This document discusses Java classes, objects, and constructors. It explains that a class contains instance variables and methods, and defines common behaviors for objects. An object is instantiated from a class using the new reserved word and can then access the class's instance variables and methods. The document provides an example of creating a Java class with an instance variable and method to demonstrate how objects are made from classes and can then access class members.

1. JAVA CLASSES, OBJECTS AND
CONSTRUCTORS
JAVA LECTURE
BY: LJRABOY

2. Object-Oriented Approach
• Class
– Instance variables – attributes of an object
– Methods – functions that manipulates objects
• Are subprograms
• Divide-and-conquer model
• One large programs – subdivide the program into smaller
subprograms
• Process needed in the program
• Methods are usually named using a action words such as
computeSum(), setValueOfX(), getValueOfY() and etc
– A user-defined method can be used after the declaration
of main() as a static method.
– Static method – does not need an object to access its
methods and instance variables.

3. Common example of methods
• main()
• System.out.println();
• JOptionPane.showMessageDialog();
• String str = JOptionPane.showInputDialog();
• Int y = Integer.parseInt(str);

4. The Class
• The class contains both the variables and its methods
• Class is represented by its own filename
• example of a declaration:
public class MyClass{
public static void main(String args *[])
{
}
}

5. The Class (cont)
• Is a general category of an object
• it contains the very common behavior of an
object.
• A class does not necessarily needs main()
because a class can be used in terms of using
objects. Together with its instance variables
and instance methods.

6. The Object
• Instantiate an object using a new reserved
word.
• So all the instance variables and instance
methods of the class can now be accessed by
an objects.
• This instance variables and methods can be
accessed using the dot “.” operator.
ObjectName.instance_Variable
ObjectName.instance_method()

7. Getting Started
• Creating a class
– Instance variables
– Instance methods

8. Example:
• Start with creating a new Java Basic
Application
• Name your class:
– Example: MyClassMain
– Select File, New  File

9. Select Java Class

10. Provide another filename for your
class

11. Two files for the project

12. Inside MyClassMain class

13. Inside MyOwnClass class
• Public  access modifier ,
can be accessed anywhere
in the package
• Reserved word class
• MyOwnClass  User-
defined class name
• public MyOwnClass(){
}  constructor
Constructor – used for
initialization

14. Modifying MyOwnClass

15. MyClassMain class

16. Format of using a method
public return_type methodname(parameters)
{
// body of the method
}
Where:
public is an access modifier
Return_type – any primitive type/class type
Methodname – user-defined method name
Parameters – optional, values to be passed to the method