The Packer class and the associated PackAs interface provide a new API for access to the GridbagLayoutManager and its associated use of the GridbagContraints class. The main purpose of this API was to provide a mechanism which was similar to the TCL/TK pack command which provides very similar capabilities. However, by eliminating the use of the GridbagContraints object, by the developer, several classes of "reuse" bugs and clearer code also occurs.
There are several different things that you need to understand about the underlying GridbagLayoutManager's core behaviors to effectively use the Packer APIs. Below is a list of topics to look over.
There are also some ways to use Packer which can simplify your code and make it easier to reorganize your GUI layouts. Look at the following pages for some hints and pointers.