Inheritance - Data Structures and Problem Solving Using Java - page 136

4.4.4 interfaces are abstract classes

Because an interface is an abstract class, all the rules of inheritance apply.
Specifically,

1. The IS-A relationship holds. If class C implements interface I ,

then C IS-A I and is type-compatible with I . If a class C implements
interfaces I 1 , I 2 , and I 3 , then C IS-A I 1 , C IS-A I 2 , and C IS-A I 3 ,
and is type-compatible with I 1 , I 2 , and I 3 .

2. The instanceof operator can be used to determine if a reference is

type-compatible with an interface.

3. When a class implements an interface method, it may not reduce

visibility. Since all interface methods are public, all implementations
must be public.

4. When a class implements an interface method, it may not add checked

exceptions to the throws list. If a class implements multiple interfaces in
which the same method occurs with a different throws list, the throws list
of the implementation may list only checked exceptions that are in the
intersection of the throws lists of the interface methods.

5. When a class implements an interface method, it must implement the

exact signature (not including the throws list); otherwise, it inherits an
abstract version of the interface method and has provided a non-
abstract overloaded, but different, method.

6. A class may not implement two interfaces that contain a method with

the same signature and incompatible return types, since it would be
impossible to provide both methods in one class.

7. If a class fails to implement any methods in an interface, it must be

declared abstract.

8. Interfaces can extend other interfaces (including multiple interfaces).

fundamental inheritance in java

4.5

Two important places where inheritance is used in Java are the Object class
and the hierarchy of exceptions.

4.5.1 the Object class

Java specifies that if a class does not extend another class, then it implicitly
extends the class Object (defined in java.lang ). As a result, every class is
either a direct or indirect subclass of Object .