1 Chapter 6: Extending classes and Inheritance

2 Basics of Inheritance One of the basic objectives of Inheritance is code reuse If you want to extend the functionality of an old class, you do not need to reinvent the wheel. All what you need is to inherit from that class and add the new features you want to introduce. The original class is called super, base or parent class while the new one is called derived or child class. When you create an object of the derived class it always contain a complete object of the parent one with all its data members and methods in addition to its own stuff.

3 Basics of Inheritance (Cont.) Inheritance is all about what you can access not what you have (Why??). To inherit from a base class you use the keyword “extends” when you define your derived class An object of the derived class do NOT have access (do not inherit) to the following: –Members or methods declared as private –Constructors of the parent class regardless their attributes –Members or methods declared as being friendly unless the child is defined in the same package In the constructor of the child class, you need to call the base class constructor using the keyword super. This call must be the first line If you do not do so, the compiler will insert a call for the default constructor Ex: Animal, Dog, TestDerived

4 Basics of Inheritance (Cont.) If you define a member or a method in the child class with the same exact name as the one in the parent THEN the derived class name will override (hide) the parent class one Overriding the base class method Ex: “TestDerived2” Inheritance also allows us to apply a principal called Polymorphism

5 Polymorphism The ability of a single variable of a given type to reference objects of other types Advantages: Among others, the ability to change program behavior at run time To polymorphically call a method, it must be defined in both the base and the derived class Also, the object type needs to be of the parent class but the stored object can be any of its children or grandchildren Ex: “TryPolymorphism” You can inherit from a child class and this can continue deeper and deeper to create an inheritance tree Ex: “TryPolymorphism2”

6 Abstract Classes A class can be declare as an abstract one using the key word “abstract” Abstract classes can NOT be instantiated One or more methods in an abstract class should be declared as abstract An abstract method is just a method definition without any implementation. This type of methods is used in order to allow children classes to override that method and make use of polymorphic calls Exercise: Convert the class Animal to an abstract one

7 Universal Superclass The Object class is the root for all classes in Java. (check its methods) We can determine the type of an object using the getClass() method of Object which returns an object of type Class and the getName() method of Class. To execute a method unique to a particular class you need to explicitly cast down the inheritance hierarchy. Ex: “LayEggs”

8 Casting & interfaces To identify the type of an object stored in a parent class member, you can use the instanceof operator In case you create an inheritance hierarchy just to take advantage of polymorphic calls, you might define the parent class as an Interface To take the concept of abstract classes one step farther, Java provides a way to make every method abstract by creating an interface instead of a class. An Interfaces does not define what a method does rather it just defines its form

9 Interfaces (Cont.) An interface can contain either constants or abstract methods or both To inherit from the interface, we use the keyword “implements” instead of “extends” Constants in an interface are referenced exactly as we do with static variables of a class Implementing an interface requires implementing all the methods defined into it Ex: “ TryConversions2”