Classes and objects

Why this matters

This is the single most important idea in OOP. Everything else — inheritance, polymorphism, abstraction — sits on top of it.

Syntax

class ClassName {
    dataType field;          // state
    returnType method() { }  // behaviour
}
ClassName obj = new ClassName();

Example

class Student {
    String name;
    int rollNumber;

    void display() {
        System.out.println(rollNumber + " - " + name);
    }
}

public class Main {
    public static void main(String[] args) {
        Student s1 = new Student();
        s1.name = "Ananya Rao";
        s1.rollNumber = 12;

        Student s2 = new Student();
        s2.name = "Arjun";
        s2.rollNumber = 15;

        s1.display();
        s2.display();
    }
}

Step by step

1. 1`class Student` defines a new type with two fields and one method.
2. 2`new Student()` asks the JVM for memory and builds one object.
3. 3`s1` is a reference variable that points to that object.
4. 4`s1.name = "Ananya Rao";` sets the field of that particular object.
5. 5s1 and s2 are separate objects, each with its own copy of the fields.
6. 6`s1.display()` runs the method using s1's data.

Key points

• Class = blueprint, object = instance of that blueprint
• Fields hold state, methods define behaviour
• new creates the object in memory
• Each object has its own copy of instance fields

Common mistakes

• Forgetting `new` and getting a NullPointerException
• Thinking a class occupies memory by itself — objects do
• Putting two public classes in one file

Try it yourself

• Create a Book class with title, author and price, then print two books.
• Add a method to Student that prints whether the student passed.