Interface (Java)

Please note that the content of this book primarily consists of articles
available from Wikipedia or other free sources online. An interface in
the Java programming language is an abstract type that is used to
specify an interface (in the generic sense of the term) that classes
must implement. Interfaces are declared using the interface keyword, and
may only contain method signatures and constant declarations (variable
declarations that are declared to be both static and final). An
interface may never contain method definitions. As interfaces are
implicitly abstract, they cannot be directly instantiated except when
instantiated by a class that implements the said interface. The class
must implement all of the methods described in the interface, or be an
abstract class. Object references in Java may be specified to be of an
interface type; in which case, they must either be null, or be bound to
an object that implements the interface. One benefit of using interfaces
is that they simulate multiple inheritance. All classes in Java (other
than java.lang.Object, the root class of the Java type system) must have
exactly one base class; multiple inheritance of classes is not allowed.