Question

我正在使用Head First Java中详细介绍的double actionlistener练习，并了解内部类，但由于某些原因，我的代码没有编译。当我尝试为JButton调用addActionListener方法时，我得到参数无效错误。

TwoButtons.java：

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

public class TwoButtons {
    JFrame frame;
    JLabel label;

    public static void main(String[] args){
        TwoButtons gui = new TwoButtons();
        gui.go();
    }

    public void go(){
        frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JButton labelButton = new JButton("Change Label");
        labelButton.addActionListener(new LabelListener());

        JButton colorButton = new JButton("Change circle");
        colorButton.addActionListener(new ColorListener());

        label = new JLabel("I'm a label");
        MyDrawPanel drawPanel = new MyDrawPanel();

        frame.getContentPane().add(BorderLayout.SOUTH, colorButton);
        frame.getContentPane().add(BorderLayout.CENTER, drawPanel);
        frame.getContentPane().add(BorderLayout.EAST, labelButton);
        frame.getContentPane().add(BorderLayout.WEST, label);

        frame.setSize(300,300);
        frame.setVisible(true);

    }

    class LabelListener implements ActionListener{
        public void actionPerformed(ActionEvent event){
            label.setText("OUch!");
        }
    }//close inner class

    class ColorListener implements ActionListener{
        public void actionPerformed(ActionEvent event){
            frame.repaint();
        }
    } //close inner class

}

MyDrawPanel.java :(没有错误）

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


public class MyDrawPanel extends JPanel{
    public void paintComponent(Graphics g){
        Graphics2D g2d = (Graphics2D) g;

        int red = (int)(Math.random()*255);
        int green = (int)(Math.random()*255);
        int blue = (int)(Math.random() * 255);
        Color startColor = new Color(red,green,blue);

        red = (int)(Math.random()*255);
        green = (int)(Math.random()*255);
        blue = (int)(Math.random() * 255);
        Color endColor = new Color(red, green, blue);

        GradientPaint gradient = new GradientPaint(70,70, startColor, 150,150, endColor);
        g2d.setPaint(gradient);
        g2d.fillOval(70, 70, 100, 100);

    }

}

Answer 1

您需要导入ActionEvent和ActionListener：

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

它们位于java.awt.event.*而不是java.awt.*。

如果您使用像Eclipse这样的IDE，您可以通过简单的操作轻松导入所有需要的类。

为什么我不能在两个内部类上实现两个ActionListener？

1 个答案: