Scrolls not appears

Question

I want add JScrolpane to JPanel, but that not appears. In JLabel works fine and its very easy. I am using JPanel beacuse I'll add some image proccessing stuff to my program. There is my code:

public void draw(){  
 panel=new JPanel(){
        protected void paintComponent(Graphics g){
            Graphics g2 = g.create();
            g2.drawImage(image, 0, 0, this);
            g2.dispose();             
        }         
 };   
 panel.setBorder(BorderFactory.createEtchedBorder());
 panel.setPreferredSize(new Dimension(400, 330));
 s=new JScrollPane(panel);
 s.setPreferredSize(new Dimension(400,285));
 this.getContentPane().add(s,BorderLayout.CENTER);
 add(panel);
 revalidate();  
 repaint();     
 }

As long as we are giving free advice ;-): Don't call `setPreferredSize()`. Either override `getPreferredSize()` or use an appropriate `LayoutManagerµ . — Guillaume Polet, Jan 14 '13 at 18:15
@Michal in your case, you should obviously override `getPreferredSize()` and return the size of the image you paint. If at some point you were to switch to another image with a different size, all you would need to call is `revalidate(); repaint();` and it would all work perfectly. — Guillaume Polet, Jan 16 '13 at 14:58

score 3 · Answer 1 · answered Jan 14 '13 at 18:08

 s=new JScrollPane(panel);
 s.setPreferredSize(new Dimension(400,285));
 this.getContentPane().add(s,BorderLayout.CENTER);
 add(panel); // ****** ????????????
 revalidate();  
 repaint();     
}

You're adding the JPanel to the GUI not the JScrollPane, so you really shouldn't expect to see any scrollpanes if they've not been added anywhere.

Solution: Add your JScrollPane, s, that holds the JPanel to the GUI, not JPanel itself.

score 3 · Answer 2 · edited May 23 '17 at 10:24

You dont honor the paint chain, call super.paintComponent(g) as first call in overridden paintComponent method. i.e

@Override
protected void paintComponent(Graphics g){ 
      super.paintComponent(g); 
      //draw here
}

or visual artifacts may occur.

Also note the @Override annotation which should be used with overridden methods in order to gain the advantage of compiler checking that we overrode the method correctly.
There is no need for getContentPane().add(..) simply call add on JFrame instance as add(..) along with remove(..) and setLayout(..) have been forward to the JFrames contentPane
Also not a good idea to go extending JFrame for nor reason, simply create an instance and use that i.e:
```
JFrame frame=new JFrame("Title here");
...
frame.add(..);
...
frame.pack();
frame.visible(true);
```
Also draw onto the Graphics object passed into paintComponent g dont go creating your own in paintComponent. Unless of course you're doing if for the reason @HFOE mentioned in below comment :).
No need for this parameter in drawImage(..) unless your JPanel implements on ImageObserver or the image may not be fully loaded when painting occurs. Simply use null.
And just for the cherry on top use some Graphics2D and RenderHints as seen here. This will allow for a better quality image to be drawn and text.

It's OK to create a new Graphics object if there's a risk that you may make changes to it that you don't want propagated elsewhere in your GUI such as in your JPanel's children. By changes I mean things such as setting an AffineTransform or Stroke. Otherwise 1+ — Hovercraft Full Of Eels, Jan 14 '13 at 18:14
@HovercraftFullOfEels +1 good point updated answer accorodingly — David Kroukamp, Jan 14 '13 at 18:20

score 0 · Accepted Answer · answered Jan 16 '13 at 14:49

I got it! The main reason for me was no-layout manager and no-declarated size of this. There is my code:

public void draw(){
 panel=new JPanel(new BorderLayout()){
 @Override
        protected void paintComponent(Graphics g){
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g;
            g2d.drawImage(img, 0, 0, null);
        }         
 };   
 panel.setBounds(0, 0, img.getWidth(), img.getHeight());
 panel.setPreferredSize(new Dimension(img.getWidth(),obraz.getHeight()));
 JScrollPane scrolls=new JScrollPane(panel,ScrollPaneConstants.VERTICAL_SCROLLBAR_AS_NEEDED,ScrollPaneConstants.HORIZONTAL_SCROLLBAR_AS_NEEDED);

 add(scrolls);
 validate();  
 repaint();     
 }

But if someone will want copy this code in the future, there is one thing to be known- a size of Jpanel isn't refreshing (it's always one size of first opened image), but I have scrolsl and I am satisfied for now ;). Thanks a lot for everyone.

Consider calling `revalidate()` rather than `validate()` – Guillaume Polet Jan 16 '13 at 14:58 — Guillaume Polet, Jan 16 '13 at 14:58

Scrolls not appears

3 Answers3