1. Inheritance
Lakshmish Ramaswamy

2. Multiple Inheritance
A class being derived from more than one base class
Ex: A StudentEmployee Class being derived both from Student and Employee
Some OO languages (ex: C++) support multiple inheritance
Java does not allow multiple inheritance

3. Issues with Multiple Inheritance
Does StudentEmployee get two copies of name, address and phone number?
If StudentEmployee does not override toString method, which implementation would be used?
If a method is declared to be final in one base class and not in the other, would the StudentEmployee be allowed to override the method?

4. Interface Alternative to multiple inheritance
The ultimate abstract class
Consists only of public abstract methods & public static final fields
No implementation of any method
Class providing definitions of all abstract methods is said to implement the interface
Multiple interfaces do not suffer from same problems as multiple inheritance
Class can implement multiple interfaces but extend from a single class

5. Specifying Interface
Similar to a class declaration but uses the keyword interface
Lists methods that need to be implemented
Class implementing an interface should do two things
Declare that it implements the interface using “implements”
Provide implementations of at least one method declared in interface
A method implementing an interface may remain abstract

6. Example of Interface

7. Example of Implementing Class

8. Multiple Interfaces Java allows multiple interfaces
List all the interfaces a class is implementing
Provide implementations of all methods in all interfaces
public class X implements a, b { … }

9. Interfaces as Abstract Classes
IS-A relationship holds
instanceof operator can be used to test type compatibility
Implementing class cannot reduce the visibility of methods
Implementing class may not add checked exceptions to throws list
In case of multiple interfaces implementation can throw exception that is listed in all interfaces

10. Interfaces as Abstract Classes
Class implementing an interface should override with the exact signature
Cannot implement two interfaces with same signature but different return types
Class that partially implements an interface should be declared abstract
Interfaces can extend other interfaces

11. Fundamental Inheritance in Java
If class does not explicitly extend another class, it implicitly extends Object class
Object contains several methods
Object is not abstract class
All methods are implemented
toString, equals and hashCode are some of the important methods