JComponent size issue

Question

我有一个JComponent子类，用于在屏幕上绘制形状。在构造函数中，我试图将ballX和ballY设置为JComponent的X和Y大小值的一半，并且我认为我做错了。我现在查了很多，并找不到补救办法。代码如下。请记住，这是我第一次真正的Swing/Graphics2D风险投资。

public class PongCanvas extends JComponent { 
//Vars to hold XY values and Dimension values. 

    private int batXDim, batYDim; 
    private int b1X, b1Y; 
    private int b2X, b2Y; 
    private int ballRad, ballX, ballY; 

    public PongCanvas() {//Instantiate vars. 
     batXDim = 20; 
     batYDim = 100; 

     b1X = 0; 
     b1Y = 0; 

     b2X = 0; 
     b2Y = 0; 

     ballRad = 20; 
     ballX = getWidth()/2; 
     ballY = getHeight()/2; 
    } 

    public void paint(Graphics g) {//Main paint Method. 
     //Cast Graphics to Graphics2D. 
     Graphics2D g2 = (Graphics2D) g; 
     //Enable antialiasing. 
     g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, 
       RenderingHints.VALUE_ANTIALIAS_ON); 
     //Draw background. 
     g2.setPaint(Color.black); 
     g2.fillRect(0, 0, getWidth(), getHeight()); 
     //Draw ball. 
     g2.setPaint(Color.white); 
     g2.fillOval(ballX, ballY, ballRad, ballRad); 
     //Draw bat 1. 
     g2.fillRect(b1X, b1Y, batXDim, batYDim); 
     //Draw bat 2. 
     g2.fillRect(b2X, b2Y, batXDim, batYDim); 
    } 
} 

Answer 1

覆盖getPreferredSize()在JComponent来回报您的首选大小，并与Dimension的宽度和高度的一半开始。为此，KineticModel在DisplayPanel中调用setPreferredSize()。

附录：通过解释，当前的做法，因为从getWidth()和getHeight()结果无效直到validate()被称为封闭的容器上，通常为pack()结果失败。

Answer 2

我同意trashgod的回答。 （+1）

移动ballX和BALLY在paintComponent(g)这样

if (ballX==0 && ballY==0) { 
    ballX = getWidth()/2; 
    ballY = getHeight()/2; 
}