Java Classes and Objects
Java is a fully object-oriented programming (OOP) language, meaning
programs are built using classes and objects that encapsulate data and
behavior together. This approach mirrors real-world entities—for example, a
Car object has attributes like color and speed, and behaviors like drive or
brake.
What is a Java Class?
A class is a blueprint or template used to create objects. It
defines:
- Attributes (fields): data the object holds
-
Methods: actions the object can perform
Create a Class in Java
Use the class keyword to define a class. By convention:
- Class names start with an uppercase letter
- The filename must match the public class name
public class Book {
String title = "Java Basics";
}
Here, Book is a class with one attribute title.
What Is an Object in Java?
An object is an instance of a class. It represents a specific entity
created from the class blueprint.
Create and Use an Object
You create an object using the new keyword, then access its fields or
methods with the dot (.) operator.
public class Book {
String title = "Java Basics";
public static void main(String[] args) {
Book myBook = new Book(); //
create object
System.out.println(myBook.title); //
access attribute
}
}
Output
Java Basics
Creating Multiple Objects
A single class can produce many objects, each representing a different
instance.
public class Laptop {
String brand = "Dell";
public static void main(String[] args) {
Laptop laptop1 = new Laptop();
Laptop laptop2 = new Laptop();
System.out.println(laptop1.brand);
System.out.println(laptop2.brand);
}
}
Output
Dell
Dell
Each object has its own memory allocation, even if values are the
same.
Using Multiple Classes in Java
In real projects, classes are usually separated across files for better
organization. One class defines data and behavior, while another contains
the main() method to run the program.
File: Student.java
public class Student {
String name = "Anita";
int marks = 85;
}
File: SchoolApp.java
public class SchoolApp {
public static void main(String[] args) {
Student s = new Student();
System.out.println(s.name + " scored " +
s.marks);
}
}
Output
Anita scored 85
Compile and Run Multiple Classes
When classes are in separate files but the same folder:
javac Student.java
javac SchoolApp.java
java SchoolApp
