JAVA: Inheritance

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Web Design & Development CS506

Lesson 5

Inheritance

In general, inheritance is used to implement a "is-a" relationship. Inheritance saves code rewriting for a

client thus promotes reusability.

In java parent or base class is referred as super class while child or derived class is known as sub

class.

Comparison with C++

Java only supports single inheritance. As a result a class can only inherit from one class at one

time.

Keyword extends is used instead of ":" for inheritance.

All functions are virtual by default

All java classes inherit from Object class (more on it later).

To explicitly call the super class constructor, use super keyword. It's important to remember that

call to super class constructor must be first line.

Keyword super is also used to call overridden methods.

Example Code: using inheritance

We'll use three classes to get familiar you with inheritance. First one is Employee class. This will act

as super class. Teacher class will inherit from Employee class and Test class is driver class that contains

main method. Let's look at them one by one

class Employee{

protected int id;

protected String name;

//parameterized constructor

public Employee(int id, String name){

this.id = id;

this.name = name;

}

//default constructor

public Employee(){

// calling parameterized constructor of same (Employee)

// class by using keyword this

this (10, "not set");

}

//setters

public void setId (int id) {

this.id = id;

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}

public void setName (String name) {

this.name = name;

}

//getters

public int getId () {

return id;

}

public String getName () {

return name;

}

// displaying employee object on console

public void display(){

System.out.println("in employee display method");

System.out.println("Employee id:" + id + " name:" + name);

}

//overriding object's class toString method

public String toString() {

System.out.println("in employee toString method");

return "id:" + id + "name:" + name;

}

}//end class

The Teacher class extends from Employee class. Therefore Teacher class is a subclass of

Employee. The teacher class has an additional attribute i.e. qualification.

class Teacher extends Employee{

private String qual;

//default constructor

public Teacher () {

//implicit call to superclass default construct

qual = "";

}

//parameterized constructor

public Teacher(int i, String n, String q){

//call to superclass param const must be first line

super(i,n);

qual = q;

}

//setter

public void setQual (String qual){

this.qual = qual;

}

//getter

public String getQual(){

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return qual;

}

//overriding display method of Employee class

public void display(){

System.out.println("in teacher's display method");

super.display(); //call to superclass display method

System.out.println("Teacher qualification:" + qual);

}

//overriding toString method of Employee class

public String toString() {

System.out.println("in teacher's toString method");

String emp = super.toString();

return emp +" qualification:" + qual;

}

}//end class

Objects of Employee & Teacher class are created inside main method in Test class. Later calls are made

to display and toString method using these objects.

class Test{

public static void main (String args[]){

System.out.println("making object of employee");

Employee e = new Employee(89, "khurram ahmad");

System.out.println("making object of teacher");

Teacher t = new Teacher (91, "ali raza", "phd");

e.display(); //call to Employee class display method

t.display(); //call to Teacher class display method

// calling employee class toString method explicitly

System.out.println("Employee: " +e.toString());

// calling teacher class toString implicitly

System.out.println("Teacher: " + t);

} //end of main

}//end class

Output

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Object The Root Class

The Od Java classes. For user defined classes, its not necessary to mention the Object class as a

super class, java doesbject class in Java is a superclass for all other classes defined in Java's class

libraries, as well as for user-define it automatically for you.

The class Hierarchy of Employee class is shown below. Object is the super class of Employee

class and Teacher is a subclass of Employee class. We can make another class Manager that can also

extends from Employee class.

Object

Employe

Teacher

Manager

Polymorphism

"Polymorphic" literally means "of multiple shapes" and in the context of OOP, polymorphic

means "having multiple behavior".

A parent class reference can point to the subclass objects because of is-a relationship. For example a

Employee reference can point to:

o Employee Object

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o Teacher Object

o Manager Object

A polymorphic method results in different actions depending on the object being referenced

o Also known as late binding or run-time binding

Example Code: using polymorphism

This Test class is the modification of last example code. Same Employee & Teacher classes are

used. Objects of Employee & Teacher class are created inside main methods and calls are made to

display and toString method using these objects.

class Test{

public static void main (String args[]){

// Make employee references

Employee ref1, ref2;

// assign employee object to first employee reference

ref1 = new Employee(89, "khurram ahmad");

// is-a relationship, polymorphism

ref2 = new Teacher (91, "ali raza", "phd");

//call to Employee class display method

ref1.display();

//call to Teacher class display method

ref2.display();

// call to Employee class toString method

System.out.println("Employee: " +ref1.toString());

// call to Teacher class toString method

System.out.println("Teacher: " + ref2.toString());

} //end of main

}//end class

Output

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Type Casting

In computer science, type conversion or typecasting refers to changing an entity of one datatype into

another. Type casting can be categorized into two types

1. Up-casting

Converting a smaller data type into bigger one

Implicit we don't have to do something special

No loss of information

Examples of

-- Primitives

int a = 10;

double b = a;

-- Classes

Employee e = new Teacher( );

2. Down-casting

Converting a bigger data type into smaller one

Explicit need to mention

Possible loss of information

Examples of

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-- Primitives

double a = 7.65;

int b = (int) a;

-- Classes

Employee e = new Teacher( ); // up-casting

Teacher t = (Teacher) e; // down-casting

References:

Java tutorial: http://java.sun.com/docs/books/tutorial/java/javaOO/

Stanford University

Example code, their explanations and corresponding figures for handout 5-1,5-2 are taken from

the book JAVA A Lab Course by Umair Javed. This material is available just for the use

of VU students of the course Web Design and Development and not for any other

commercial purpose without the consent of author.

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