Question

我的程序现在遇到了很大麻烦。我不明白这个错误消息“MyEventA不是抽象的，并且不会覆盖java.awt.event.ActionListener中的抽象方法actionPerformed（java.awt.event.actionEvent）。”我试过在网上查看我的教科书，但我仍然无法使用它。

如果有人能帮助我弄清楚我的代码有什么问题，我真的很感激。我一直在研究这个问题一个半小时但仍然没有修复它。 :(提前致谢！

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

public class MyEventA extends JFrame implements ActionListener
{
    private JButton plus;
    private JButton minus;
    private JButton reset;
    private JButton quit;

    public MyEventA()
{
    add(new JLabel("Current Value", JLabel.LEFT), BorderLayout.NORTH);
    JTextField jtfCurrent = new JTextField("0");
    add(jtfCurrent, BorderLayout.EAST);
    JPanel jpSouth = new JPanel();
    plus = new JButton("+");
    minus = new JButton("-");
    reset = new JButton("Reset");
    quit = new JButton("Quit");
    ButtonListener b1 = new ButtonListener(jtfCurrent);
    jtfCurrent.addActionListener(b1);
    plus.addActionListener(this);
    minus.addActionListener(this);
    reset.addActionListener(this);
    quit.addActionListener(this);
    jpSouth.add(plus);
    jpSouth.add(minus);
    jpSouth.add(reset);
    jpSouth.add(quit);
    add(jpSouth, BorderLayout.SOUTH);
}

class ButtonListener implements ActionListener {
    private JTextField writeInto;   // text field reference
    private int count = 0;

    public ButtonListener(JTextField tf) {
        writeInto = tf;
        count = 0;
    }

    public void actionPerformed(ActionEvent ae){
        if(ae.getActionCommand().equals("+")){
            count++;
            writeInto.setText("" + count);
        }
        else if(ae.getActionCommand().equals("-")){
            count--;
            writeInto.setText("" + count);
        }
        else if(ae.getActionCommand().equals("Reset")){
            count = 0;
            writeInto.setText("" + count);
        }
        else {
            System.exit(0);    
        }
    }

    public static void main(String[] args){
        MyEventA events = new MyEventA();
        events.setTitle("Part 2 Using getSource");
        events.pack();
        events.setDefaultCloseOperation(EXIT_ON_CLOSE);
        events.setSize(300,100);
        events.setLocation(200,200);
        events.setVisible(true);
    }
}
}

Answer 1

通常，当一个（非抽象）类implements接口时，会签订一个契约，建议该类实现接口中提供的所有方法。

您的课程MyEventA implements界面ActionListener，需要覆盖方法（void actionPerformed(ActionEvent)）。由于MyEventA不是抽象的，编译器告诉您需要在此处实现该方法。

如果您想知道如何实现该方法，那么您在实现的子类（ButtonListener）中有一个很好的示例。在这里，您可以看到ButtonListener实现了相同的接口ActionListener，并实现了它的方法。

Answer 2

如果要实现ActionListener接口，则必须覆盖actionPerformed（java.awt.event.ActionEvent）方法。如果未覆盖接口中的方法，编译器会要求将类声明为abstarct。

Answer 3

因为MyEventA已经实现了ActionListener，并且因为它不是一个抽象类，所以必须以与actionPeformed (ActionEvent)中相同的方式实现/定义方法ButtonListener

但我认为你想要做的事实上没有MyEventA实施ActionListener，而是将new ButtonListener ()传递给你的按钮，如

plus.addListener (new ButtonListener ());

