Swing JPanel won't repaint

Question

I have a simple object which extends JPanel, when the update() method is called on this object it it meant to add some labels to the panel and then repaint. However the labels do not show up after the update method is called, below is the code for update:

public void update(){
        GridBagConstraints constraints = new GridBagConstraints();

        if(cardsHidden){
            for(int i = 0; i < 2; i++){
                constraints.gridx = i;
                constraints.gridy = 0;
                JLabel card = new JLabel(PlayingCards.cardImages[PlayingCards.CARD_BACK_INDEX]);
                add(card, constraints);
            }
        }
        else{
            Card[] holeCards = player.getHoleCards();
            for(int i = 0; i < holeCards.length; i++){
                constraints.gridx = i;
                constraints.gridy = 0;
                JLabel card = new JLabel(holeCards[i].getImageIcon());
                add(card, constraints); 
            }
        }

        validate();
        repaint();
    }

any ideas?

Thanks

EDIT

solved:

It turns out that the HoleCardsPanel wasn't adding to its parent frame properly. Once that was fixed the adding of new JLabels works fine. I also:

• added the call to the update() method to the event dispatch thread using SwingUtillities.invokeLater
• had to call validate() from the uppermost component (in this case the JFrame) as Devon_C_Miller suggests in his answer.

Answer 1

It depends on what you want to happen and what layout managers are in use, but the basic rules are:

1. Make sure update is called on the EDT. If it's not (SwingUtilities.isEventDispatchThread()returns false) you will need to use SwingUtilities.invokeLater to schedule the update on the EDT. For example:

   SwingUtilities.invokeLater(new Runnable() {
       public void run() {
           update();
       }});
   

2. Call invalidate(). Most things that change a component will do this for you. So you only need to call this if the following does not work on its own.

3. Call validate() on the highest affected component. This is probably the muddiest bit of Java's rendering cycle. The call to invalidate marks the component and all of its ancestors as needing layout. The call to validate performs the layout of the component and all of its descendants. One works "up" and the other works "down". You need to call validate on the highest component in the tree that will be affected by your change.

   Also, calling validate on a top-level component (JWindow, JDialog, JFrame) will not necessarily resize that component. To make that happen, you'll need to call pack() or setSize().

4. If your changes alter the size or position of containers, The resized containers will repaint, but they will not erase the space the used to occupy. Calling repaint() on the parent of the container will cause it to repaint the background, correcting the damage.