Question

What’s the purpose of java.util.Set interface if it contains absolutely equal methods in comparison with java.util.Collection?
There is no prohibition for multi-implementing in Java, and Set realization (HashSet) could work fine without Set interfaces.
So why do we need that useless baggage?

Answer 1

JavaDoc describing contract of Collection (emphasis mine):

A collection represents a group of objects, known as its elements. Some collections allow duplicate elements and others do not.

On Set:

A collection that contains no duplicate elements.

In other words, Set abstraction describes a (ekhem!) subset of Collections.