Q:

JSplitPane splitting 50% precisely

In Swing,
what's the best way to make the JSplitPane to split two jpanels with 50% size each.
It looks like if I don't set preferred sizes on the panels it always makes the first panel almost invisible (2%) and the second one (98%)
Thanks in advance

A:

You should use setDividerLocation(double proportionalLocation) to determine the initial space distribution of the JSplitPane, and then call setResizeWeight(double) with the same value to ensure that the panes are resized in proportion.
Also, be aware: Calling setDividerLocation(double) before the JSplitPane is visible will not work correctly, as the space calculation is based on the Component's current size.  Instead you need to involve a nasty hack, such as overriding the JPanel's paint method that contains the JSplitPane:
private boolean painted;

@Override
public void paint(Graphics g) {
    super.paint(g);

    if (!painted) {
        painted = true;
        splitPane.setDividerLocation(0.25);
    }
}

A:

Use
setResizeWeight(.5d);

[...] A value of 0, the default, indicates the right/bottom component gets all the extra space (the left/top component acts fixed), where as a value of 1 specifies the left/top component gets all the extra space (the right/bottom component acts fixed). [...]

A:

I had a similar problem and I solved it by using a component listener in the parent container and set the divider location on the first resize. Initialise a variable firstResize to true and add this into the parent container constructor:
addComponentListener(new ComponentAdapter(){
    @Override
    public void componentResized(ComponentEvent e) {
        if(firstResize){
            splitPane.setDividerLocation(0.5);
            firstResize = false;
        }
    }
});

This should cause the divider to be centred when the parent container size is first set.

