Framework collection objects in Java are instantiated from classes that implement the core interfaces of the framework. Thus, a Java interface in the Collections Framework specifies the type of such an object, and provides a generic representation of the operations that apply across different implementations of the interface. Collection interface declares several methods The Collection interface declares several methods. This is not unusual. From a technical standpoint, all interfaces declare none, one, or more methods.
Most interfaces declare several methods.
In general, there is no technical requirement for a specification of the behavior of the interface methods when implemented in a class. In fact, because a method that is declared in an interface is abstract, it specifically refrains from defining the behavior of the method. The interface definition simply declares the interfaces for all the methods that it declares. The Sun documentation for the Collection interface, however, goes a step beyond the minimum technical requirements for an interface. The documentation describes the general behavior that must be exhibited by each of the methods belonging to an object instantiated from a class that implements the Collection interface.