Classes and Objects

Reading Time: 9 Minutes
Difficulty: Beginner

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Topic Summary

A class is a blueprint that describes what something looks like and what it can do. An object is an actual thing built from that blueprint. In Java, you define classes and then create as many objects as you want from them — each object gets its own copy of the data.

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What You'll Learn

• How to write a class in Java with fields and methods
• How to create objects using the new keyword
• How to access fields and methods on an object
• How multiple objects from the same class work independently

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Prerequisites

• Introduction to OOP (Lesson 01)
• Basic Java syntax (variables, methods)

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Explanation

Anatomy of a Class

A class in Java has three main parts:

1. Fields (instance variables) — the data the object stores
2. Constructors — special methods to create the object (next lesson)
3. Methods — the behaviors the object can perform

Here is the general syntax:

class ClassName {
    // 1. Fields
    dataType fieldName;

    // 2. Methods
    returnType methodName() {
        // body
    }
}

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Fields (Instance Variables)

Fields are variables declared inside a class but outside any method. Every object gets its own private copy of these fields.

class Student {
    String name;    // field
    int age;        // field
    double gpa;     // field
}

These are called instance variables because each instance (object) of the class holds its own values for them.

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Methods

Methods define what the object can do. They live inside the class body.

class Student {
    String name;
    int age;
    double gpa;

    // method — behavior of a Student object
    void introduce() {
        System.out.println("Hi, I'm " + name + ", age " + age);
    }

    void study() {
        System.out.println(name + " is studying hard!");
    }
}

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Creating Objects with new

To create an object from a class, use the new keyword:

Student s1 = new Student();

Breaking this down:

• Student — the type (which class to use as blueprint)
• s1 — the variable name (reference to the object)
• new Student() — actually creates the object in memory

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Accessing Fields and Methods

Use the dot (.) operator to access an object's fields and methods:

// Setting fields
s1.name = "Alice";
s1.age = 16;
s1.gpa = 3.9;

// Calling methods
s1.introduce();
s1.study();

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Multiple Objects from the Same Class

You can create as many objects as you want. Each object is independent — changing one doesn't affect another.

Student s1 = new Student();
s1.name = "Alice";
s1.age = 16;

Student s2 = new Student();
s2.name = "Bob";
s2.age = 17;

// Alice and Bob are completely separate objects
s1.introduce(); // Hi, I'm Alice, age 16
s2.introduce(); // Hi, I'm Bob, age 17

Think of it like this: you have one Student mold (class), but you can stamp out Alice, Bob, Charlie — as many individual students (objects) as you need.

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Object Reference vs Object

An important concept: the variable s1 doesn't hold the object — it holds a reference (like a pointer/address) to where the object lives in memory.

Student s1 = new Student();
Student s2 = s1;  // s2 now points to the SAME object as s1!

s1.name = "Alice";
System.out.println(s2.name); // prints "Alice" — same object!

This is different from primitive types like int, which store the actual value.

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The null Reference

Before you assign an object, a reference variable holds null — meaning it points to nothing.

Student s1 = null; // s1 points to nothing
s1.introduce();    // NullPointerException! Can't call method on null

Always make sure your reference points to an actual object before using it.

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Real-World Analogy

Think of a Student Enrollment Form at a school.

• The blank form template = the Class (it defines what information goes where: name, age, grade)
• Each filled-in form for a real student = an Object (Alice's form, Bob's form)
• Each form has its own information, but they all follow the same structure

The school can have thousands of filled forms (objects), all based on the same blank template (class).

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Code Example

// Defining the Student class (blueprint)
class Student {
    // Fields (instance variables)
    String name;
    int age;
    String subject;
    double gpa;

    // Method: introduce the student
    void introduce() {
        System.out.println("Hello! I'm " + name + ", " + age + " years old.");
        System.out.println("I study " + subject + " and my GPA is " + gpa);
    }

    // Method: check if student passed
    void checkResult() {
        if (gpa >= 2.0) {
            System.out.println(name + " has PASSED!");
        } else {
            System.out.println(name + " has FAILED. Study harder!");
        }
    }
}

public class Main {
    public static void main(String[] args) {
        // Creating Object 1
        Student alice = new Student();
        alice.name = "Alice";
        alice.age = 16;
        alice.subject = "Computer Science";
        alice.gpa = 3.9;

        // Creating Object 2
        Student bob = new Student();
        bob.name = "Bob";
        bob.age = 17;
        bob.subject = "Mathematics";
        bob.gpa = 1.8;

        // Creating Object 3
        Student charlie = new Student();
        charlie.name = "Charlie";
        charlie.age = 15;
        charlie.subject = "Physics";
        charlie.gpa = 3.2;

        // Using each object independently
        alice.introduce();
        alice.checkResult();
        System.out.println("---");

        bob.introduce();
        bob.checkResult();
        System.out.println("---");

        charlie.introduce();
        charlie.checkResult();
    }
}

Output

Hello! I'm Alice, 16 years old.
I study Computer Science and my GPA is 3.9
Alice has PASSED!
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Hello! I'm Bob, 17 years old.
I study Mathematics and my GPA is 1.8
Bob has FAILED. Study harder!
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Hello! I'm Charlie, 15 years old.
I study Physics and my GPA is 3.2
Charlie has PASSED!

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Common Mistakes

• ❌ Mistake: Forgetting new when creating an object (Student s1 = Student()) → ✅ Fix: Always use new: Student s1 = new Student()
• ❌ Mistake: Accessing a field before setting it (getting default values like 0 or null) → ✅ Fix: Always set field values after creating the object, or use a constructor (next lesson)
• ❌ Mistake: Thinking two references are two objects (Student s2 = s1 is NOT a new object) → ✅ Fix: Use new to create a truly new, separate object

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Best Practices

• Name your class with a capital first letter and use a singular noun: Student, not students or student
• Keep related fields and methods together in the same class
• Don't create giant classes — if a class does too many things, split it up
• Use meaningful field names: studentName is better than n

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Interview Questions

Q: What is the difference between a class and an object in Java?
A: A class is a blueprint/template that defines fields and methods. An object is an actual instance created from that blueprint using the new keyword. A class exists once in code; objects can be created many times from it.

Q: What is an instance variable?
A: An instance variable (or field) is a variable declared inside a class but outside any method. Each object (instance) of the class gets its own separate copy of instance variables.

Q: What is a NullPointerException?
A: A NullPointerException occurs when you try to access a field or call a method on a reference that is null (not pointing to any object). It's one of the most common runtime errors in Java.

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Quick Revision

✔ A class is a blueprint with fields (data) and methods (behavior)
✔ An object is created from a class using the new keyword
✔ Use the dot operator (.) to access fields and methods on an object
✔ Each object has its own independent copy of instance variables
✔ A reference variable stores the address of an object, not the object itself

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Related Topics

• Constructors
• this Keyword
• Encapsulation

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Next Lesson

03 - Constructors