Java的事件驱动入门（二）——点击按钮触发事件

港控/mmm° 2024-04-19 16:21 63阅读 0赞

# 前言 #

生成一个圆，点击“放大”按钮，这个圆可以被放大；点击“缩小”按钮，这个圆可以被缩小。

# 效果 #

![在这里插入图片描述][20190919210705732.gif]

# 代码 #

ControlCircle2.java:

package Test;
    
    import javax.swing.*;
    import java.awt.*;
    import java.awt.event.ActionEvent;
    import java.awt.event.ActionListener;
    
    public class ControlCircle2 extends JFrame {
        
        private JButton jbtEnlarge=new JButton("放大");
        private JButton jbtShrink=new JButton("缩小");
        private CirclePanel canvas=new CirclePanel();
    
        public ControlCircle2(){
        
            JPanel panel=new JPanel();
            panel.add(jbtEnlarge);
            panel.add(jbtShrink);
    
            add(canvas, BorderLayout.CENTER);
            add(panel,BorderLayout.SOUTH);
    
            jbtEnlarge.addActionListener(new EnlargeListener());
            jbtShrink.addActionListener(new ShrinkListener() );
        }
    
        public static void main(String[] args) {
        
            ControlCircle2 frame=new ControlCircle2();
            frame.setTitle("ControlCircle2");
            frame.setSize(200,200);
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        }
    
        class EnlargeListener implements ActionListener{
        
            @Override
            public void actionPerformed(ActionEvent e) {
        
                canvas.enlarge();
            }
        }
        class ShrinkListener implements ActionListener{
        
            @Override
            public void actionPerformed(ActionEvent e) {
        
                canvas.shrink();
            }
        }
        class CirclePanel extends JPanel{
        
            private int radius=5;
            public void enlarge(){
        
                radius++;
                repaint();
            }
            public void shrink(){
        
                radius--;
                repaint();
            }
            @Override
            protected void paintComponent(Graphics g) {
        
                super.paintComponent(g);
                g.drawOval(getWidth()/2-radius,getHeight()/2-radius,2*radius,2*radius);
            }
        }
    
    }

# 解析 #

当我们点击了“放大”按钮的时候，放大的事件监听器被触发，同理，当点击“缩小”的按钮时，缩小的监听器也会被触发。

所以监听器的设置是比较重要的，代码如下：

jbtEnlarge.addActionListener(new EnlargeListener());
        jbtShrink.addActionListener(new ShrinkListener() );

两个按钮都被设置了事件监听器，里面的EnlargeListener和ShrinkListener就是监听器，需要手动定义，代码如下：

class EnlargeListener implements ActionListener{
        @Override
     	   public void actionPerformed(ActionEvent e) {
     	       canvas.enlarge();
        }
     }
    
    	class ShrinkListener implements ActionListener{
        @Override
        public void actionPerformed(ActionEvent e) {
            canvas.shrink();
        }
    }

当“放大”事件被触发的时候，enlarge方法被调用，圆的半径会自增一次，并且调用repaint()方法重新绘制一个圆，“缩小”事件同理，代码如下：

class CirclePanel extends JPanel{
        private int radius=5;
        public void enlarge(){
            radius++;
            repaint();
        }
        public void shrink(){
            radius--;
            repaint();
        }
        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.drawOval(getWidth()/2-radius,getHeight()/2-radius,2*radius,2*radius);
        }
    }

[20190919210705732.gif]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/18/f3f6f4cc6f8e48eca77eae3a0a6b9c1f.gif