java - 为什么jPanel不清除？

Question

该程序在jPanel上绘制输入尺寸的椭圆形。当我改变绘制的圆圈的大小时，旧的圆圈不会消失。为什么？

代码:

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {  

    jPanel.Repaint();
    try{
        jLabel6.setText("");
        int a=Integer.parseInt(jTextField1.getText()); 

        Graphics2D gfx=(Graphics2D)jPanel1.getGraphics();
        gfx.setColor(Color.red);
        gfx.fillOval(100,50,a,a);
        gfx.fillOval(400,50,a,a);

    }catch(NumberFormatException e){
        jLabel6.setText("Incorrect data");
    }
}

Answer 1

使用覆盖的JPanel#paintComponent()方法而不是使用 JPanel#getGraphics() 方法。

不要忘记在重写的 paintComponent() 方法中调用 super.paintComponent(g);。

JPanel jPanel = new JPanel() {
    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.red);
        g.fillOval(100, 50, c, c);
        g.fillOval(400, 50, c, c);
    }
};

<小时/>

--编辑--

代码中的更改:

// This is the code where you have create JPanel object
JPanel jPanel = new JPanel() {
    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);

        try{
            int c=Integer.parseInt(jTextField1.getText());//read the value

            g.setColor(Color.red);
            g.fillOval(100, 50, c, c);
            g.fillOval(400, 50, c, c);

        }catch(NumberFormatException e){//handle the exception} 
    }
};


private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {  

    try{
        jLabel6.setText("");
        jPanel.repaint(); // simply call repaint method
    }catch(NumberFormatException e){
        jLabel6.setText("Incorrect data");
    }
}