Z-order on Java Swing components

You are looking for setComponentZOrder

Here's an example.

Look at the setComponentZOrder(...) method of the Container class.

You can also check out the Overlap Layout which has a brief description on how ZOrder works.

If you want to overlay components over other components, you should use JLayeredPane, which will let you add components to specific layers. This is precisely what's used for things like tooltips and drag-and-drop operations.

Here's a tutorial for it as well.