Answer 4

重写如下，它对我有用：

import javax.swing.*;

import java.awt.*;
import java.awt.event.*;

public class MyEventA extends JFrame implements ActionListener {
    private JButton plus;
    private JButton minus;
    private JButton reset;
    private JButton quit;

    public MyEventA() {
        add(new JLabel("Current Value", JLabel.LEFT), BorderLayout.NORTH);
        JTextField jtfCurrent = new JTextField("0");
        add(jtfCurrent, BorderLayout.EAST);
        JPanel jpSouth = new JPanel();
        plus = new JButton("+");
        minus = new JButton("-");
        reset = new JButton("Reset");
        quit = new JButton("Quit");
        ButtonListener b1 = new ButtonListener(jtfCurrent);
        jtfCurrent.addActionListener(b1);
        plus.addActionListener(this);
        minus.addActionListener(this);
        reset.addActionListener(this);
        quit.addActionListener(this);
        jpSouth.add(plus);
        jpSouth.add(minus);
        jpSouth.add(reset);
        jpSouth.add(quit);
        add(jpSouth, BorderLayout.SOUTH);
    }

    class ButtonListener implements ActionListener {
        private JTextField writeInto; // text field reference
        private int count = 0;

        public ButtonListener(JTextField tf) {
            writeInto = tf;
            count = 0;
        }

        public void actionPerformed(ActionEvent ae) {
            if (ae.getActionCommand().equals("+")) {
                count++;
                writeInto.setText("" + count);
            } else if (ae.getActionCommand().equals("-")) {
                count--;
                writeInto.setText("" + count);
            } else if (ae.getActionCommand().equals("Reset")) {
                count = 0;
                writeInto.setText("" + count);
            } else {
                System.exit(0);
            }
        }
    }

    @Override
    public void actionPerformed(ActionEvent e) {// was to be implemented

        javax.swing.JOptionPane.showMessageDialog(null, "clicked");

    }

    public static void main(String[] args) {// main method should be in the outer class
        MyEventA events = new MyEventA();
        events.setTitle("Part 2 Using getSource");
        events.pack();
        events.setDefaultCloseOperation(EXIT_ON_CLOSE);
        events.setSize(300, 100);
        events.setLocation(200, 200);
        events.setVisible(true);
    }

}

如果您想使用actionPerformed课程中的ButtonListener，请使用以下代码：

import javax.swing.*;

import java.awt.*;
import java.awt.event.*;

public class MyEventA extends JFrame implements ActionListener {
    private JButton plus;
    private JButton minus;
    private JButton reset;
    private JButton quit;   


    public MyEventA() {
        add(new JLabel("Current Value", JLabel.LEFT), BorderLayout.NORTH);
        JTextField jtfCurrent = new JTextField("0");
        add(jtfCurrent, BorderLayout.EAST);
        JPanel jpSouth = new JPanel();
        plus = new JButton("+");
        minus = new JButton("-");
        reset = new JButton("Reset");
        quit = new JButton("Quit");     


        ButtonListener b1 = new ButtonListener(jtfCurrent);
        jtfCurrent.addActionListener(b1);
        plus.addActionListener(new ButtonListener(jtfCurrent));
        minus.addActionListener(new ButtonListener(jtfCurrent));
        reset.addActionListener(new ButtonListener(jtfCurrent));
        quit.addActionListener(new ButtonListener(jtfCurrent));
        jpSouth.add(plus);
        jpSouth.add(minus);
        jpSouth.add(reset);
        jpSouth.add(quit);
        add(jpSouth, BorderLayout.SOUTH);
    }

    class ButtonListener implements ActionListener {
        private JTextField writeInto; // text field reference
        private int count = 0;
        public ButtonListener() {

        }
        public ButtonListener(JTextField tf) {
            writeInto = tf;
            count = 0;
        }

        public void actionPerformed(ActionEvent ae) {
            if (ae.getActionCommand().equals("+")) {
                count++;
                writeInto.setText("" + count);
            } else if (ae.getActionCommand().equals("-")) {
                count--;
                writeInto.setText("" + count);
            } else if (ae.getActionCommand().equals("Reset")) {
                count = 0;
                writeInto.setText("" + count);
            } else {
                System.exit(0);
            }
        }

    }

    @Override
    public void actionPerformed(ActionEvent e) {

        javax.swing.JOptionPane.showMessageDialog(null, "clicked");

    }

    public static void main(String[] args) {
        MyEventA events = new MyEventA();
        events.setTitle("Part 2 Using getSource");
        events.pack();
        events.setDefaultCloseOperation(EXIT_ON_CLOSE);
        events.setSize(300, 100);
        events.setLocation(200, 200);
        events.setVisible(true);
    }

}

事件处理问题 - Java

4 个答案: