1. Programming With Java ICS201 University Of Ha’il1 Chapter 7 Inheritance

2. Programming With Java ICS201 University Of Ha’il2 Introduction  Inheritance is the process by which a new class is created from another class  The new class is called a derived class  The original class is called the base class  A derived class automatically has all the instance variables and methods that the base class has, and it can have additional methods and/or instance variables as well.  Inheritance is especially advantageous because it allows code to be reused, without having to copy it into the definitions of the derived classes.

3. Programming With Java ICS201 University Of Ha’il3 Introduction  The extends clause in a class declaration establishes an inheritance relationship between two classes. It has the following syntax: class ClassName2 extends ClassName1 { // body of the class }

4. Programming With Java ICS201 University Of Ha’il4 Derived Class (subclass)  A derived class, also called a subclass (or child class), is defined by starting with another already defined class, called a base class or superclass or parent class, and adding (and/or changing) methods, instance variables, and static variables  The derived class inherits all the public methods, all the public and private instance variables, and all the public and private static variables from the base class.  The derived class can add more instance variables, static variables, and/or methods.

5. Programming With Java ICS201 University Of Ha’il5 Example (Inheritance) //This example shows how to declare a simple class inheritance hierarchy class Grandparents{ } class Parents extends Grandparents{ } class Uncles extends Grandparents{ } class Sisters extends Parents{ } class Brothers extends Parents{ } class Cousins extends Uncles{ } class InheritanceHierarchy { public static void main( String[ ] a) { Grandparents Grandfather = new Grandparents(); System.out.println( "Instantiated Grandparents” ); Grandfather = new Parents(); System.out.println( "Instantiated Parents" ); Grandfather = new Sisters(); System.out.println( "Instantiated Sisters" ); Grandfather = new Brothers(); System.out.println( "Instantiated Brothers" ); Grandfather = new Uncles(); System.out.println( "Instantiated Uncles" ); Grandfather = new Cousins(); System.out.println( "Instantiated Cousins" ); }

6. Programming With Java ICS201 University Of Ha’il6 Example (Inheritance) Output: Instantiated Grandparents Instantiated Parents Instantiated Sisters Instantiated Brothers Instantiated Uncles Instantiated Cousins

7. Programming With Java ICS201 University Of Ha’il7 Inherited Members  A derived class automatically has all the instance variables, all the static variables, and all the public methods of the base class  Members from the base class are said to be inherited.  Definitions for the inherited variables and methods do not appear in the derived class  The code is reused without having to explicitly copy it, unless the creator of the derived class redefines one or more of the base class methods.

8. Programming With Java ICS201 University Of Ha’il8 Inheritance and Variables Inherit Variable and Methods  A class inherits the state (determined by variables) and behavior (determined by methods) defined by all of its super classes. Therefore an object has one copy of every instance variable of its own class and its super classes. Variable Hiding  If a variable of a class has a same name (type maybe different) as a superclass variable, in that case class variable hides the superclass variable.  You can access a hidden variable by using super keyword super.variable

9. Programming With Java ICS201 University Of Ha’il9 Example (Inheritance and variables) class C1 { static int x; } class C2 extends C1 { static String x; } class Cc { public static void main(String[] args) { C1 p = new C1(); p.x = 55; System.out.println( “p.x=" + p.x); C2 q = new C2(); q.x = "This is a String"; System.out.println( "q.x=" + q.x); } Output: p.x=55 q.x=This is a String

10. Programming With Java ICS201 University Of Ha’il10 Example (Inheritance and variables) class M100 { int x = 100; } class M200 extends M100 { int x = 200; void display() { System.out.println( "x=" + x); System.out.println( "super.x=" +super.x); } class SuperKeyword { public static void main(String[] args) { M200 m200 = new M200(); m200.display(); } Output: x=200 super.x=100

11. Programming With Java ICS201 University Of Ha’il11 Homework Write an application that illustrates how to access a hidden variable. Class G declares a static variable x. Class H extends G and declares an instance variable x. A display() method in H displays both of these variables.

