Microtuning like this also depends on the jdk version, jvm, and the mode (such as hotspot) that you are running in and probably many other factors. Rarely/never will such code be a bottleneck, and so most seasoned developers don't know or care about the performance difference.
If the object you are calling is a shared object calling getSomething() multiple times may return a different answer each time you call it. If possible I like to design objects to be immutable so I don't have to worry about such things. Assigning to a temporary variable would ensure that the value isn't changed underneath you (that assumes this would be a bad thing). As always design considerations such as these should take a front seat to performance.
If it is important to find out the difference
you should use a profiling tool. You can learn about them at
www.javaperformancetuning.com [ January 18, 2007: Message edited by: steve souza ]