What are the most effective ways to optimize a Java program with a large data structure?

When optimization is the issue, you need to know when to use which of the seemingly equivalent structures. E.g. HashMap has pretty much same functionality as ConcurrentHashMap, but the latter is synchronized, while the first isn't. Meaning HashMap operations are generally faster in single-thread environment, but you *must* sync multi-thread writes to it by yourself, while ConcurrentHashMap does that already. If virtual threads are used, this is even more troublesome, as each use of *synchronized* keyword pins the virtual thread (disallowing its dismount from the carrier thread) - blocking the carrier as well...

Garbage Collector will only clean up objects that aren't referenced by any strong reference (all "normal" references/variables are strong - except specifically declared weak ones). If you continually create objects (e.g. in a loop or a long-running thread) and don't free them, by assigning null to their pointer variables (esp. if you use class-scoped variables), GC won't free them, until your whole class goes out-of-scope - which might *never* happen for some objects. So you end up with an inefficient, bloated memory hog, instead of a fast, efficient program. If/when GC finally gets to remove such objects, the cleaning process of large, fragmented pieces of memory may create considerable performance hiccups.

Using parallelism can help speed up the equivalent synchronous code, but this should always be benchmarked against the original code. This is because mostly in code, the bottleneck would be either the database or the memory. It's rarely the CPU. And using parallelism generally helps optimise CPU usage, sometimes at the cost of more RAM or disk usage, which are already much slower than CPU. This may overall decrease the performance of the entire code block and during benchmarking, this would get manifested as the parallel version performing poorly. Try to use parallelism when using horizontal scaling using multiple machines.

Beware that whenever you use parallel processing (e.g multiple threads) accessing the same data, any writes (sometimes reads as well) to that data must be synchronized. If you use structures that imply parallelism in their names (e.g. ConcurrentHashMap), their methods will do that for you automatically. If such structures aren't available for what you're trying to accomplish, you'll have to sync your data access by yourself - taking special care that your synced blocks are: * absolutely waterproof, * yet still short and efficient. That requires experience, understanding of underlying instruction set, meticulous what-if thinking and is extremely difficult to test and debug. If this is new to you, better stick to what is already available.