12. Programming With Java ICS201 University Of Ha’il12 Method Overriding  Method overriding occurs when a class declares a method that has the same type signature as a method declared by one of its super classes.  When a method in a subclass overrides a method in a superclass, the method in the super class is hidden relative to the subclass object.  Method overriding is a very important capability because it forms the basis for run-time polymorphism (means one interface, multiple implementation).

13. Programming With Java ICS201 University Of Ha’il13 Method Overriding  The access permission of an overridden method can be changed from private in the base class to public (or some other more permissive access) in the derived class.  However, the access permission of an overridden method can not be changed from public in the base class to a more restricted access permission in the derived class.

14. Programming With Java ICS201 University Of Ha’il14 Method Overriding  Given the following method header in a base case: private void doSomething()  The following method header is valid in a derived class: public void doSomething()  However, the opposite is not valid  Given the following method header in a base case: public void doSomething()  The following method header is not valid in a derived class: private void doSomething()

15. Programming With Java ICS201 University Of Ha’il15 Note  Do not confuse overriding a method in a derived class with overloading a method name  When a method is overridden, the new method definition given in the derived class has the exact same number and types of parameters as in the base class.  When a method in a derived class has a different signature from the method in the base class, that is overloading.  Note that when the derived class overloads the original method, it still inherits the original method from the base class as well.

16. Programming With Java ICS201 University Of Ha’il16 Example (Method Overriding) class A1 { void hello() { System.out.println( "Hello from A1" ); } class B1 extends A1 { void hello() { System.out.println( "Hello from B1" ); } class C1 extends B1 { void hello() { System.out.println( "Hello from C1"); } class MethodOverriding { public static void main(String[] arg) { C1 obj = new C1(); obj.hello(); A1 a = new C1() ; a.hello(); } Output: Hello from C1

17. Programming With Java ICS201 University Of Ha’il17 The final Modifier  If the modifier final is placed before the definition of a method, then that method may not be redefined in a derived class.  If the modifier final is placed before the definition of a class, then that class may not be used as a base class to derive other classes.

18. Programming With Java ICS201 University Of Ha’il18 Example (Inheritance and Methods) class I1 { void hello( String s ) { System.out.println( "I1:"+ s ); } class J1 extends I1 { void hello( String s ) { super.hello(s); System.out.println( "J1:"+ s ); } class K1 extends J1 { void hello( String s ) { super.hello(s); System.out.println( "K1:"+ s ); } class SuperForMethod { public static void main( String[] ar ) { I1 obj; System.out.println( "Initiating I1" ); obj = new I1(); obj.hello( "morning" ); System.out.println( "Initiating J1" ); obj = new J1(); obj.hello( "noon" ); System.out.println( "Initiating K1" ); obj = new K1(); obj.hello( "evening" ); }

19. Programming With Java ICS201 University Of Ha’il19 Example (Inheritance and Methods) Output: Initiating I1 I1:morning Initiating J1 I1:noon J1:noon Initiating K1 I1:evening J1:evening K1:evening

20. Programming With Java ICS201 University Of Ha’il20 Exercise (Inheritance and Methods)  Write an application that illustrates how a method can invoke a superclass method. Class I2 is extended by J2. Class J2 is extended by K2. Each of these classes defines a getDescription() method that returns a string. That string includes a description of the class plus descriptions of each superclass. Instantiate each of these classes and invoke the getDescription() mehod.

21. Programming With Java ICS201 University Of Ha’il21 Inheritance and Methods  When we define a method with the same signature as a base class methods, we effectively override that base method. That means when we send the same message to base or subclass, the methods defined for that class will be executed. This is called overriding. This is what we do in C++ when we define a method as virtual.  To access the parent method we can use modifier super followed by the dot and the parent method name.  Super.parentMethod()

22. Programming With Java ICS201 University Of Ha’il22 The super Constructor  A derived class uses a constructor from the base class to initialize all the data inherited from the base class  In order to invoke a constructor from the base class, it uses a special syntax: public derivedClass(int p1, int p2, double p3) { super(p1, p2); instanceVariable = p3; }  In the above example, super(p1, p2); is a call to the base class constructor

23. Programming With Java ICS201 University Of Ha’il23 The super Constructor  A call to the base class constructor can never use the name of the base class, but uses the keyword super instead.  A call to super must always be the first action taken in a constructor definition.  An instance variable cannot be used as an argument to super.

