JComponent size issue

Question

我有一個JComponent子類，用於在屏幕上繪製形狀。在構造函數中，我試圖將ballX和ballY設置爲JComponent的X和Y大小值的一半，並且我認爲我做錯了。我現在查了很多，並找不到補救辦法。代碼如下。請記住，這是我第一次真正的Swing/Graphics2D風險投資。

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

覆蓋getPreferredSize()在JComponent來回報您的首選大小，並與Dimension的寬度和高度的一半開始。爲此，KineticModel在DisplayPanel中調用setPreferredSize()。

附錄：通過解釋，當前的做法，因爲從getWidth()和getHeight()結果無效直到validate()被稱爲封閉的容器上，通常爲pack()結果失敗。

Answer 2

我同意trashgod的回答。 （+1）

移動ballX和BALLY在paintComponent(g)這樣

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