OOP using Java

1. Constructors
Lecture 8
Asif Shahzad, Assistant Professor
COMSATS University Islamabad, Lahore Campus

3. • It’s a special method that is called at the time of
object construction/instantiation.
• Constructors are generally used to initialize
instance attributes.
• Constructor name match with class name, they do
not return any value. They mostly have ‘public’
access modifier but its not a requirement.
• Demo: Initialize id and name of Person object.
Introduction to Constructors. Continue..

4. • If you have deployed a policy on modification, you
should call set method from the constructor, to
enforce that policy at the time of object
construction.
• We can change attributes values later too, using
set methods or by direct access (depending how
you have defined attributes).
• To enforce certain attributes are initialized at the
time of object construction, make them private
and initialize in constructors.
Introduction to Constructors

5. Constructor Overloading
• We can define multiple constructors inside a class,
its called constructor overloading.
• By constructor overloading, we let others to
create object of our class in multiple ways.
• We can call constructors from each others to
avoid code duplication.
• Demo: Constructor in Person that initialize id and
name. Another that initialize id only.

6. Method Overloading
• Its not only the constructors that can be
overloaded. We can defined multiple methods of
same name in a class but they must be different
by:
• Number of parameters
• Type of parameters
• Order of parameters
• The return type of such method is kept same,
generally*

7. Default Constructor
• Its not mandatory do define constructors in a class,
when we don’t define a constructor, compiler
automatically provide default constructor.
• Default constructor is no-argument constructor.
• Default constructor is not provided when we define a
custom constructor. If you want to create an object
passing no arguments, we must define a no-argument
constructor.
• When we do not add no-argument constructor
ourselves, actually we are enforcing a policy to create an
object that must pass the asked data.

8. Initializing attributes in Constructor
• Generally no-argument constructor have empty
body, but its not mandatory. You can initialize
attributes inside constructor: for example:
public Player(){
this.luckyNumber = new Random().nextInt(10);
}
• Its not mandatory to initialize only those
attributes that object has received in constructor
parameters. You can initialize others attributes
too, when needed.