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

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Actually, it’s not just HashSet. All implementations of the Set interface in Java 6 are based on an underlying Map. This is not a requirement; it’s just the way the implementation is. You can see for yourself by checking out the documentation for the various implementations of Set.

Your main questions are

But, why is it still used? Is there
any reason to use it besides making it
easier to maintain the codes?

I assume that code maintenance is a big motivating factor. So is preventing duplication and bloat.

Set and Map are similar interfaces, in that duplicate elements are not allowed. (I think the only Set not backed by a Map is CopyOnWriteArraySet, which is an unusual Collection, because it’s immutable.)

Specifically:

From the documentation of Set:

A collection that contains no
duplicate elements. More formally,
sets contain no pair of elements e1
and e2 such that e1.equals(e2), and at
most one null element. As implied by
its name, this interface models the
mathematical set abstraction.

The Set interface places additional
stipulations, beyond those inherited
from the Collection interface, on the
contracts of all constructors and on
the contracts of the add, equals and
hashCode methods. Declarations for
other inherited methods are also
included here for convenience. (The
specifications accompanying these
declarations have been tailored to the
Set interface, but they do not contain
any additional stipulations.)

The additional stipulation on
constructors is, not surprisingly,
that all constructors must create a
set that contains no duplicate
elements (as defined above).

And from Map:

An object that maps keys to values.
A map cannot contain duplicate keys; each key can map to at most one value.

If you can implement your Sets using existing code, any benefit (speed, for example) you can realize from existing code accrues to your Set as well.

If you choose to implement a Set without a Map backing, you have to duplicate code designed to prevent duplicate elements. Ah, the delicious irony.

That said, there’s nothing preventing you from implementing your Sets differently.

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