java - 方法的final局部变量 'avoid repeat invocations'怎么办？

Question

Creating a GUI with JFC/Swing > Perform Custom Painting : Refining the Design

我正在阅读上面链接的教程，部分示例代码让我感到困惑。根据moveSquare方法中的代码注释，将位置信息存储为最终局部变量将

'avoid repeat invocations of the same methods'

这对我来说完全没有意义，我希望有人能解释一下评论的含义。 (请参阅上面的链接以获取完整的源代码和教程评论)

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);
}  
}

Answer 1

这意味着调用 getX() 或其他方法的结果将保存在一个变量中并重复使用，这样您就不必在每次需要 X 或 Y 时都继续调用这些方法。

这有三个好处:

1. 由于变量名，代码更具可读性
2. 由于不必一次又一次地调用这些方法，性能得到了提高
3. 将来的更改可能会更容易，因为您只需在一个地方更改方法名称。