Interfaces
Interfaces are reference types like classes, but can have only abstract method declarations and constants as members. Interfaces are defined in the same way as classes, with a source code file that has a .java file type that is compiled to a class file. Interfaces are also members of packages, just like classes. Java uses interfaces extensively to provide a convenient alternative to multiple inheritance. A Java class can implement any number of interfaces.
A class that implements an interface must provide implementations for all the methods that the interface defines. An object of this class then has a sort of alternative identity and can be referred to as being of the interface type. For example, the Runnable interface, a member of ...
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