Can't paint on directly on JFrame

问题

I have an assignment to create a paint program in Java. I have managed to create something but not exactly what I wanted.

My problem is that I cannot create a JFrame in my IDE(NetBeans 7.0.1) from the options that the IDE gives me, and call the paint classes correctly.

To be more specific I want to press a button from one panel(ex. Panel1) and paint in Panel2,in the same frame.

That's the calling of the class:

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

    PaintFlower102 f = new PaintFlower102();
}

Part of Class:

    super("Drag to Paint");
    getContentPane().add(new Label ("Click and Drag"),BorderLayout.SOUTH);
    // add(new JButton("Brush 20"),BorderLayout.NORTH);
    addMouseMotionListener( new MouseMotionAdapter() {

        @Override
        public void mouseDragged(MouseEvent event) {
            xval=event.getX();
            yval=event.getY();
            repaint();
        }
    });

    setSize(500, 500);
    setVisible(true);
    setDefaultCloseOperation(PaintFlower102.DISPOSE_ON_CLOSE);
}

public void paint(Graphics g) {      
    g.fillOval(xval, yval, 10, 10);   
}

The problem is that if I do not put the extend JFrame in the class this doesn't work. And if I do, it creates a new frame in which I can draw.

回答1:

Suggestions:

Don't ever paint directly in a JFrame except under rare circumstances of absolute need (this isn't one of them).
Instead paint in a JPanel or JComponent or other derivative of JComponent.
Paint in the class's paintComponent(Graphics g) method, not in paint(Graphics g).
Read the Java tutorials on this as it's all explained well there. Check out Trail: 2D Graphics and Performing Custom Painting.

回答2:

I might be wrong, but I think that you need to include super.paintComponent(g), and override the paintComponent method like Hovercraft Full Of Eels said.

public void paintComponent(Graphics g) {
    super.paintComponent(g);       

    // Draw Oval
   g.fillOval(xval, yval, 10, 10);
}

来源：https://stackoverflow.com/questions/10301380/cant-paint-on-directly-on-jframe

标签

java

swing

paint