Objectives

• Introduction to Inheritance and Composition (Subclasses and SuperClasses)

• Overriding (and extending), and inheriting methods and constructors from the SuperClass of an inherited class

• Understand what an interface class is and how to use it

• Introduction to abstract classes

Encapsulation

• Combine data and operations in a single unit known as a class

• The class is a self-contained entity. The internal state of an object cannot be manipulated directly

Inheritance and Composition

• When a new class is defined it may be a special case, or a generalization of an existing class

• Inheritance (is a) and composition (has a) can then be used to build a class that has the properties ( elements, methods) of the existing class with additions and/or changes.

Inheritance• The object created by and Inherited class

“IS A” object that could be considered as an incidence of the super class (or base class)

• A dog “IS A” pet• A square “IS A” rectangle• BUT a wall “HAS A” rectangle

(composition, we will revisit this later)

Inheritance Heirarchy

Superclass or base class

Subclass or derived class

Shape

Square

RectangleCircle

Inheritance in Java

• To implement inheritance

public class Square extends Rectangle

{

// implements class Square

}

Why Inherit?

• The base class already has elements constructors and methods to work with the object created by the class

• The inherited class will automatically have the default constructor, and the public or protected elements and methods of the super class

• NOTE: private elements and methods are not inherited.

• The class can then be extended by adding or overriding methods or adding variable elements

protected

• What is the meaning of protected

• Protected– Protected elements can be accessed by inherited

classes– Protected elements cannot be accessed outside

the base class and it’s inherited classes

Types of Inheritance

• Java supports single inheritance, that is inheritance from one class

• Some other languages also support multiple inheritance (inheriting from more than one class)

Inheritance: constructors• Your derived class will automatically

inherit the default constructor from your base class

• Other constructors must be included in your derived class

• You can efficiently add to the constructors (including the default constructor) in the base class by implementing them in your derived class using super()

Using super() to extend Constructors

• If you want the constructor in your derived class to do everything the constructor of your base class does, and a few extras, the constructor in your derived class should look like

• public derivedClassName( parameter list ){

super( parameter list );// added functionality if any

}• For the default constructor the parameter list will be empty

super() and parameters

• When we build a constructor in an inherited class it can have– The same number and kind of parameters as a

constructor in the base class. • Extend using super(parameter list) and the same parameter list

as the constructor in the base class

– A different number or kind of parameters as a constructor in the base class

• This is a case of overloading. We have increased the overloading options for the constructors. The new overloaded constructor will be available to the derived class and not the base class

super() and private members

• Inheritance does not allow direct access to private members of the base class

• private members of the base class need to be initialized by the constructors of the derived classes

• super() allows the base class to initialize its private members, without providing the derived class direct access to them

Inheritance and Methods

• The public methods of the base class can be directly used on objects of the derived class.

• Any of the public methods in the base class can be extended, overridden, or overloaded in the derived class

• Additional methods can be added to the derived class that are not part of the base class

Methods and the use of super• In the derived class a method to be overridden or overloaded has

the following syntax

public methodtype methodname( parameter list1 ){

super.methodname( parameter list2 );// added functionality if any

}

• Parameter list2 must always match the parameter list for an instance of this method in the base class

• To override (extend) a method from the base class the parameter list1 must match a parameter list of the method in the superclass.

• To overload parameter list1 does not match the parameter list in any occurrence of the method in the base class

Abstract Methods

• An abstract method is a method that– Specifies a method name type and parameter

list– Provides no code to implement the method – For example:

• public void abstract print()

• Public abstract insert(int insertItem)

Abstract Classes• Can contain

– instance variables – Constructors– abstract and non abstract methods

• Any class containing an abstract method must be an abstract class

• You cannot declare an instance of an abstract class• You can declare an instance of a class inherited

from an abstract class ONLY if all the abstract methods in the abstract class are implemented in the derived class

Interfaces• An interface is a class that contains only

abstract methods and/or named constants

• Java does not support multiple inheritance, A derived class can inherit from only one base class

• A class can use more than one interface.

• A class can inherit from a single class and use interfaces

Use of an interface

• In Java a class implements an interface• Within the class implementing the interface all

methods in the interface must be definedpublic class ClassName implements InterfaceName{

//Define variable elements of class// Constructors for class ClassName//code defining each of the methods in the interface// Any other methods for class ClassName

}