>These kinds of things are impossible with C++

What? Anything you can do in C, you can do in C++ at the same performance level (pay only for what you use); just be aware of what you are doing and using.

> I remember just refactoring all of foreach-loops/iterators into for-loops and got insane speedups.

Wow! Can you please provide a simple example when/where it happens?

> It's possible to write efficient Java but you have to not use some of the language features to do so.

Please tell us more.

Let's say you write an Iterable class so that you can use foreach on your data structure. It's quite possible iteration won't be completely inlined and the overhead of calling hasNext & next can become non-significant.

An even worse case that made massive speedups is just inlining everything. I believe modern IDEs can automatically inline all invocations of a function. You'd be surprised how much performance you can get with that.

As for writing efficient Java, it comes down to using primitive and value types.

Although, my experience is mostly with Java 8. I have no idea how streams or other new syntax works. After optimizing a really massive Java code base I've never used it since.

But you still have to make the next and hasNext calls in a for loop. What guarantees that they would be inline in a for loop?

I inline them manually.

How much was the performance improvement?

The biggest improvement I measured was about 15x. I'd say you can get 3x most of the time (go down to 33% of previous runtime).

That is insane speedup! Thanks for sharing. How do I find next and hasNext is the bottleneck? Should I run profiler to see if they are being called a large number of times?

Unfortunately, any profiler I tried does not show these issues. Because the code you are profiling will be optimized completely differently. Although it makes sense to do the optimizations/inlining inside the top-level functions that profiler reports to take a lot of time (even if the iteration happens in deeper levels).

You just have to try it and see. I remember it was quite easy in IntelliJ, I believe there's an option to inline all invocations of a function. I just went crazy with that option but did it systematically to really find where the bottleneck is and make a minimal change.