精炼设计

原文： https://docs.oracle.com/javase/tutorial/uiswing/painting/refining.html

出于演示目的，将绘制逻辑完全包含在MyPanel类中是有意义的。但是，如果您的应用程序需要跟踪多个实例，那么您可以使用的一种模式是将该代码分解为一个单独的类，以便可以将每个方块视为单个对象。这种技术在 2D 游戏编程中很常见，有时也被称为“精灵动画”。

单击“启动”按钮以使用 Java™Web Start （下载 JDK 7 或更高版本）运行 SwingPaintDemo4。或者，要自己编译并运行示例，请参考示例索引。

package painting;
import javax.swing.SwingUtilities;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.BorderFactory;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics; 
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseMotionListener;
import java.awt.event.MouseMotionAdapter;
public class SwingPaintDemo4 {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI(); 
            }
        });
    }
    private static void createAndShowGUI() {
        System.out.println("Created GUI on EDT? "+
        SwingUtilities.isEventDispatchThread());
        JFrame f = new JFrame("Swing Paint Demo");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
        f.add(new MyPanel());
        f.setSize(250,250);
        f.setVisible(true);
    } 
}
class MyPanel extends JPanel {
    RedSquare redSquare = new RedSquare();
    public MyPanel() {
        setBorder(BorderFactory.createLineBorder(Color.black));
        addMouseListener(new MouseAdapter(){
            public void mousePressed(MouseEvent e){
                moveSquare(e.getX(),e.getY());
            }
        });
        addMouseMotionListener(new MouseAdapter(){
            public void mouseDragged(MouseEvent e){
                moveSquare(e.getX(),e.getY());
            }
        });
    }
    private void moveSquare(int x, int y){
        // Current square state, stored as final variables 
        // to avoid repeat invocations of the same methods.
        final int CURR_X = redSquare.getX();
        final int CURR_Y = redSquare.getY();
        final int CURR_W = redSquare.getWidth();
        final int CURR_H = redSquare.getHeight();
        final int OFFSET = 1;
        if ((CURR_X!=x) || (CURR_Y!=y)) {
            // The square is moving, repaint background 
            // over the old square location. 
            repaint(CURR_X,CURR_Y,CURR_W+OFFSET,CURR_H+OFFSET);
            // Update coordinates.
            redSquare.setX(x);
            redSquare.setY(y);
            // Repaint the square at the new location.
            repaint(redSquare.getX(), redSquare.getY(), 
                    redSquare.getWidth()+OFFSET, 
                    redSquare.getHeight()+OFFSET);
        }
    }
    public Dimension getPreferredSize() {
        return new Dimension(250,200);
    }
    public void paintComponent(Graphics g) {
        super.paintComponent(g);       
        g.drawString("This is my custom Panel!",10,20);
        redSquare.paintSquare(g);
    }  
}
class RedSquare{
    private int xPos = 50;
    private int yPos = 50;
    private int width = 20;
    private int height = 20;
    public void setX(int xPos){ 
        this.xPos = xPos;
    }
    public int getX(){
        return xPos;
    }
    public void setY(int yPos){
        this.yPos = yPos;
    }
    public int getY(){
        return yPos;
    }
    public int getWidth(){
        return width;
    } 
    public int getHeight(){
        return height;
    }
    public void paintSquare(Graphics g){
        g.setColor(Color.RED);
        g.fillRect(xPos,yPos,width,height);
        g.setColor(Color.BLACK);
        g.drawRect(xPos,yPos,width,height);  
    }
}

在这个特定的实现中，我们完全从头开始创建了一个RedSquare类。另一种方法是通过使RedSquare成为它的子类来重用java.awt.Rectangle的功能。无论如何实现RedSquare，重要的是我们为类提供了一个接受Graphics对象的方法，并且该方法是从面板的paintComponent方法调用的。这种分离使代码保持清洁，因为它实际上告诉每个红色方块自己绘制。