The lecture introduces object-oriented programming (OOP) concepts, particularly focusing on access control modifiers such as private, package, protected, and public. It explains the relationship between classes and objects, including how objects are instances of classes and can access class members. Additionally, it covers the purpose of constructors in Java for initializing objects and demonstrates how constructors can take parameters.

1. Object-Oriented Programming
Dr. Riaz UlAmin
Lecture : Introduction to OOP

2. More about field modifiers (1)
 Access control modifiers
– private: private members are accessible only in the class
itself
– package: package members are accessible in classes in the
same package and the class itself
– protected: protected members are accessible in classes in
the same package, in subclasses of the class, and in the
class itself
– public: public members are accessible anywhere the class
is accessible

3. public class Pencil {
public String color = “red”;
public int length;
public float diameter;
private float price;
public static long nextID = 0;
public void setPrice (float newPrice) {
price = newPrice;
}
}
public class CreatePencil {
public static void main (String args[]){
Pencil p1 = new Pencil();
p1.price = 0.5f;
}
}
Pencil.java
CreatePencil.java
%> javac Pencil.java
%> javac CreatePencil.java
CreatePencil.java:4: price has private
access in Pencil

6.  An object is an instance of a class.
 A class is a template or blueprint from
which objects are created. So, an object is
the instance(result) of a class.

10. Access Members of a Class
 We can use the name of objects along with
the . operator to access members of a class.
For example,

12.  In the above example, we have created a class named
Bicycle. It includes a field named gear and a method
named braking(). Notice the statement,
 Bicycle sportsBicycle = new Bicycle(); Here, we have
created an object of Bicycle named sportsBicycle. We then
use the object to access the field and method of the class.
 sportsBicycle.gear - access the field gear
 sportsBicycle.braking() - access the method braking()

15. Constructors
 A constructor in Java is a special method
that is used to initialize objects. The
constructor is called when an object of a
class is created.

17. Constructor Parameters
 Constructors can also take parameters,
which is used to initialize attributes.
 The following example adds an int y
parameter to the constructor. Inside the
constructor we set x to y (x=y). When we
call the constructor, we pass a parameter to
the constructor (5), which will set the value
of x to 5: