关于paintComponent的小细节（Graphics 类）

浅浅的花香味﹌ 2022-08-27 03:28 141阅读 0赞

题目:编写一个程序，使用箭头键绘制线段。

对于这个题目，我开始写的代码是这样的

import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;
    
    public class Exercise16_9 extends JFrame {
        
    	private KeyLinePanel p = new KeyLinePanel();
    	public Exercise16_9() {
    		add(p);
    		
    		p.setFocusable(true);
    	}
    	public static void main(String[] args) {
    		Exercise16_9 frame = new Exercise16_9();
    		frame.setTitle("Exercise16_9");
    		frame.setSize(300, 200);
    		frame.setLocationRelativeTo(null);
    		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    		frame.setVisible(true);
    	}
    	static class KeyLinePanel extends JPanel {
    		private int x1 = 50;
    		private int y1 = 50;
    		private int x2 = 50;
    		private int y2 = 50;
    		public KeyLinePanel() {
    			addKeyListener(new KeyAdapter() {
    				public void keyPressed(KeyEvent e) {
    					switch(e.getKeyCode()) {
    						case KeyEvent.VK_UP: y2 = y1 - 2; break;
    						case KeyEvent.VK_DOWN: y2 = y1 + 2; break;
    						case KeyEvent.VK_LEFT: x2 = x1 - 2; break;
    						case KeyEvent.VK_RIGHT: x2 = x1 + 2; break;
    					}
    					repaint();
    				}
    			});
    		}
    		protected void paintComponent(Graphics g) {
    		    
    			super.paintComponent(g);
    			g.drawLine(x1, y1, x2, y2);
    			x1 = x2;
    			y1 = y2;
    		}
    	}
    }

结果运行时发现，运行结果更预期结果不一样，然后我去掉了

super.paintComponent(g);

这条语句，就O了。

我注意到了

super.paintComponent(g);

## 这个方法，用这个方法调用父类中定义的paintComponent方法，该方法的作用是确保在显示新的图画之前清空视图区域 ##