Inheritance and Polymorphism in java simple and clear

Inheritance and Polymorphism in java simple and clear with an example

1. Lecturer :Ashna nazm hamasalh [email protected] Kirkuk institute for computer science lecturer .miss .Ashna Nazm Hamasalh 1

2. Chapter One Introduction to OOP in JAVA lecturer .miss .Ashna Nazm Hamasalh 2

3. Introduction This tutorial will help you to understand about Java OOP’S concepts with examples. Let’s discuss about what are the features of (Object Oriented Programming). Writing object-oriented programs involves creating classes, creating objects from those classes, and creating applications, which are stand-alone executable programs that use those objects. lecturer .miss .Ashna Nazm Hamasalh 3

4. A class is a template, blue print, or contract that defines what an object’s data fields and methods will be. An object is an instance of a class. You can create many instances of a class. A Java class uses variables to define data fields and methods to define actions. Additionally, a class provides methods of a special type, known as constructors, which are invoked to create a new object. A constructor can perform any action, but constructors are designed to perform initializing actions, such as initializing the data fields of objects. lecturer .miss .Ashna Nazm Hamasalh 4

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6. Objects are made up of attributes and methods. Attributes are the characteristics that define an object; the values contained in attributes differentiate objects of the same class from one another. To understand this better let’s take example of Mobile as object. Mobile has characteristics like model, manufacturer, cost, operating system etc. So if we create “Samsung” mobile object and “IPhone” mobile object we can distinguish them from characteristics. The values of the attributes of an object are also referred to as the object’s state. lecturer .miss .Ashna Nazm Hamasalh 6

7. There are three main features of OOPS. 1)  Inheritance 2)Polymorphism 3 ) Encapsulation lecturer .miss .Ashna Nazm Hamasalh 7

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9. Chapter Two   Inheritance, Polymorphism, Abstract, Interface and Package lecturer .miss .Ashna Nazm Hamasalh 9

10. Inheritance Inheritance can be defined as the process where one class acquires  the   properties(methods  and fields) of another. With the use of inheritance the  information   is made  manageable in a hierarchical order. The class which inherits the properties of other is known as subclass (derived class, child class) and the class whose properties are inherited is known as superclass(base class, parent class ). lecturer .miss .Ashna Nazm Hamasalh 10

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12. extends Keyword extends is the keyword used to inherit the properties of a class. Below given is the syntax of extends keyword. lecturer .miss .Ashna Nazm Hamasalh 12 class   Super {   .....   .....   }     class   Sub   extends   Super {   .....   .....     }

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14. Sample Code Below given is an example demonstrating Java inheritance. In this example you can observe two classes namely Calculation and My_Calculation . Using extends keyword the My_Calculation inherits the methods addition() and Subtraction() of Calculation class. Copy and paste the program given below in a file with name My_Calculation.java lecturer .miss .Ashna Nazm Hamasalh 14

15. public class  My_Calculation  extends Calculation{     public void multiplication( int  x,  int  y ){ z=x*y ; System.out.println ("The product of the given numbers:"+z ); } public static void main(String  args []){ int  a=20, b=10 ; My_Calculation  demo = new  My_Calculation ();   demo.addition (a, b);   demo.Substraction (a, b);   demo.multiplication (a, b); } } class  Calculation {   int  z;   public  void addition( int  x,  int  y){   z= x+y ;   System.out.println ("The sum of the given numbers:"+z);   }   public  void  Substraction ( int   x,int  y){   z=x-y ;   System.out.println ("The difference between the given numbers:"+z);   }    } Child class parent class lecturer .miss .Ashna Nazm Hamasalh 15

16. After executing the program it will produce the following result. The  sum  of the given numbers: 30 The  difference  between the given numbers: 10 The  product  of the given numbers: 200 lecturer .miss .Ashna Nazm Hamasalh 16

17. In the given program when an object to   My_Calculation  class is created,  a  copy  of the  contents of the super class is made with in it.  That  is why, using the object  of the  subclass you can access the members  of  a super class. lecturer .miss .Ashna Nazm Hamasalh 17

18. The Superclass reference variable can hold the subclass object, but using that variable you can access only the members of the superclass, so to access the members of both classes it is recommended to always create reference variable to the subclass . If you consider the above program you can instantiate the class as given below as well. But using the superclass reference variable (   cal  in this case ) you cannot call the method  multiplication() , which belongs to the subclass  My_Calculation . Calculation  cal =new  My_Calculation ();   demo.addition (a, b);   demo.Subtraction (a, b);   lecturer .miss .Ashna Nazm Hamasalh 18

19. lecturer .miss .Ashna Nazm Hamasalh 19 Note: A subclass inherits all the members (fields, methods, and nested classes) from its superclass. Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass .

20. The  super  keyword The   super  keyword is similar to   this  keyword following are the scenarios  where the  super keyword is used . • It is used to differentiate the members of superclass from the members of subclass , if they have same names . • It is used to invoke the superclass constructor from subclass. lecturer .miss .Ashna Nazm Hamasalh 20

21. Differentiating the members If a class is  inheriting  the properties of another class. And if the members of  the superclass  have the names  same as  the  sub class , to differentiate these variables we use  super  keyword as shown below.   super.variable   super.method (); lecturer .miss .Ashna Nazm Hamasalh 21

22. Sample Code This section provides you a program that demonstrates the usage of the super keyword. In the given program you have two classes namely Sub_class and Super_class , both have a method named display() with different implementations, and a variable named num with different values. We are invoking display() method of both classes and printing the value of the variable num of both classes, here you can observe that we have used super key word to differentiate the members of super class from sub class. Copy and paste the program in a file with name Sub_class.java. lecturer .miss .Ashna Nazm Hamasalh 22

23. lecturer .miss .Ashna Nazm Hamasalh 23 class   Super_class { int   num = 20 ; //display   method   of   superclass public   void   display (){ System . out . println ( "This   is   the   display   method   of   superclass" ); } }

24. public   class   Sub_class   extends   Super_class   {   int   num = 10 ; //display   method   of   sub   class   public   void   display (){   System . out . println ( "This   is   the   display   method   of   subclass" );   } public   void   my_method (){   //Instantiating   subclass   Sub_class   sub = new   Sub_class ();   //Invoking   the   display()   method   of   sub   class   sub . display ();   //Invoking   the   display()   method   of   superclass super . display (); //printing   the   value   of   variable   num   of   subclass   System . out . println ( "value   of   the   variable   named   num   in   sub   class :" +   sub . num );     //printing   the   value   of   variable   num   of superclass   System . out . println ( "value   of   the   variable   named   num   in   super   class :" + super . num );   } public   static   void   main ( String   args []){   Sub_class   obj   =   new   Sub_class ();   obj . my_method (); }   }   lecturer .miss .Ashna Nazm Hamasalh 24

25. Compile and execute the above code .   On executing the program you will get the following result: This   is   the   display   method   of   subclass This   is   the   display   method   of   superclass value   of   the   variable   named   num   in   sub   class:10 value   of   the   variable   named   num   in   super  class:20   lecturer .miss .Ashna Nazm Hamasalh 25

26. Invoking   Superclass  constructor If   a   class   is   inheriting   the   properties   of   another   class,   the   subclass   automatically acquires   the  default constructor of  the  super class. But if you want to  call  a parameterized   constructor   of   the   super   class,   you   need   to   use   the   super   keyword as shown   below. lecturer .miss .Ashna Nazm Hamasalh 26

27. super(values); Sample Code The program given in this section demonstrates how to use the super keyword to invoke the parameterized constructor of the superclass. This program contains a super class and a sub class , where the super class contains a parameterized constructor which accepts a string value, and we used the super keyword to invoke the parameterized constructor of the super class . Copy and paste the below given program in a file with name Subclass.java lecturer .miss .Ashna Nazm Hamasalh 27

28. Compile   and   execute the   above   code   On   executing   the  program   you   will get   the   following   result:   The   value   of   the   variable   named   age   in   super   class   is:   24 class   Superclass {     int   age ;     Superclass ( int   age ){   this . age = age ;   }    public   void   getAge (){   System . out . println ( "The   value   of   the   variable   named   age   in   super   class   is:   " + age );   }   } public   class   Subclass   extends   Superclass   {   Subclass ( int   age ){   super ( age ); } public   static   void   main ( String   argd []){   Subclass   s =   new   Subclass ( 24 );   s . getAge ();   } } lecturer .miss .Ashna Nazm Hamasalh 28

29. Types of inheritance There are various types of inheritance as demonstrated below. lecturer .miss .Ashna Nazm Hamasalh 29

30. What is polymorphism in programming?   Polymorphism is the capability of a method to do different things based on the object that it is acting upon . In other words, polymorphism allows you define one interface and have multiple implementations. I know it sounds confusing. Don’t worry we will discuss this in detail. lecturer .miss .Ashna Nazm Hamasalh 30

31. It is a feature that allows one interface to be used for a general  class of  actions. An  operation may exhibit different behavior in different instances. The  behavior depends on the types of data used in the operation. It  plays an important role in allowing objects having different internal structures  to share the same external interface. Polymorphism  is extensively used in implementing inheritance. Following concepts demonstrate different types of polymorphism in java. 1)   Method Overloading 2)   Method Overriding lecturer .miss .Ashna Nazm Hamasalh 31

32. Method Overloading: In Java, it is possible to define two or more methods of same name in a  class,provided  that there argument list or parameters are different. This concept  is known  as Method Overloading. I have covered method overloading and Overriding below. To  know  more  about polymorphism  types refer my post  Types of polymorphism in java: Static, Dynamic, Runtime and Compile time Polymorphism . lecturer .miss .Ashna Nazm Hamasalh 32

