Java doesn't mandate the usage of a heap - it can in certain cases avoid doing so and allocate on the stack (escape analysis).

Besides, as mentioned java's allocations are much closer to something like using an arena in a low-level language, then a "slow" malloc. It uses thread-local allocation buffers, where you have a large buffer with a pointer pointing to the start of the free region. Allocation is just a pointer bump, not even needing synchronization since it is per a single thread. As it gets full, the GC moves out still alive objects in the background and resets the buffer.

This is pretty much the most efficient one can be after per-object type arenas and simply not allocating.