[wuch]

Controlling memory layout is one area that is very important for performance, where Java does not exactly shines. You essentially have to store your data in arrays of primitive types, to avoid memory overhead of objects (pointer to class, lock, gc bits) and additional indirection during access. While not exactly impossible, Java makes it quite inconvenient and costly to write code that takes control of memory layout.

[pjmlp]

Hence why there are ongoing improvements targeted to Java 10, to change exactly that.

Until then there are the IBM and Azul extensions, off heap memory, or optimizing in native code that 1% after using the profiler, where it really matters.

[therpe1]

It's worth pointing out that:

1) Controlling memory layout is not as big a deal as people think. The JVM is highly optimized (really it's probably the most optimized system ever devised) for allocating and deallocating objects on the heap.

2) It is in fact possible to control memory layout today because Java has access to offheap memory where you can have all the control you want.

This has actually been the case for nearly half a decade but for some reason a lot of people are convinced that "big arrays" can knock over the JVM.