在梁的第15章中Java 编程简介（第七版），他介绍了一个程序，可以在 JPanel 上制作一个（2-D）球，并通过单击放大/缩小按钮来放大它。我修改了程序，以便它还可以 1）在用户单击/选项+单击时放大/缩小球，2）允许您通过按下按钮来选择球的颜色，3）允许您移动球用鼠标拖动圆圈。
最后的修改给我带来了一段时间的麻烦，因为我想在一开始将球居中，但随后允许用户用鼠标移动球。我提出的解决方案是让 PaintComponent 方法在第一次绘制时仅设置球相对于 getWidth() 和 getHeight() 的 x 和 y 坐标。为此，我向 BallCanvas 类添加了一个 PaintCount 变量，并创建了一个 if 语句，以便它仅在第一次执行。当我最初试图弄清楚如何做到这一点时，我看到了其他解决方案，例如这里给出的解决方案：为什么我无法访问面板的 getWidth() 和 getHeight() 函数？，但我发现我的解决方案要简单得多。
所以问题是：我所做的是否被认为是糟糕的编码风格？专业程序员会嘲笑这个解决方案吗？或者可以吗？
更重要的是，是否有更好（但也相对简单）的方法来做到这一点，而不涉及设置计数器？
以下是相关代码：
BallCanvas 的开头：
public static class BallCanvas extends JPanel {
private int radius = 20;
private Color color = Color.BLACK;
private int ballX;
private int ballY;
private int paintCount = 0;
...
move 方法（响应 MouseDragged 事件）：
public void move(MouseEvent e){
ballX = e.getX() - radius;
ballY = e.getY() - radius;
repaint();
}
PaintComponent 方法：
protected void paintComponent(Graphics g){
super.paintComponent(g);
g.setColor(color);
if(paintCount < 1){
ballX = getWidth()/2 - radius;
ballY = getHeight()/2 - radius;
}
g.fillOval(ballX, ballY, 2*radius, 2*radius);
paintCount++;
}
完整程序：
// Reference: Liang's Intro to Java Programming
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class ControlBall extends JFrame{
private JButton jbtRed = new JButton("Red");
private JButton jbtGreen = new JButton("Green");
private JButton jbtBlue = new JButton("Blue");
private JButton jbtBlack = new JButton("Black");
private BallCanvas canvas = new BallCanvas();
private JMenuBar menuBar = new JMenuBar();
private JMenu menu = new JMenu("Edit");
private JMenuItem miEnlarge = new JMenuItem("Enlarge");
private JMenuItem miShrink = new JMenuItem("Shrink");
public ControlBall(){
menuBar.add(menu);
menu.add(miEnlarge);
menu.add(miShrink);
JPanel panel = new JPanel();
panel.add(jbtRed);
panel.add(jbtGreen);
panel.add(jbtBlue);
panel.add(jbtBlack);
this.add(canvas, BorderLayout.CENTER);
this.add(panel, BorderLayout.SOUTH);
this.add(menuBar, BorderLayout.NORTH);
jbtRed.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
canvas.setColor(Color.RED);
}
});
jbtGreen.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
canvas.setColor(Color.GREEN);
}
});
jbtBlue.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
canvas.setColor(Color.BLUE);
}
});
jbtBlack.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
canvas.setColor(Color.BLACK);
}
});
miEnlarge.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
canvas.enlarge();
}
});
miShrink.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
canvas.shrink();
}
});
canvas.addMouseListener(new MouseListener() {
public void mouseClicked(MouseEvent e){
canvas.changeSize(e);
}
public void mousePressed(MouseEvent e){}
public void mouseReleased(MouseEvent e){}
public void mouseEntered(MouseEvent e){}
public void mouseExited(MouseEvent e){}
});
canvas.addMouseMotionListener(new MouseMotionAdapter() {
public void mouseDragged(MouseEvent e) {
canvas.move(e);
}
});
}
public static void main(String[] args){
JFrame frame = new ControlBall();
frame.setTitle("ControlBall");
frame.setLocationRelativeTo(null);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(400, 200);
frame.setVisible(true);
}
public static class BallCanvas extends JPanel {
private int radius = 20;
private Color color = Color.BLACK;
private int ballX;
private int ballY;
private int paintCount = 0;
public BallCanvas(){
System.out.println(getWidth() + " " + getHeight());
}
public BallCanvas(int initialRadius){
radius = initialRadius;
}
public void setColor(Color color){
this.color = color;
repaint();
}
public void changeSize(MouseEvent e){
int numClicks = e.getClickCount();
if(e.isAltDown()){
