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A collections library can specify a List<E> Java interface, with different implementations such as ArrayList<E> and LinkedList<E>. A method accepting List<E> can iterate or access elements according to the documented list contract without requiring one particular storage representation. Each implementation may also extend its own appropriate class superclass.

The benefit is separating a usable contract from its implementation. A parameter restricted to a concrete ArrayList<E> would exclude a linked list implementation and unnecessarily expose the storage choice. Requiring all providers to extend one concrete class would couple them to its implementation and consume Java's single class-inheritance relationship. Interfaces permit multiple implemented contracts, though clients must rely on the stated contract rather than assuming every optional operation is supported or has the same time complexity.