message = "Numerator: " + num + "\n" +
                            "Denominator: " + den + "\n" +
                            "Quotient: ";
                JOptionPane.showMessageDialog(null, message,"Calculation",JOptionPane.INFORMATION_MESSAGE);
            }*/
            yesNo = JOptionPane.showConfirmDialog(null,"Would you like to make another calculation?","Continue?",JOptionPane.YES_NO_OPTION);
        }while(yesNo == JOptionPane.YES_OPTION);
    }
    public static int IsValid(String input)
    {
        int num = 0; // This method is passed the String that the JOptionPane receives and using Integer.parseInt to make sure it's an integer, and assigns it to int "num".
        try{num = Integer.parseInt(input);}
        catch(NumberFormatException ex)
        {
            JOptionPane.showMessageDialog(null, "Error: Invalid number. Please enter a valid number.","Invalid Input",JOptionPane.WARNING_MESSAGE);
        }
        return num;
        // My main issue is that it continues with the code instead of jumping back to the JOptionPane to ask the user for input a second time.
    }

基本上，我想调用IsValid（分子）并将其赋给int“num”，如果IsValid的catch语句被执行，则跳回到do循环的开头，以便它再次显示JOptionPane。如果针对分母执行catch语句，则跳回到该分母，而不是最开始。我不能将JOptionPane放入方法中，因为方法将无法判断我是发送给它分子还是分母。我对java还很陌生，我确信有更简单的方法来实现这一点，所以我们非常感谢任何帮助。谢谢！

Answer 1

如果输入无效，对isvalid的调用将中断程序。最自然的方法是让它抛出一个异常。然后程序捕获异常并退出。

package so20190423;

import java.awt.HeadlessException;

import javax.swing.JOptionPane;

public class Snippet {
    public static void main(String[] args) {
            String numerator = "";
            String denominator = "";
            String message = "";
            int num = 0;
            int den = 0;
            int quo = 0;
            int yesNo = 0;
            try {
                do
                {
                    // Displays a JOptionPane asking the user to input a numerator.
                    numerator = JOptionPane.showInputDialog(null, "Please enter a numerator.", "Enter a Numerator",JOptionPane.QUESTION_MESSAGE);
                    if(numerator == null)// This breaks the loop if  the user clicks the "cancel" button.
                        break;
                    num = IsValid(numerator); // Calls the IsValid method, passing it the String entered by the user to validate if it is a proper integer.
                    denominator = JOptionPane.showInputDialog(null, "Please enter a denominator.", "Enter a Denominator",JOptionPane.QUESTION_MESSAGE);
                    if(denominator == null) // same as above but for the denominator
                        break;
                    den = IsValid(denominator);
                    /*if(den != 0 && num<den) 
                    { // This section is unrelated to my problem
                        quo = num/den;
                        message = "Numerator: " + num + "\n" +
                                    "Denominator: " + den + "\n" +
                                    "Quotient: ";
                        JOptionPane.showMessageDialog(null, message,"Calculation",JOptionPane.INFORMATION_MESSAGE);
                    }*/
                    yesNo = JOptionPane.showConfirmDialog(null,"Would you like to make another calculation?","Continue?",JOptionPane.YES_NO_OPTION);
                }while(yesNo == JOptionPane.YES_OPTION);
            }
            catch(NumberFormatException ex)
            {
                JOptionPane.showMessageDialog(null, "Error: Invalid number. Please enter a valid number.","Invalid Input",JOptionPane.WARNING_MESSAGE);
            }
        }
        public static int IsValid(String input) throws NumberFormatException
        {
            int num = 0; // This method is passed the String that the JOptionPane receives and using Integer.parseInt to make sure it's an integer, and assigns it to int "num".
            num = Integer.parseInt(input);
            return num;
            // My main issue is that it continues with the code instead of jumping back to the JOptionPane to ask the user for input a second time.
        }
}

