The interface methods must all have public accessibility when implemented in the class (or its subclasses). A class can neither narrow the accessibility of an interface method nor specify new exceptions in the method's throws clause, as attempting to do so would amount to altering the interface's contract, which is illegal. The criteria for overriding methods also apply when implementing interface methods.
Note that interface methods cannot be declared static, because they comprise the contract fulfilled by the objects of the class implementing the interface. Interface methods are always implemented as instance methods.
However, note that regardless of how many interfaces a class implements directly or indirectly, it only provides a single implementation of a member that might have multiple declarations in the interfaces.
An interface can extend other interfaces, using the extends clause. Unlike extending classes, an interface can extend several interfaces. The interfaces extended by an interface (directly or indirectly), are called super interfaces. Conversely, the interface is a subinterface of its superinterfaces. Since interfaces define new reference types, superinterfaces and subinterfaces are also supertypes and subtypes, respectively.
A subinterface inherits all methods from its superinterfaces, as their method declarations are all implicitly public. A subinterface can override method prototype declarations from its superinterfaces. Overridden methods are not inherited. Method prototype declarations can also be overloaded, analogous to method overloading in classes.
Thinking in terms of types, every reference type in Java is a subtype of Object type. This means that any interface type is also a subtype of Object type.
