Interfaces allow the nominative typing in Java to work across disjoint class hierarchies.
This is due to the "single inheritance" limitation on a class hierarchy and "closed types". I will hold my tongue on subtype polymorphism and Java's implementation of it ;-)
There are other solutions to this problem such as dynamic typing, structural typing, multiple inheritance, and traits, etc. Each approach has advantages and dis-advantages. Interfaces were just the approach that Java took.
