1. Session 2: Introduction to Object Oriented Programming
Java Training
Session 2: Introduction to Object Oriented Programming

2. Objectives Classes, Objects, Methods, Instance Variables
Instantiating an Object of a Class
Initializing Objects with Constructors
Methods with Parameters
Instance Variables, set Methods and get Methods
Primitive Types vs. Reference Types
Static
Object Vs. Class

3. Objects Inspired by real life. Look around you, everything is object.
Example:
Marker in my hand.
Computer
Table
Chair
etc…

4. Object (Cont.) So, What is an object?
Or what is the characteristics of an object?
In simplest possible term – An object is a collection of Fields/Properties/(State) and Method/(Behavior).
Let’s elaborate with example.

5. Illustration
Meto
Methods
Methods
Fields
Methods
Methods

6. A Pen Fields Method Height Radius Color Can open it’s cap Can write
Can close it’s cap

7. Car Fields Methods Dimensions Speed Gear Direction Number of Wheels
Number of Seats
Number of Doors
Methods
Open door
Start car
Accelerate
Break

8. Car Car Object!! openDoor start accelerate break changeDirection
decelerate
break
changeDirection
Car
Type
Wheels
Doors
Speed

9. Object in Action
So far we have understood that object has some fields and some methods.
To begin coding one more thing.. Constructor!!
Each java object have some special method. (we will elaborate on this in future, after understanding inheritance)
One of them is constructor.
It’s method that is called when a object is created( more java term is “instantiated” ).
Let’s see some codes now ..

10. Class: GradeBook, File: GradeBook.java
Instance Variables –
Note the naming convention
public class GradeBook { private String courseName; private String courseTeacher; public GradeBook(String name) courseName = name; } public void setCourseName(String name)
public String getCourseName() { return courseName; } public void displayMessage() System.out.println("CourseName is: " + courseName); public String getCourseTeacher() return courseTeacher;
Constructor
Public method -
Carefully note the naming convention

11. Class: GradeBookTest, File: GradeBookTest.java
public class GradeBookTest { public static void main(Stringargs[ ]) // Instantiate or create two GradeBook objects GradeBook gb1 = new GradeBook(“OOPL”); GradeBook gb2 = new GradeBook(“DS”); System.out.printf(“gb1 course name is: %s\n”, gb1.getCourseName()); gb1.setCourseName(“DLD”); System.out.printf(“gb1 course teacher is: %s\n”, gb1.getCourseTeacher()); // The above statement is trying to print something that is not initialized yet. }

12. Executing the Program To compile the source files type
javacGradeBook.javaGradeBookTest.java
Or, javac *.java
It produces two class files named GradeBook.class and GradeBookTest.class
To run the program type
java GradeBookTest
We used GradeBookTest as the argument to java as class GradeBookTest contains the “main” method.
If we use “java GradeBook” then the JVM will throw an ERROR (more dangerous than an EXCEPTION) and will terminate immediately.
Exception in thread “main” java.lang.NoSuchMethodError: main

13. Output of “java GradeBookTest”
The String courseTeacher is “null”.
But using a reference which is “null” will cause unexpected results
or exceptions in many cases

14. Some Words on Multiple Classes in the Same Directory
There is a special relationship between classes that are compiled in the same directory on disk.
By default, such classes are considered to be in the same package – known as the default package.
Classes in the same package are implicitly imported into the source code files of other classes in the same package.
Thus, an import declaration is not required when one class in a package uses another in the same package.

15. Primitive Types vs. Reference Types
Data types in Java are divided into two categories
Primitive types
boolean, byte, char, short, int, long, float and double
By default, all primitive-type instance variables are initialized to 0 (except boolean which is initialized to “false”).
Local primitive-type variables are not initialized by default.
Reference types
All nonprimitive types e.g. class variables
By default, all reference-type instance variables are initialized to the special value “null”.
“null” is a keyword in java.
Local reference-type variables are not initialized by default.
Java does not permit using a non-initialized variable before assigning a value to that variable.
intn; // local variable of a method, not member of any class
System.out.printf(“Value if n is: %d\n”, n); // compiler error
(Example provided in LocalTest.java)

16. Object Vs Class Class is the definition Object is the instance Example
It’s just the definition
No real value is attach
Object is the instance
An object is the entity in ram that contains data.
Example
We all belongs to human class, but each of us is a different human object.

17. Basic flow of class and object
Define Class
Instantiate
using “new”
Create object
Use the Object

18. Static!! Static fields and methods are unique to a class.
One class has one single memory allocation for a static field.
Static methods are accessed without instantiating the class.
Ex: System.out.println(“Hello Java”);
We will elaborate it later on.
Example File: StaticTest.java and StaticMainTest.java

19. this Reference
In a class there is always a reference present name this
Which points to current object.
Can’t use this in a static method as static methods are not related to a object.

20. Core Conception of OOP
Encapsulation
Polymorphism
Inheritance

21. Encapsulation
The most important thing software engineers want to achieve.
It is a mechanism for restricting access to some of the object's components.
Provide user methods to interact but user directly can not change the fields.

22. Encapsulation - Example
Say we have a class name Student which has a field name mark and a method name calculateGrade()
We will encapsulate the algorithm related to calculating grade depending on the method.
Example: StudentTest.java

23. Let’s try some examples
Write a class of Triangle.
Fields
Base
Height
Methods
Constructor, Getter and Setters
calculateArea

24. Let’s try some examples
Now take use input of base and height
Then create object depending on the user input.

25. Let’s try points Now write a class of Point Fields Methods X Y
Constructor, Getter and Setters
Print() // just print the coordinates
distance(Point) //calculate the distance between this point the provided point

26. Back to triangle Add a method name compare(Triangle)
That will compare the area of this triangle and the provided triangle.

27. Few things
JRE – the package containing only JVM and related software. Can’t compile java codes with a jre.
JDK – the package containing software to run and compile java code.
Java Doc – All class function’s detail is provided in java doc format.
Java Tutorial – Official java tutorial from oracle.