Question

我试图弄清楚如何编写一个try和catch语句来验证我的字符串，以显示当你运行程序时，你会看到一条错误消息，告诉用户你没有输入任何内容。

以下是代码：

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Q2L04 extends JFrame {


private JButton q2l;
private JButton L2Q;
private JButton exit;
private JPanel panel;
private final int WINDOW_WIDTH=500;
private final int WINDOW_HEIGHT=300;
private JTextField QuartstoLiters;
private JTextField Answer;

public Q2L04() {

        super("Q2L04");

        setSize(WINDOW_WIDTH,WINDOW_HEIGHT);



        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        QuartstoLiters=new JTextField(10);

        Answer= new JTextField(10);


            q2l= new JButton("Q2L");
            L2Q=new JButton("L2Q");
            exit=new JButton("EXIT");




            JPanel buttonsPanel=new JPanel();
                buttonsPanel.setLayout(new GridLayout(2,0));
                buttonsPanel.add(q2l);
                buttonsPanel.add(L2Q);






        setLayout(new BorderLayout());
            add(buttonsPanel, BorderLayout.WEST);
            add(exit,BorderLayout.EAST);
            add(QuartstoLiters,BorderLayout.NORTH);
            add(Answer,BorderLayout.SOUTH);





            panel=new JPanel();

            panel.setBackground(Color.YELLOW);
            add(panel);
            setVisible(true);




q2l.addActionListener(new QuartstoLiters());

exit.addActionListener(new ExitButton());

L2Q.addActionListener(new LiterstoQuarts());

以下是动作侦听器

private class QuartstoLiters implements ActionListener {
public void actionPerformed (ActionEvent e) {
    double  math = 0;
    String userQuarts=QuartstoLiters.getText();
    int Stringtoint=Integer.parseInt(userQuarts);
    double liters=0.946353;


  // the math to conver quartstoliters
    // 1 quart =0.946353 liters
   // quarts * 0.946353= numbers of liters
    math=Stringtoint*liters;
// convert the double to a string
Answer.setText(Double.toString(math));

这是try和catch语句

   try 
     {
        {

    if (userQuarts!= null)
    throw(new Exception());

   }


}
catch (Exception exe){
JOptionPane.showMessageDialog(null,"No input data was entered","Error",JOptionPane.ERROR_MESSAGE);

  return;


    }

  }


 }

用户不会在输入QuartstoLiters文本字段中输入任何内容，然后会显示没有输入数据的错误消息。

Answer 1

我不知道您的代码是什么样的。这是我的完整测试代码。

我准备了两个版本的actionlistener类，QuartstoLiters和QuartstoLiters2。

前者是具有检查变量状态的版本（textfield-txtQuartstoLiters），后者使用try catch机制。当parseInt方法中的空字符串时抛出NumberFormatException。

package com.tobee.tests.swing;

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Q2L04 extends JFrame {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            public void run() {
                JFrame frame = new Q2L04();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                //frame.getContentPane().add(mainPanel);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }

        });
    }

    private JButton q2l;
    private JButton L2Q;
    private JButton exit;
    private JPanel panel;
    private final int WINDOW_WIDTH = 500;
    private final int WINDOW_HEIGHT = 300;
    private JTextField txtQuartstoLiters;
    private JTextField Answer;

    public Q2L04() {

        super("Q2L04");

        setSize(WINDOW_WIDTH, WINDOW_HEIGHT);

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        txtQuartstoLiters = new JTextField(10);
        txtQuartstoLiters.setText("0");

        Answer = new JTextField(10);

        q2l = new JButton("Q2L");
        L2Q = new JButton("L2Q");
        exit = new JButton("EXIT");

        JPanel buttonsPanel = new JPanel();
        buttonsPanel.setLayout(new GridLayout(2, 0));
        buttonsPanel.add(q2l);
        buttonsPanel.add(L2Q);

        setLayout(new BorderLayout());
        add(buttonsPanel, BorderLayout.WEST);
        add(exit, BorderLayout.EAST);
        add(txtQuartstoLiters, BorderLayout.NORTH);
        add(Answer, BorderLayout.SOUTH);

        panel = new JPanel();

        panel.setBackground(Color.YELLOW);
        add(panel);
        setVisible(true);

        q2l.addActionListener(new QuartstoLiters2());

        //exit.addActionListener(new ExitButton());

        //L2Q.addActionListener(new LiterstoQuarts());
    }

    class QuartstoLiters implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            double math = 0;
            int Stringtoint = 0;
            final double liters = 0.946353;

            String userQuarts = txtQuartstoLiters.getText();

            if(!userQuarts.equals(""))
            {
                Stringtoint = Integer.parseInt(userQuarts);
            }
            else
            {
                JOptionPane.showMessageDialog(null, "No input data was entered", "Error", JOptionPane.ERROR_MESSAGE);
            }

            //try {
            //  if (userQuarts == null) throw (new Exception());

            //} catch (Exception exe) {
            //  JOptionPane.showMessageDialog(null, "No input data was entered", "Error", JOptionPane.ERROR_MESSAGE);
            //
            //  return;

            //}

            // the math to conver quartstoliters
            // 1 quart =0.946353 liters
            // quarts * 0.946353= numbers of liters
            math = Stringtoint * liters;
            // convert the double to a string
            Answer.setText(Double.toString(math));
        }
    }

    class QuartstoLiters2 implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            double math = 0;
            int Stringtoint = 0;
            final double liters = 0.946353;

            String userQuarts = txtQuartstoLiters.getText();


            try {
                Stringtoint = Integer.parseInt(userQuarts);
            }  catch (NumberFormatException exe) {

                if(userQuarts.equals(""))
                    JOptionPane.showMessageDialog(null, "Empty string not allowed", "Number Error", JOptionPane.ERROR_MESSAGE);
                else
                    JOptionPane.showMessageDialog(null, "Only number is allowed", "Number Error", JOptionPane.ERROR_MESSAGE);

                return;
            }

            // the math to conver quartstoliters
            // 1 quart =0.946353 liters
            // quarts * 0.946353= numbers of liters
            math = Stringtoint * liters;
            // convert the double to a string
            Answer.setText(Double.toString(math));
        }
    }

}

您可以通过两种情况对文本字段值进行验证检查。

try {
    Stringtoint = Integer.parseInt(userQuarts);
}  catch (NumberFormatException exe) {

    if(userQuarts.equals(""))
        JOptionPane.showMessageDialog(null, "Empty string not allowed", "Number Error", JOptionPane.ERROR_MESSAGE);
    else
        JOptionPane.showMessageDialog(null, "Only number is allowed", "Number Error", JOptionPane.ERROR_MESSAGE);

    return;
}

它将显示一个空字符串或不显示带有if语句的数字消息，具体取决于用户输入。

和输出图像。

用户在JFrame中不输入任何内容并获取错误消息

1 个答案: