Java Swing Tips

This post collects some tips for Java AWT/Swing features that I found less than obvious in my recent programming. I’m using Oracle OpenJDK 10 on Windows 10, but all Oracle Java Documentation links refer to Java SE 8 as that is the target version of my projects.

Border Insets

The Border implementations returned by BorderFactory do not allow specifying insets in addition to the (non-empty) Border itself, but there is a simple mechanism for this: create a CompoundBorder with a visible outer border and an empty inner border taking up the desired inset space.

Mouse Event Handling

To handle multiple mouse events, it’s common and convenient to derive from the general classes MouseAdapter or MouseInputAdapter. But while such classes may contain handlers for all kinds of mouse events, those handlers will not actually be called unless you also add your adapter-derived instance to your component using the corresponding specific listener. That is, you must supply it separately to addMouseListener, addMouseMotionListener, and/or addMouseWheelListener as needed. I frequently forget the latter two and then marvel why mouse motion and mouse wheel events aren’t being handled, even though I had implemented them on my adapter-derived class.

Resize Event Handling

To lay out components manually in a container without a layout manager, you’ll need to handle the componentResized event on the container and reposition and/or resize your components as desired. Importantly, once you’ve done that you also need to call revalidate() and repaint() on the components, or the positions and/or sizes may not be updated correctly. The “may” is important: in my experience the layout works sometimes even without revalidate/repaint, so it’s easy to forget these calls and become puzzled over the resulting intermittent layout failures.

Some Popup Tips

The documentation for PopupFactory.getPopup is somewhat confusing. The optional owner parameter is described as “Component mouse coordinates are relative to, may be null.” This seems to imply that when an owner is present the x and y parameters would be relative it, but that is not the case. Their documentation correctly states that both are absolute screen coordinates, and that is the case regardless of any owner. Also note that the contract for the PopupFactory class, described at the top of the page, is quite serious. You must indeed call Popup.hide on any popup obtained from a factory before you request another popup from the same factory, or else a myriad of popup remnants will be smeared across the screen!