if(radius >= 6){
this.radius -= 5*numClicks;
} else{
// do nothing
}
} else{
this.radius += 5*numClicks;
}
repaint();
}
public void enlarge(){
this.radius += 5;
repaint();
}
public void shrink(){
if(radius >= 10){
this.radius -= 5;
}
repaint();
}
public void move(MouseEvent e){
ballX = e.getX() - radius;
ballY = e.getY() - radius;
repaint();
}
protected void paintComponent(Graphics g){
super.paintComponent(g);
g.setColor(color);
if(paintCount < 1){
ballX = getWidth()/2 - radius;
ballY = getHeight()/2 - radius;
}
g.fillOval(ballX, ballY, 2*radius, 2*radius);
paintCount++;
}
}
}
有几点值得注意：
覆盖getPreferredSize()建立面板的初始几何形状。
使用该几何形状来确定球的初始位置。
Invoke pack() and then设置位置和可见性。
Use Action封装菜单和控件共享的代码。
始终如一地使用适配器。
Use 初始线程正确。
看到这个，它从多个角度审视了一个相关的例子。
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class ControlBall extends JFrame {
private JButton jbtRed = new JButton("Red");
private JButton jbtGreen = new JButton("Green");
private JButton jbtBlue = new JButton("Blue");
private JButton jbtBlack = new JButton("Black");
private BallCanvas canvas = new BallCanvas();
private JMenuBar menuBar = new JMenuBar();
private JMenu menu = new JMenu("Edit");
private JMenuItem miEnlarge = new JMenuItem("Enlarge");
private JMenuItem miShrink = new JMenuItem("Shrink");
public ControlBall() {
menuBar.add(menu);
menu.add(miEnlarge);
menu.add(miShrink);
JPanel panel = new JPanel();
panel.add(jbtRed);
panel.add(jbtGreen);
panel.add(jbtBlue);
panel.add(jbtBlack);
this.add(canvas, BorderLayout.CENTER);
this.add(panel, BorderLayout.SOUTH);
this.add(menuBar, BorderLayout.NORTH);
jbtRed.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
canvas.setColor(Color.RED);
}
});
jbtGreen.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
canvas.setColor(Color.GREEN);
}
});
jbtBlue.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
canvas.setColor(Color.BLUE);
}
});
jbtBlack.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
canvas.setColor(Color.BLACK);
}
});
miEnlarge.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
canvas.enlarge();
}
});
miShrink.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
canvas.shrink();
}
});
canvas.addMouseListener(new MouseAdapter() {
@Override
public void mouseClicked(MouseEvent e) {
canvas.changeSize(e);
}
@Override
public void mouseDragged(MouseEvent e) {
canvas.move(e);
}
});
}
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
@Override
public void run() {
JFrame frame = new ControlBall();
frame.setTitle("ControlBall");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
public static class BallCanvas extends JPanel {
private static final int SIZE = 400;
private int radius = 20;
private Color color = Color.BLACK;
private int ballX = SIZE / 2 - radius;
private int ballY = SIZE / 2 - radius;
public BallCanvas() {
System.out.println(getWidth() + " " + getHeight());
}
public BallCanvas(int initialRadius) {
radius = initialRadius;
}
public void setColor(Color color) {
this.color = color;
repaint();
}
public void changeSize(MouseEvent e) {
int numClicks = e.getClickCount();
if (e.isAltDown()) {
if (radius >= 6) {
this.radius -= 5 * numClicks;
} else {
// do nothing
}
} else {
this.radius += 5 * numClicks;
}
repaint();
}
public void enlarge() {
this.radius += 5;
repaint();
}
public void shrink() {
if (radius >= 10) {
this.radius -= 5;
}
repaint();
}
public void move(MouseEvent e) {
ballX = e.getX() - radius;
ballY = e.getY() - radius;
repaint();
}
@Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
g.setColor(color);
g.fillOval(ballX, ballY, 2 * radius, 2 * radius);
}
@Override
public Dimension getPreferredSize() {
return new Dimension(SIZE, SIZE);
}
}
}