Interfaces are used to define collections of methods that are implemented by classes. However, they may also define constants, inner classes, and inner interfaces. Their syntax is as follows:

modifiers interface InterfaceName extends  {
 // Interface body
}

Valid modifiers are public and abstract. A public interface may be accessed outside of its package. All interfaces are implicitly abstract. The extends clause consists of extends followed by a comma-separated list of interfaces. If interface X extends interfaces Y and Z, then X inherits all of the constants and methods of Y and Z.
The interface body consists of constant declarations, abstract method declarations, inner classes, and inner interfaces. Inner classes and interfaces are covered later in this module. A constant definition is the declaration and initialization of a variable that is public, static, and final. The variables modifiers are optional.
An abstract method declaration is specified as follows:
modifiers returnType methodName(arguments) throwsClause;
Note the semicolon replaces the method body. Methods are implicitly abstract and public. They may not be declared static, native, final, or synchronized.