What is the difference between Collection and Set interfaces in Java?

Question

The declared method signatures seem to be equal. So what is the purpose of introducing Set interface rather then implementing Collection directly? Or is it just sort of a marker interface here?

Answer 1

Set guarantees that the collection will contain unique elements (no duplicates). A Collection does not guarantee this.

The Set interface contains only methods inherited from Collection and adds the restriction that duplicate elements are prohibited.

Set also adds a stronger contract on the behavior of the equals and hashCode operations, allowing Set instances to be compared meaningfully even if their implementation types differ. Two Set instances are equal if they contain the same elements.

The Set Interface

Note, Set does not preserve the order in which elements were inserted. You LinkedHashSet if you need to preserve insertion order of the elements. https://stackoverflow.com/a/821104/4587961