Question

我试图让我的第二个按钮减去起始编号，但我一直收到ButtonListener1所在的错误（第23和47行），我无法运行我的代码。
我不明白为什么它不起作用如果我应该在私人课程或主要课程中添加按钮和操作，请打电话给我。

package addsubtract;

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

public class SubtractAdd extends JApplet {

    private int APPLET_WIDTH = 300, APPLET_HEIGHT = 35;
    private int num;
    private JLabel label;
    private JButton add;
    private JButton subtract;

    public void init ()
    {
        num = 50;

        add = new JButton ("Add");
        add.addActionListener (new ButtonListener());
        subtract = new JButton ("Subtract");
        subtract.addActionListener ((ActionListener) new ButtonListener1());

        label = new JLabel("Number: " + Integer.toString (num));

        Container cp = getContentPane();
        cp.setBackground (Color.PINK);
        cp.setLayout (new FlowLayout());
        cp.add(add);
        cp.add(subtract);
        cp.add(label);

        setSize (APPLET_WIDTH, APPLET_HEIGHT);

    }

    private class ButtonListener implements ActionListener
    {
        public void actionPerformed (ActionEvent event)
        {
            num++;
            label.setText("Number: " + Integer.toString(num));

        }

    private class ButtonListener1 implements ActionListener
    {   
        public void actionPerfomred (ActionEvent event)    
        {
            num--;
            label.setText("Number: " + Integer.toString(num));

        }
    }  
    }
}

Answer 1

我认为你不需要私人课程。另外，我认为它们会导致您查明范围问题（无法从其中访问num）。

相反，您可以创建匿名类

add = new JButton ("Add");
add.addActionListener (new ActionListener() {
    @Override
    public void actionPerformed (ActionEvent event) {
        label.setText("Number: " + (++num));
    }
});
subtract = new JButton ("Subtract");
subtract.addActionListener (new ActionListener() {
    @Override
    public void actionPerformed (ActionEvent event) {
        label.setText("Number: " + (--num));
    }
});

或者让类实现接口

public class SubtractAdd extends JApplet implements ActionListener {

    public void init() {

        add = new JButton ("Add");
        add.addActionListener (this);
        subtract = new JButton ("Subtract");
        subtract.addActionListener(this);

    }

    @Override
    public void actionPerformed (ActionEvent event) {
        Object source = event.getSource();
        if (source == add) {
            label.setText("Number: " + (++num));
        } else if (source == subtract) {
            label.setText("Number: " + (--num));
        }
    });

为什么GUI的第二个按钮不起作用？

1 个答案: