As the name suggests, HashSet and HashMap are Hashing based collections. Similarities between HashSet and HashMap are...

1 - Thread Safety : Both HashMap and HashSet are not synchronized collections. Therefore they are not good for thread-safe operations. To make these thread-safe we need to explicitly use synchronized versions.

2 - Order of Elements : None of these classes guarantee the order of elements. These are unordered collections.

3 - Internal Implementation : A HashMap backs up a HashSet internally. So HashSet uses a HashMap for performing its operations.

4 - Performance : Both of these collections provide constant time performance for basic operations such as insertion and removal of elements.