Why does HashSet implementation in Sun Java use HashMap as its backing?

Question

Looking at the source of Java 6, HashSet is actually implemented using HashMap, using dummy object instance on every entry

Answer 1

Your question: I think that wastes 4 byte (on 32-bit machines) for the size of the entry itself.

Just one Object variable is created for the entire datastructure of hashset and doing that would save yourself from re-writing the entire hashMap kind of code again.

private static final Object PRESENT = new Object();

All the keys are having one value i.e PRESENT object.