java - (作业)对话框窗口的问题以及关闭对话框时执行操作

Question

我正在做一项家庭作业，该作业有四个文本字段和一个文本区域，以及一个将文本字段和文本区域保存到文本文件的按钮，每行一个元素。然后，应出现一个对话框通知用户文件已保存。当对话框关闭时，它应该清空文本字段和文本区域。但是，我在使用该程序时遇到了一些问题。

关于对话框窗口，当我尝试编译时，程序显示以下错误:

emailProg.java:81: error: no suitable method found for showMessageDialog(emailProg.sendAction, String)

JOptionPane.showMessageDialog(this, "Saved");
           ^

其次，我不确定如何在关闭对话框后清空文本字段和文本区域。我知道清空文本字段可以通过使用以下代码来完成:

[textfield].setText("");

但我不确定如何在关闭对话框后执行此操作。

这是我的代码:

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

public class emailProg extends JFrame {
    private JPanel panNorth;
    private JPanel panCenter;
    private JPanel panSouth;

    private JLabel toLabel;
    private JLabel ccLabel;
    private JLabel bccLabel;
    private JLabel subLabel;
    private JLabel msgLabel;

    private JTextField toField;
    private JTextField ccField;
    private JTextField bccField;
    private JTextField subField;
    private JTextArea msgArea;

    private JButton send;

//The Constructor
public emailProg() {
    setTitle("Compose Email");
    setLayout(new BorderLayout());

    panNorth = new JPanel();
    panNorth.setLayout(new GridLayout(4, 2));
    JLabel toLabel = new JLabel("To:");
    panNorth.add(toLabel);
    JTextField toField = new JTextField(15);
    panNorth.add(toField);
    JLabel ccLabel = new JLabel("CC:");
    panNorth.add(ccLabel);
    JTextField ccField = new JTextField(15);
    panNorth.add(ccField);
    JLabel bccLabel = new JLabel("Bcc:");
    panNorth.add(bccLabel);
    JTextField bccField = new JTextField(15);
    panNorth.add(bccField);
    JLabel subLabel = new JLabel("Subject:");
    panNorth.add(subLabel);
    JTextField subField = new JTextField(15);
    panNorth.add(subField);
    add(panNorth, BorderLayout.NORTH);

    panCenter = new JPanel();
    panCenter.setLayout(new GridLayout(2, 1));
    JLabel msgLabel = new JLabel("Message:");
    panCenter.add(msgLabel);
    JTextArea msgArea = new JTextArea(5, 15);
    panCenter.add(msgArea);
    add(panCenter, BorderLayout.CENTER);

    panSouth = new JPanel();
    panSouth.setLayout(new FlowLayout());
    JButton send = new JButton("Send");
    panSouth.add(send);
    add(panSouth, BorderLayout.SOUTH);

    send.addActionListener (new sendAction());
}

private class sendAction implements ActionListener {
    public void actionPerformed (ActionEvent event) {
        try {
            PrintWriter outfile = new PrintWriter("email.txt");
            outfile.print("To: ");
            outfile.println(toField.getText());
            outfile.print("CC: ");
            outfile.println(ccField.getText());
            outfile.print("Bcc: ");
            outfile.println(bccField.getText());
            outfile.print("Subject: ");
            outfile.println(subField.getText());
            outfile.print("Message: ");
            outfile.println(msgArea.getText());

            JOptionPane.showMessageDialog(this, "Saved");
        }
        catch(FileNotFoundException e) {
        System.out.println("File not found.");
        }
    }
}

public static void main(String[] args) {
    emailProg win = new emailProg();
    win.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    win.pack();
    win.setVisible(true);
}

}

感谢您提供的任何帮助。

Answer 1

JOptionPane.showMessageDialog(...) 需要一个 Component 作为其第一个参数。在您的情况下，您从扩展 ActionListener 的类中调用它，因此 this 并不引用组件。您可以考虑为此参数传递 null。像这样的事情:

JOptionPane.showMessageDialog(null, "Saved");

另外，作为旁注，请考虑阅读 Java Naming Convention 。类名最好以大写字母开头。

编辑:如果您仔细查看代码，您会在构造函数中创建与全局变量同名的局部变量，并将它们添加到面板中。例如，您有一个全局 private JTextField toField;，但是在构造函数中您正在执行以下操作:

JTextField toField = new JTextField(15);
panNorth.add(toField);

所以你的全局变量仍然保持为空。当您尝试使用此变量在 actionPerformed() 代码中执行任何操作时，您将遇到 NullPointerException。

这是更新后的代码供您引用。请注意，我进行了某些更改，尤其是对类名称的更改，并添加了 SwingUtilities.invokeLater(....) 来执行您的代码。要了解为什么这是必要的，请阅读 "The Event Dispatch Thread"和 "Concurrency in Swing"

import java.awt.BorderLayout;
import java.awt.FlowLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.FileNotFoundException;
import java.io.PrintWriter;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JTextArea;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

public class EmailProg extends JFrame {
    private JPanel panNorth;
    private JPanel panCenter;
    private JPanel panSouth;

    private JLabel toLabel;
    private JLabel ccLabel;
    private JLabel bccLabel;
    private JLabel subLabel;
    private JLabel msgLabel;

    private JTextField toField;
    private JTextField ccField;
    private JTextField bccField;
    private JTextField subField;
    private JTextArea msgArea;

    private JButton send;

    // The Constructor
    public EmailProg() {
        setTitle("Compose Email");
        setLayout(new BorderLayout());

        panNorth = new JPanel();
        panNorth.setLayout(new GridLayout(4, 2));
        toLabel = new JLabel("To:");
        panNorth.add(toLabel);
        toField = new JTextField(15);
        panNorth.add(toField);
        ccLabel = new JLabel("CC:");
        panNorth.add(ccLabel);
        ccField = new JTextField(15);
        panNorth.add(ccField);
        bccLabel = new JLabel("Bcc:");
        panNorth.add(bccLabel);
        bccField = new JTextField(15);
        panNorth.add(bccField);
        subLabel = new JLabel("Subject:");
        panNorth.add(subLabel);
        subField = new JTextField(15);
        panNorth.add(subField);
        add(panNorth, BorderLayout.NORTH);

        panCenter = new JPanel();
        panCenter.setLayout(new GridLayout(2, 1));
        msgLabel = new JLabel("Message:");
        panCenter.add(msgLabel);
        msgArea = new JTextArea(5, 15);
        panCenter.add(msgArea);
        add(panCenter, BorderLayout.CENTER);

        panSouth = new JPanel();
        panSouth.setLayout(new FlowLayout());
        send = new JButton("Send");
        panSouth.add(send);
        add(panSouth, BorderLayout.SOUTH);

        send.addActionListener(new SendAction());
    }

    private class SendAction implements ActionListener {
        public void actionPerformed(ActionEvent event) {
            try {
                PrintWriter outfile = new PrintWriter("email.txt");
                outfile.print("To: ");
                outfile.println(toField.getText());
                outfile.print("CC: ");
                outfile.println(ccField.getText());
                outfile.print("Bcc: ");
                outfile.println(bccField.getText());
                outfile.print("Subject: ");
                outfile.println(subField.getText());
                outfile.print("Message: ");
                outfile.println(msgArea.getText());

                JOptionPane.showMessageDialog(null, "Saved");
            } catch (FileNotFoundException e) {
                System.out.println("File not found.");
            }
        }
    }

    public static void main(String[] args) {

        //Make sure that all your operations happens through the EDT
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                EmailProg win = new EmailProg();
                win.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                win.pack();
                win.setVisible(true);

            }
        });
    }
}