为什么java.util.BitSet不实现java.util.Set

Question

Why java.util.BitSet does not implement java.util.Set<Integer> interface. Especially when it can easily do so. Also BitSet is used to maintain if an integer was "switched on or not", Set also does exactly the same. So BitSet is a Set<Integer> . Referring to some comments saying that

Now take a look at the methods of BitSet. The purpose of the class is to perform logical operations on a collection of bits that are in a certain order.

I would still say that BitSet is a Set<Integer> which also provides additional set of operations.

Answer 1

Since the comments don't make it obvious enough:

You say "BitSet is effectively an efficient set. BitSet is a Set" - no it is not. A Set is a collection which contains no duplicate elements.

Now imagine a Set<Bit> . Since a bit can only be 1 or 0, that will be a pretty boring set of a maximum size of 2.

Now take a look at the methods of BitSet . The purpose of the class is to perform logical operations on a collection of bits that are in a certain order. That has nothing to do with a Set .

Answer 2

java.util.BitSet is in the JDK since version 1.0, java.util.Set entered the stage with JDK 1.2. So BitSet cannot implement Set .

If you need a Set<> implementation backed by a BitSet , that does not seem to hard to implement using a backing BitSet .