JPanel Preferred Size Not Being Set

Question

When is the preferred size of a JPanel set if you don't set it explicitly?

I have the following code:

private static class ScrollableJPanel extends JPanel implements Scrollable {
    @Override
    public Dimension getPreferredScrollableViewportSize() {
                 return getPreferredSize();
    }
            .
            .
            .
}

and this code:

    myPanel = new ScrollableJPanel();
            // Add a bunch of controls to the panel.
    final JScrollPane scroll = new JScrollPane(myPanel);
    scroll.setAutoscrolls(true);
    scroll.setHorizontalScrollBarPolicy(
        ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);

However, the preferred size of myPanel at the time getPreferredScrollableViewportSize() is being called seems to be an arbitrary value, (4225, 34), and prefSizeSet = false.

According to the Java documentation, this is a reasonable way to define this method. However, the actual size of myPanel is much higher than 34 pixels, so the view port is way too small.

What am I doing wrong here? Should I set the preferred size of myPanel explicitly? I tried that, but I want to use the actual size determined by the layout manager, not just make a guess that will be overridden later.

Note that my question isn't how to set an exact size for the JPanel. I want the layout manager to do that. Nor do I want to have to explicitly set the preferred size. I thought that was also handled by the layout manager. What I want to know is why the layout manager is not setting a reasonable value for the preferred size itself so that I can return a reasonable PreferredScrollableViewportSize.

Answer 1

The Preferred size of a JPanel is based on the components in the panel. The Layout manager calculates a preferred size of the panel, based on the preferred size of the items in the panel, and if some of those things are panels, then it is a recursive calculation. If there are no sub components in the panel, then the preferred size will have to be set explicitly.

Answer 2

In AWT (and then Swing), Container.getPreferredSize() either:

• returns the size explicitly set by setPreferredSize(),
• or the size calculated by the LayoutManager if setPreferredSize() has not been called explicitly

Hence, what getPreferredSize() returns highly depends on which LayoutManager you use for your ScrollableJPanel.

Be aware that most LayoutManagers will use getPreferredSize() on all direct children of the Container upon which getPreferredSize() was called. This is a recursive process (if children have children themselves) and may involve several different LayoutManagers.