Objectives

1
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

2
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

3
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.

4
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)

5
Inheritance Heirarchy

• Superclass or base class
• Subclass or derived class

Shape
Rectangle
Circle
Square
6
Inheritance in Java

• To implement inheritance
• public class Square extends Rectangle
• // implements class Square

7
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

8
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 its inherited classes

9
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)

10
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()

11
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

12
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

13
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

14
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

15
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

16
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)

17
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

18
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

19
Use of an interface

• In Java a class implements an interface
• Within the class implementing the interface all
  methods in the interface must be defined
• public 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