Java compressed object pointers.

Shipilev (JVM performance engineer) explains it in great detail here: https://shipilev.net/jvm/anatomy-quarks/23-compressed-refere...

But the short of it: Java uses 32-bit numbers for object references if it can address whole heap with it. Given default 8-byte alignment of object we have 2^32 * 8 = 32g of addressable heap.

Once we are beyond that number 64-bit references are used and suddenly we are wasting a lot of heap space (Java is very much objects everywhere). Usually up to around 40% of heap is references.