It is possible to precisely position widgets on the screen using x and y pixel coordinates that are relative to the applet's panel. You should only do this when the applet will only run on your web pages so you can control the applet's size. Free-standing windows may even be resized by the user on the fly. If you've used a layout manager to position your components, they'll be adjusted immediately to fit the new window size. This behavior is extremely hard to duplicate if you don't use a layout manager.
Components use varying amounts of space on different platforms.
You should probably only use these techniques when you are writing a single-platform, compiled Java application. You should not use them for applets. Laying out components manually is a lot of work, especially given the lack of Visual Basic like interface drawing environments.