Inheritance in Java-Java Programminng-Lab Lecture, Lecture notes of Java Programming

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Inheritance in Java

Access Specifiers

 private keyword  Used for most instance variables  private variables and methods are accessible only to methods of the class in which they are declared  Declaring instance variables private is known as data hiding  public keyword  Exposes variables or methods outside the Class.  Declaring public methods is know as defining the class’ public interface.  Protected keyword  Provides limited access  Provides access to sub classes but not to the classes outside a package

Inheritance

 Inheritance allows a software developer to reuse classes by

deriving a new class from an existing one

 The existing class is called the parent class, or superclass, or

base class

 The derived class is called the child class or subclass.

 As the name implies, the child inherits characteristics of the

parent

 That is, the child class inherits the methods and data defined

for the parent class

Inheritance Relationship

 Inheritance should create an is-a relationship , meaning the

child is a more specific version of the parent

Class Hierarchy

 Good class design puts all common features as high in

the hierarchy as reasonable

 inheritance is transitive

 An instance of class Parrot is also an instance of Bird, an instance of Animal, …, and an instance of class Object

Class inheritance

 The inclusion of members of a base class in a derived class so that they are accessible in that derived class is called class inheritance.

 An inherited member of a base class is one that is accessible within the derived class.

 If a base class member is not accessible in a derived class, then it is not an inherited member of the derived class.

Inheriting Data Members

Hidden Data Members

 Variables of same name both in base class and derived

class

 Use of keyword super

Example

// A simple example of inheritance. // Create a superclass. class A { int i, j; void showij() { System.out.println("i and j: " + i + " " + j); } } // Create a subclass by extending class A. class B extends A { int k; void showk() { System.out.println("k: " + k); } void sum() { System.out.println("i+j+k: " + (i+j+k)); } }

class SimpleInheritance {

public static void main(String args[]) {

A superOb = new A();

B subOb = new B();

// The superclass may be used by itself.

superOb.i = 10;

superOb.j = 20;

System.out.println("Contents of superOb: ");

superOb.showij();

System.out.println();

OUTPUT

 The output from this program is shown here:

 Contents of superOb:

 i and j: 10 20

 Contents of subOb:

 i and j: 7 8

 k: 9

 Sum of i, j and k in subOb:

 i+j+k: 24

Example

public class Animal {

public Animal(String aType) {

type = new String(aType);

}

public String toString() {

return “This is a “ + type;

}

private String type;

}

Constructors of the Derived Class

public class Dog extends Animal { public Dog(String aName) { super(“Dog”); name = aName; breed = “Unknown”; } public Dog(String aName, String aBreed) { super(“Dog”); name = aName; breed = aBreed; } private String name; private String breed; }

Overriding a Base Class Method

public String toString() {

return “It’s “ + name + “ the “ + breed;