Advanced Java - Lec #5 - Inheritance and Polymorphism.pptx

1
In the name of Allah
Ghalib University
Computer Science Faculty
General Department
Advanced Java
Lecture Title
Inheritance and Polymorphism
Subject: Advanced Java
Instructor: Saddiqi
Date: Saturday, July 5, 2025
Lecture #5

List of Contents
• Introduction
• Super-classes and Sub-classes
• Using the super Keyword
• The Object Class and Its toString() Method
• Polymorphism
• The Object’s equals Method
• Summary
Saturday, July 5, 2025 Instructor: Saddiqi- Ghalib University 2

Introduction
• Object-oriented programming allows you to define new classes from
existing classes. This is called inheritance.
• Inheritance is an important and powerful feature for reusing
software.
• Suppose you need to define classes to model circles, rectangles, and
triangles. These classes have many common features.
• What is the best way to design these classes so as to avoid redundancy
and make the system easy to comprehend and easy to maintain?
• The answer is to use inheritance.

Super-classes and Sub-classes
• Inheritance enables you to define a general class (i.e., a superclass)
and later extend it to more specialized classes (i.e., subclasses).
• In Java terminology, a class C1 extended from another class C2 is
called a subclass, and C2 is called a superclass.
• A superclass is also referred to as a parent class or a base class.
• A subclass as a child class, an extended class, or a derived class.
• A subclass inherits accessible data fields and methods from its
superclass and may also add new data fields and methods.

Cont.
• The Circle class (Listing 11.2) extends the GeometricObject class
(Listing 11.1) using the following syntax:
• The keyword extends (lines 1–2) tells the compiler that the Circle
class extends the GeometricObject class, thus inheriting the methods
getColor, setColor, isFilled, setFilled, and toString.

Using the super Keyword
• The keyword super refers to the superclass and can be used to invoke
the superclass’s methods and constructors.
• It can be used in two ways:
• To call a superclass constructor.
• To call a superclass method.
• The syntax to call a superclass’s constructor is:
• The syntax to call a superclass’s method is:

The Object Class and Its toString() Method
• Every class in Java is descended from the Object class.
• If no inheritance is specified when a class is defined, the superclass
of the class is Object by default.
• For example, the following two class definitions are the same:

Cont.
• It is important to be familiar with the methods provided by the Object
class so that you can use them in your classes.
• Here, we introduce the toString method in the Object class.
• The signature of the toString() method is:
• Invoking toString() on an object returns a string that describes the
object.

Cont.
• By default, it returns a string consisting of
• a class name of which the object is an instance,
• an at sign (@),
• and the object’s memory address in hexadecimal.
• For example, consider the following code for the Loan class:
• The output for this code displays something like Loan@15037e5.
• This message is not very helpful or informative. Usually you should override the
toString method so that it returns a descriptive string representation of the object.

Cont.
• For example, the toString method in the Object class was overridden
in the GeometricObject class in lines 46–49 in Listing 11.1 as
follows:

The Object’s equals Method
• Like the toString() method, the equals(Object) method is another useful
method defined in the Object class.
• Its signature is
• This method tests whether two objects are equal. The syntax for invoking
it is:
• The default implementation of the equals method in the Object class is:

Polymorphism
• Polymorphism means that a variable of a supertype can refer to a
subtype object.
• A class defines a type. A type defined by a subclass is called a
subtype, and a type defined by its superclass is called a supertype.
• Therefore, you can say that Circle is a subtype of GeometricObject
and GeometricObject is a supertype for Circle.

Summary
• Inheritance is an important and powerful feature for reusing
software.
• A superclass is also referred to as a parent class or a base class.
• A subclass as a child class, an extended class, or a derived class.
• The super keyword can be used in two ways:
• To call a superclass constructor.
• To call a superclass method.
• Every class in Java is descended from the Object class.
• Polymorphism means that a variable of a supertype can refer to a
subtype object.

The End
Thank You

Advanced Java - Lec #5 - Inheritance and Polymorphism.pptx

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