java中的一个画图小细节

女爷i 2022-08-27 03:28 191阅读 0赞

有如下程序：（这个程序是用键盘画出直线的）

import javax.swing.*;
    import java.awt.event.*;
    import java.awt.*;
    
    public class Exercise16_9 extends JFrame {
    	private DrawLine k = new DrawLine();
    	
    	public static void main(String[] args) {
    		JFrame frame = new Exercise16_9();
    		frame.setSize(600,600);
    		frame.setTitle("Exercise16_9");
    		frame.setLocationRelativeTo(null);
    		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    		frame.setVisible(true);
    	}
    	
    	public Exercise16_9() {
    		add(k);
    		k.setFocusable(true);
    		
    	}
    	
    	static class DrawLine extends JPanel {
    		private boolean center = false;
    		Polygon p = new Polygon();
    		int x;
    		int y;
    		
    		public DrawLine() {
    //			p.addPoint(200,200);
    			addKeyListener(new KeyAdapter() {
    				public void keyPressed(KeyEvent e) {
    					if(e.getKeyCode()==KeyEvent.VK_UP) {
    						y -= 5;
    						p.addPoint(x,y);
    						repaint();
    					}
    					else if(e.getKeyCode()==KeyEvent.VK_DOWN) {
    						y += 5;
    						p.addPoint(x,y);
    						repaint();
    					}
    					else if(e.getKeyCode()==KeyEvent.VK_LEFT) {
    						x -= 5;
    						p.addPoint(x,y);
    						repaint();
    					}
    					else if(e.getKeyCode()==KeyEvent.VK_RIGHT) {
    						x += 5;
    						p.addPoint(x,y);
    						repaint();
    					}
    				}
    			});
    			
    		}
    		
    		protected void paintComponent(Graphics g) {
    			super.paintComponent(g);
    			
    			if(!center) {
    				p.addPoint(getWidth()/2,getHeight()/2);
    				center = true;
    				x = p.xpoints[0];
    				y = p.xpoints[0];
    				
    			}
    			g.drawPolyline(p.xpoints,p.ypoints,p.xpoints.length);
    		}
    	}
    }

在运行的时候，发现多了一条奇怪的直线，结果我把

g.drawPolyline(p.xpoints,p.ypoints,p.xpoints.length);

改为

g.drawPolyline(p.xpoints,p.ypoints,p.npoints);

就可以了。为了解释这个原因，

我反编译了polygon类：

观察到他的addPoint方法：

public void addPoint(int x, int y) {
            if (npoints >= xpoints.length || npoints >= ypoints.length) {
                int newLength = npoints * 2;
                // Make sure that newLength will be greater than MIN_LENGTH and
                // aligned to the power of 2
                if (newLength < MIN_LENGTH) {
                    newLength = MIN_LENGTH;
                } else if ((newLength & (newLength - 1)) != 0) {
                    newLength = Integer.highestOneBit(newLength);
                }
    
                xpoints = Arrays.copyOf(xpoints, newLength);
                ypoints = Arrays.copyOf(ypoints, newLength);
            }
            xpoints[npoints] = x;
            ypoints[npoints] = y;
            npoints++;
            if (bounds != null) {
                updateBounds(x, y);
            }
        }

发现了

p.xpoints.length

和

p.npoints

是不一样的