Draw free hand in Java, add drawing to JPanel

Question

In my class Test I create Three panels. In my class Draw I draw by free hand.

I want to add my object d to centerPanel. When I do that nothing draws. But if i add it to the frame (using getContentPane().add) it draws. Does anyone knows where the problem is?

   topPanel = new JPanel(); 
   centerPanel = new JPanel();    
   bottomPanel = new JPanel();  

   Draw d = new Draw();
   getContentPane().add(d, BorderLayout.CENTER);  //This works
   add(topPanel, BorderLayout.PAGE_START);                 
   add(bottomPanel, BorderLayout.PAGE_END);  


   /* I WANT THIS TO WORK INSTEAD                */
   /* centerPanel.add(d);                        */  //How can I write this line of code?
   /* add(topPanel, BorderLayout.PAGE_START);    */             
   /* add(centerPanel, BorderLayout.CENTER);     */     
   /* add(bottomPanel, BorderLayout.PAGE_END);   */

Class Draw:

public class FreeHand extends JComponent, MouseListener, MouseMotionListener {      
    int x;
    int y;
    int posX;
    int posY;

    public FreeHand()
    {
        addMouseListener(this);
        addMouseMotionListener(this);
    }

    @Override
    public void mousePressed(MouseEvent me) {            
        posX = me.getX();
        posY = me.getY();
    }

    @Override
    public void mouseDragged(MouseEvent me) {
        Graphics g = getGraphics();
        g.setColor(Color.RED);
        g.drawLine(posX, posY, me.getX(), me.getY());
        posX = me.getX();
        posY = me.getY();
    }

    @Override
    public void mouseMoved(MouseEvent me) {}

    @Override
    public void mouseClicked(MouseEvent me) {}

    @Override
    public void mouseEntered(MouseEvent me) {}

    @Override
    public void mouseExited(MouseEvent me) {}

    @Override
    public void mouseReleased(MouseEvent me) {}
}

I notice you are using `getGraphics` to paint outside `paintComponent` and this is not the right way to do it. Read [my answer here](http://stackoverflow.com/questions/21322353/java-jpanel-getgraphics/21322873#21322873) for an example of how to do this correctly. (Draw on an image and then paint the image on a panel.) — Radiodef, Oct 20 '14 at 23:02

score 2 · Answer 1 · answered Oct 20 '14 at 20:37

getContentPane().add(d, BorderLayout.CENTER); //This works

That works because the content pane uses a BorderLayout and the layout manager will give all the available space to the Draw component when it is added to the CENTER.

centerPanel.add(d);                     
add(centerPanel, BorderLayout.CENTER);

This doesn't work because the BorderLayout will give all the space to the "centerPanel". But the "centerPanel uses a FlowLayout and by default the FlowLayout will respect the preferred size of any component added to it. Your Draw class doesn't have a preferred size so the size is zero.

You can change the layout manager of the centerPanel to also use a BorderLayout or you can override the getPreferredSize() method of your Draw class to return the appropriate preferred size of your panel.

The question is why do you want to create an extra "centerPanel" when you don't need it?

Thank you for your explanation. The reason I want to draw in centerPanel is that the background color of the centerPanel is set to white. I don't know how to change the background color of the Borderlayout.center? — user2939293, Oct 20 '14 at 20:42
@user2939293, `I don't know how to change the background color of the Borderlayout.center?` you are creating a custom component you can paint the background whatever you want. Or, it would be easier to extend JPanel then you just set the background. Also, you usually "accept" answers when you get help. — camickr, Oct 21 '14 at 00:58

Draw free hand in Java, add drawing to JPanel

1 Answers1