33. 1) Method Overloading 1. To call an overloaded method in Java, it is must to use the type and/or number of arguments to determine which version of the overloaded method to actually call. 2. Overloaded methods may have different return types; the return type alone is insufficient to distinguish two versions of a method. 3. When Java encounters a call to an overloaded method, it simply executes the version of the method whose parameters match the arguments used in the call. 4. It allows the user to achieve compile time polymorphism. 5. An overloaded method can throw different exceptions. 6. It can have different access modifiers. lecturer .miss .Ashna Nazm Hamasalh 33

34. public   static   void   main   ( String   args   []) { Overload   Obj  =   new   Overload (); double  result ; Obj   .demo( 10 ); Obj   .demo( 10 ,  20 ); }} Example:   class   Overload { void   demo  ( int  a ){ System . out .println   ( "a:   "  +   a); } void   demo  ( int  a,   int  b) { System . out .println   ( "a   and   b:   "   +   a +   ","  +   b); } double   demo( double   a)   { System . out .println ( "double   a:   "   +   a); return   a*a; }} lecturer .miss .Ashna Nazm Hamasalh 34

35. Output: a:   10 a   and   b:   10 , 20 Here   the   method  demo()   is overloaded   3   times:   first   having   1   int   parameter,second   one   has  2   int   parameters  and   third one   is having   double arg. The   methods are   invoked   or   called   with   the   same   type and   number   of parameters  used. lecturer .miss .Ashna Nazm Hamasalh 35

36. Rules for Method Overloading 1 . Overloading can take place in the same class or in its sub-class. 2. Constructor in Java can be overloaded 3. Overloaded methods must have a different argument list. 4. Overloaded method   should always be the part of the same class  (can also take place in sub class), with same name but different parameters. 5. The parameters may differ in their type or number, or in both. 6. They may have the same or different return types. 7. It is also known as compile time polymorphism.     lecturer .miss .Ashna Nazm Hamasalh 36

37. 2) Method Overriding Child class has the same method as of base class. In such cases child class overrides the parent class method without even touching the source code of  the base  class. This feature is known as method overriding. lecturer .miss .Ashna Nazm Hamasalh 37

38. Example: public   class   BaseClass { public   void   methodToOverride ()   //Base   class   method { System . out .println   ( "I'm   the   method   of   BaseClass " ); }} lecturer .miss .Ashna Nazm Hamasalh 38 public   class   DerivedClass   extends   BaseClass { public   void   methodToOverride ()   //Derived   Class   method { System . out .println   ( "I'm   the   method   of   DerivedClass " ); } }

39. Output: I 'm   the   method   of   BaseClass I' m   the   method  of  DerivedClass public   class   TestMethod { public   static   void  main   ( String   args  [])   { //   BaseClass   reference   and   object BaseClass  obj1   =   new   BaseClass (); //   BaseClass   reference   but   DerivedClass   object BaseClass  obj2   =   new   DerivedClass (); //   Calls   the   method   from   BaseClass   class obj1.methodToOverride(); //Calls   the   method   from   DerivedClass   class obj2.methodToOverride ();}} lecturer .miss .Ashna Nazm Hamasalh 39

40. Rules for  Method  Overriding: 1 .  applies only   to  inherited   methods 2.  object   type (NOT   reference   variable   type)  determines  which   overridden method   will  be   used at   runtime 3.  Overriding   method  can   have   different   return   type  ( refer   this ) 4.  Overriding   method  must  not   have   more   restrictive   access  modifier 5.  Abstract   methods  must   be   overridden 6. Static and final methods cannot be overridden 7. Constructors cannot be overridden 8. It is also known as Runtime polymorphism. lecturer .miss .Ashna Nazm Hamasalh 40

41. super keyword in Overriding: When invoking a superclass version of an overridden method the super keyword is used. lecturer .miss .Ashna Nazm Hamasalh 41

42. Example: lecturer .miss .Ashna Nazm Hamasalh 42 class   Vehicle   { public   void   move   ()  { System . out .println   ( "Vehicles   are   used   for   moving   from   one   place   to   another " ); }} class   Car   extends   Vehicle   { public   void   move   ()  { super .   move   ();  //   invokes   the   super   class   method System . out .println   ( "Car   is   a   good   medium   of   transport   " ); }} public   class   TestCar   { public   static   void   main   ( String   args   []){ Vehicle   b =   new   Car  ();   //   Vehicle   reference   but   Car   object b.move   ();   //Calls   the   method   in   Car   class }}

43. Output: Vehicles   are   used   for  moving   from   one   place   to  another Car   is   a   good   medium   of  transport   lecturer .miss .Ashna Nazm Hamasalh 43

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