您甚至可以将try/catch限制在while循环中，以便在输入失败后恢复并重试！

package so20190423;

import java.awt.HeadlessException;

import javax.swing.JOptionPane;

public class Snippet {
    public static void main(String[] args) {
            String numerator = "";
            String denominator = "";
            String message = "";
            int num = 0;
            int den = 0;
            int quo = 0;
            int yesNo = 0;
                do
                {
                    try {
                    // Displays a JOptionPane asking the user to input a numerator.
                    numerator = JOptionPane.showInputDialog(null, "Please enter a numerator.", "Enter a Numerator",JOptionPane.QUESTION_MESSAGE);
                    if(numerator == null)// This breaks the loop if  the user clicks the "cancel" button.
                        break;
                    num = IsValid(numerator); // Calls the IsValid method, passing it the String entered by the user to validate if it is a proper integer.
                    denominator = JOptionPane.showInputDialog(null, "Please enter a denominator.", "Enter a Denominator",JOptionPane.QUESTION_MESSAGE);
                    if(denominator == null) // same as above but for the denominator
                        break;
                    den = IsValid(denominator);
                    /*if(den != 0 && num<den) 
                    { // This section is unrelated to my problem
                        quo = num/den;
                        message = "Numerator: " + num + "\n" +
                                    "Denominator: " + den + "\n" +
                                    "Quotient: ";
                        JOptionPane.showMessageDialog(null, message,"Calculation",JOptionPane.INFORMATION_MESSAGE);
                    }*/
                    yesNo = JOptionPane.showConfirmDialog(null,"Would you like to make another calculation?","Continue?",JOptionPane.YES_NO_OPTION);
                }
                catch(NumberFormatException ex)
                {
                    JOptionPane.showMessageDialog(null, "Error: Invalid number. Please enter a valid number.","Invalid Input",JOptionPane.WARNING_MESSAGE);
                }
                }while(yesNo == JOptionPane.YES_OPTION);
        }
        public static int IsValid(String input) throws NumberFormatException
        {
            int num = 0; // This method is passed the String that the JOptionPane receives and using Integer.parseInt to make sure it's an integer, and assigns it to int "num".
            num = Integer.parseInt(input);
            return num;
            // My main issue is that it continues with the code instead of jumping back to the JOptionPane to ask the user for input a second time.
        }
}

最后一个变体，如果你想重试每个输入直到它有效（或者被用户取消）：

package so20190423;

import javax.swing.JOptionPane;

public class Snippet {
    public static void main(String[] args) {
        String numerator = "";
        String denominator = "";
        String message = "";
        int num = 0;
        int den = 0;
        int quo = 0;
        int yesNo = 0;
        do {
            boolean userCancelled = false;
            boolean numeratorIsOk = false;
            do {
                try {
                    numerator = JOptionPane.showInputDialog(null, "Please enter a numerator.", "Enter a Numerator",
                            JOptionPane.QUESTION_MESSAGE);
                    if (numerator == null) {
                        userCancelled = true;
                    } else {
                        num = IsValid(numerator);
                        numeratorIsOk = true;
                    }
                } catch (NumberFormatException ex) {
                    JOptionPane.showMessageDialog(null, "Error: Invalid number. Please enter a valid number.",
                            "Invalid Input", JOptionPane.WARNING_MESSAGE);
                }
            } while (!numeratorIsOk && !userCancelled);
            if (userCancelled) {
                break;
            }

            boolean denominatorIsOk = false;
            do {
                try {
                    denominator = JOptionPane.showInputDialog(null, "Please enter a denominator.",
                            "Enter a Denominator", JOptionPane.QUESTION_MESSAGE);
                    if (denominator == null) {
                        userCancelled = true;
                    } else {
                        den = IsValid(denominator);
                        denominatorIsOk = true;
                    }
                    /*
                     * if(den != 0 && num<den) { // This section is unrelated to my problem quo =
                     * num/den; message = "Numerator: " + num + "\n" + "Denominator: " + den + "\n"
                     * + "Quotient: "; JOptionPane.showMessageDialog(null,
                     * message,"Calculation",JOptionPane.INFORMATION_MESSAGE); }
                     */
                    yesNo = JOptionPane.showConfirmDialog(null, "Would you like to make another calculation?",
                            "Continue?", JOptionPane.YES_NO_OPTION);
                } catch (NumberFormatException ex) {
                    JOptionPane.showMessageDialog(null, "Error: Invalid number. Please enter a valid number.",
                            "Invalid Input", JOptionPane.WARNING_MESSAGE);
                }

            } while (!denominatorIsOk && !userCancelled);
            if (userCancelled) {
                break;
            }
        } while (yesNo == JOptionPane.YES_OPTION);

    }

    public static int IsValid(String input) throws NumberFormatException {
        int num = 0; // This method is passed the String that the JOptionPane receives and using
                        // Integer.parseInt to make sure it's an integer, and assigns it to int "num".
        num = Integer.parseInt(input);
        return num;
        // My main issue is that it continues with the code instead of jumping back to
        // the JOptionPane to ask the user for input a second time.
    }
}