Optimizing the memory requires a good knowledge of the problem. Maybe there is a better way to optimize for a certain problem, but there is no way to do so in general.

If you have used twice as memory as someone else, while you both implemented the same algorithm and logic, then you can ask about optimizations (for general programming) and how to reduce the amount of memory usage to get to the goal... or better: you have to get worried and consider thinking more on memory optimizations.

Even in this case; it's not in anyways related to Java. As someone else mentioned, you have to work on your solution. (but yes, considering the language structure and specifications)

The small amount of additional memory usage of Java programs may come from OOP, which we do it less in C++ in CP.

If you have used that much memory and someone else used no memory at all, I believe the difference comes from different algorithms being used, O(n) vs O(1) memory for example. But in case you have small difference, you may have used 64-bit integers where 32-bit integers were enough, for example.

Why you need optimize, just walk of in 256 MB.

1) Let the gc do it's work
2) When creating multi-dimensional arrays, say int[i][j], make sure that i <= j.