An interface in Java is an abstract type that is used to declare a behavior that classes must implement). It is similar to a protocol and is declared using the interface keyword. Interfaces may only contain method signature and constant declarations, and all methods of an interface do not contain implementation (method bodies) as of all versions below Java 8. Starting with Java 8, default and static methods may have implementation in the interface definition, and in Java 9, private and private static methods were added). A Java interface can have up to six different types).
When a class implements an interface, it must implement all of the non-default methods described in the interface, or be an abstract class). Object references in Java may be specified to be of an interface type, and in each case, they must either be null or be bound to an object that implements the interface).
An interface is a completely "abstract class" that is used to group related methods with empty bodies. To access the interface methods, the interface must be "implemented" by another class with the implements keyword (instead of extends). The body of the interface method is provided by the "implement" class.
Interfaces form a contract between the class and the outside world, and this contract is enforced at build time by the compiler. If a class claims to implement an interface, all methods defined by that interface must appear in its source code before the class will successfully compile.
In summary, an interface in Java is a way to declare a behavior that classes must implement, and it is enforced at build time by the compiler. It is similar to a protocol and is declared using the interface keyword. When a class implements an interface, it must implement all of the non-default methods described in the interface, or be an abstract class. Object references in Java may be specified to be of an interface type.
