java - 使用 Graphics2D 绘制 JPanel 程序在调整窗口大小时消失

Question

我正在尝试创建一个 JPanel 来制作一个简单的绘画程序，每当用户拖动鼠标时就会进行绘画。然而，在调整窗口大小后，我已经绘制的图形消失了。这是 PaintPanel 类:

public class PaintPanel extends JPanel {    

private static final long serialVersionUID = 4267027584083413157L;

private class MouseMotionHandler implements MouseMotionListener {

    public void mouseDragged(MouseEvent e) {
        int x = e.getX();
        int y = e.getY();

        Graphics2D g2 = (Graphics2D)getGraphics();

        g2.setPaint(color);         
        g2.fillOval(x, y, size, size);
    }


    public void mouseMoved(MouseEvent e) {

    }

}

class JSliderHandler implements ChangeListener {
    @Override
    public void stateChanged(ChangeEvent e) {
        JSlider colorChange = (JSlider)e.getSource();
        if (!colorChange.getValueIsAdjusting()) {
            colVal = (int)colorChange.getValue();
            color = new Color(colVal, colVal, colVal);
            System.out.print(colVal);
        }
    }
}

class JSpinnerHandler implements ChangeListener {
    @Override
    public void stateChanged(ChangeEvent e) {
        JSpinner thickChange = (JSpinner)e.getSource();
        size = (int)thickChange.getValue();
    }
}


private Color color;
private int colVal = 0;
private int   size;


public PaintPanel() {     

    color = new Color(colVal, colVal, colVal);
    size  = 8;

    addMouseMotionListener(new MouseMotionHandler());        
}
}

Answer 1

a simple paint program that paints whenever the user drags their mouse.

参见Custom Painting Approaches 。它演示了两种常见的自定义绘画方法:

1. 将要绘制的对象存储在 ArrayList 中，然后在 PaintComponent(...) 方法中迭代 ArrayList 以绘制每个对象

2. 直接绘制 BufferedImage，然后绘制 BufferedImage。