CS111: PROGRAMMING

LANGUAGE II

Lecture 4(a): Introduction to Inheritance Computer Science

Department

Lecture Contents

Dr. Amal Khalifa, 2014

What is Inheritance?

Super-class & sub class

The object class

Using extends keyword

@override keyword

OOP

Dr. Amal Khalifa, 2014

OOP - Inheritance

A class can extend another class, inheriting all its data

members and methods while redefining some of them

and/or adding its own.

Dr. Amal Khalifa, 2014

OOP - Inheritance

Inheritance represents the is a relationship between

data types (e.g. student/person)

Existing class is the superclass (more general)

New class is the subclass (more specific)

Java supports only single inheritance

each class is derived from exactly one direct

superclass.

Possible to Create hierarchy of classes

Dr. Amal Khalifa, 2014

Superclasses and Subclasses

Superclasses

“more

general”

subclasses

“more

specific.”

every

subclass

object is an

object of its

superclass

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The Object Class

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The Java class hierarchy begins with class Object

(in package java.lang)

Every class in Java directly or indirectly extends (or

“inherits from”) Object.

All classes in Java inherit directly or indirectly from

Object, so its 11 methods are inherited by all other

classes.

The Object Class

Dr. Amal Khalifa, 2014

The Object Class

Dr. Amal Khalifa, 2014

The Object Class

learn more about Object’s methods at: java.sun.com/javase-6/docs/api/java/lang/Object.html Or java.sun.com/docs/books/tutorial/java/IandI/ objectclass.html

Apply on Student class

Dr. Amal Khalifa, 2014

Does your class extends Object?

If you don’t explicitly specify which class a new class extends, the class extends Object implicitly.

What methods did you redefine (override)?

finalize()

toString()

equals()

To override a superclass method, a subclass must declare a method with the same signature as the superclass method or a compilation error occurs

@Override annotation

Programming Pitfalls!!

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Superclasses and Subclasses

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Inheritance issue

A subclass can inherit methods that it does not need or

should not have.

Even when a superclass method is appropriate for a

subclass, that subclass often needs a customized

version of the method.

The subclass can override (redefine) the superclass

method with an appropriate implementation.

An overrided subclass method can access the superclass method

using the keyword super and a dot (.) separator.

Book Ref.

working on Chap. 9

That’s all for today…..

Dr. Amal Khalifa, 2014