24. Programming With Java ICS201 University Of Ha’il24 Example (Inheritance & Constructor) class S1 { int s1; S1( ) { System.out.println( "print S1" ) ; s1=1; } class T1 extends S1 { int t1; T1() { System.out.println( "print T1" ); t1=2; } class U1 extends T1 { int u1; U1( ) { System.out.println( "print U1" ); u1=3; } class InhAndConstructor { public static void main( String[ ] ar) { U1 u1 = new U1(); System.out.println( "u1.s1=" + u1.s1); System.out.println( "u1.t1=" + u1.t1); System.out.println( "u1.u1=" + u1.u1); } Output: print S1 print T1 print U1 u1.s1=1 u1.t1=2 u1.u1=3

25. Programming With Java ICS201 University Of Ha’il25 Example (Inheritance & Constructor) class S2 { int s2; S2( int s2) { this.s2 = s2; } class T2 extends S2 { int t2; T2( int s2, int t2) { super (s2); this.t2 = t2; } class U2 extends T2 { int u2; U2( int s2, int t2, int u2) { super (s2,t2); this.u2 = u2; } class SuperAndConstructor { public static void main( String[] arg) { U2 u2 = new U2(1,2,3); System.out.println( "u2.s2 =" + u2.s2); System.out.println( "u2.t2 =" + u2.t2); System.out.println( "u2.u2 =" + u2.u2); } Output: u2.s2 =1 u2.t2 =2 u2.t2 =3

26. Programming With Java ICS201 University Of Ha’il26 Inheritance & Final classes  final classes – A class can be defined as ‘final’ when we do not allow any further sub classing.  This can be done for security reasons, or to allow Java to perform some optimization in accessing methods. final class Polygon extends Shape { }

27. Programming With Java ICS201 University Of Ha’il27 Abstract Class  There are 3 possible modifiers that may precede the class keyword: abstractCannot be instantiated finalCannot be extended public Can be accessed by any other class. If this keyword is missing, access to the class is limited to the current package.

28. Programming With Java ICS201 University Of Ha’il28 Example (Abstract Class) abstract class Shape { void display() { } class Circle extends Shape { void display() { System.out.println( "Circle" ); } class Rectangle extends Shape { void display() { System.out.println( "Rectangle" ); } class Triangle extends Shape { void display() { System.out.println( "Triangle" ); } class AbstractClass { public static void main( String[] arg) { Shape s = new Circle(); s.display(); s = new Rectangle(); s.display(); s = new Triangle(); s.display(); } OUTPUT: Circle Rectangle Triangle

29. Programming With Java ICS201 University Of Ha’il29 Variable Modifiers  The possible modifiers that may precede the declaration of a variable are: final Is a constant private Can be accessed only by code in the same class protected Can be accessed only by code in a subclass or the same package public Can be accessed by any other class static Is not an instance variable

30. Programming With Java ICS201 Access Modifiers

31. Programming With Java ICS201 University Of Ha’il31 Polymorphism  Polymorphism (from the Greek, meaning "many forms", or something similar) is the quality that allows one name to be used for more than one (hopefully related) but technically different purpose.  Method overriding occurs when a class declares a method having same types of signature as a method by one of its super class.  Note: Super class reference can refer to objects of its own or its subclass.

32. Programming With Java ICS201 University Of Ha’il32 Polymorphism  Methods can be overloaded. A method has the same name as another member methods, but the type and/or the count of parameters differs. class Integer { float val; void add( int amount ) { val = val + amount; } void add( int num, int den) { val = float(num) / den; }