1st of all sizing objects in java is non-trivial (as explained very well here).
if you wish to know the size of a particular object, there are at least 2 open source libraries that will do the math for you - java.sizeof and javabi-sizeof
now, as for your specific test - System.gc() is mostly ignored by modern (hotspot) jvms, no matter how many times you call it. also, is it possible your useMyDataStructure()
method does not retain a reference to the object(s) it creates? in that case measuring free memory after calling it is no good as any allocated Objects might have been cleared out.