A memory profiling tool essentially tells you what objects you are
creating, how big they are, who is holding a reference to them, and how
long they live.  Most time profilers also include a memory profiling
module that can give you this information.

Running one of these can be an education - you find out whether GC is a
big part of your execution profile, and if it is, what objects are
choking it.  In general, creating objects is pretty cheap, and not worth
optimizing away, and this will be even more true for Java 6.  That said,
there are usage patterns that result in huge amounts of junk lying about.

There are a lot of good tools to do a memory profile.  I, personally,
prefer JProfiler by e-j technologies, but YourKit also gets a lot of
good press.  Both work well with IDEA, and I believe they both work well
with Eclipse, so they would also be good tools to use to profile the
IDEs themselves.

MacOS X developers also have access to the very neat Shark and
ObjectAlloc tools.  I find JProfiler superior to the general tools for
most uses, but there are times where I want to know why we are getting
hit with a strange cost - is the system swapping, or calling into the
kernel.  For those cases, Shark is the bee's knees.

Scott