Classes & Objects

Object-Oriented Programming (OOP) is at the heart of Java. Understanding classes and objects is essential for writing Java programs. A class is a blueprint for creating objects, while an object is an instance of a class. In this lesson, you'll learn how to define classes, create objects, work with instance variables, and understand object references.

What is a Class?

A class is a template or blueprint that defines the properties (attributes) and behaviors (methods) that objects of that class will have. Think of a class as a cookie cutter and objects as the cookies made from it.

Defining a Class

To define a class in Java, use the class keyword followed by the class name. Class names should be in PascalCase.

Click Run to execute your code

In this example:

• class Student - Defines a class named Student
• String name; and int age; - Instance variables (attributes)
• Each object created from this class will have its own name and age

Creating Objects

To create an object (instance) from a class, use the new keyword followed by the class constructor. This process is called instantiation.

Click Run to execute your code

Key points about object creation:

• new Student() - Creates a new object instance
• student1 and student2 - Object references (variables that hold references to objects)
• Each object has its own copy of instance variables
• You can create multiple objects from the same class

Instance Variables

Instance variables are variables declared inside a class but outside any method. Each object has its own copy of these variables.

public class Car {
    // Instance variables
    String brand;      // Each Car object has its own brand
    String color;      // Each Car object has its own color
    int speed;         // Each Car object has its own speed
    
    // Methods can access instance variables
    void displayInfo() {
        System.out.println(brand + " " + color + " - Speed: " + speed);
    }
}

Characteristics of instance variables:

• Each object has its own copy
• Initialized to default values (0, null, false) if not explicitly set
• Accessible from any method in the class
• Exist as long as the object exists

Object References

In Java, object variables store references (memory addresses), not the objects themselves. This means multiple variables can reference the same object.

Student student1 = new Student();
student1.name = "Alice";
student1.age = 20;

Student student2 = student1;  // student2 now references the same object

System.out.println(student2.name);  // Prints "Alice"
student2.name = "Bob";
System.out.println(student1.name);  // Prints "Bob" (same object!)

The null Keyword

The null keyword represents an absence of an object reference. A reference variable that is null doesn't point to any object.

Student student = null;  // Reference variable doesn't point to any object

// Trying to use a null reference causes NullPointerException
// student.name = "Alice";  // ERROR: NullPointerException

// Always check for null before using
if (student != null) {
    student.name = "Alice";
} else {
    System.out.println("Student object is null");
}

Class vs Object Visualization

Understanding the relationship between classes and objects is crucial. Below is a visual representation:

The diagram shows how one class (the blueprint) can create multiple objects (instances), each with its own set of instance variables.

Common Mistakes

// Wrong - trying to access instance variables from class
Student.name = "Alice";  // Error! name is not static

// Correct - create an object first
Student student = new Student();
student.name = "Alice";

// Wrong - can cause NullPointerException
Student student = null;
System.out.println(student.name);  // Runtime error!

// Correct - check for null first
Student student = null;
if (student != null) {
    System.out.println(student.name);
}

// Wrong - doesn't create an object
Student student;
student.name = "Alice";  // Error! student is null

// Correct - use 'new' to create object
Student student = new Student();
student.name = "Alice";

// Wrong assumption - thinking this creates a copy
Student student1 = new Student();
student1.name = "Alice";
Student student2 = student1;  // Both reference the same object!
student2.name = "Bob";
System.out.println(student1.name);  // Prints "Bob", not "Alice"!

// Correct - create separate objects if you need copies
Student student1 = new Student();
student1.name = "Alice";
Student student2 = new Student();  // New object
student2.name = "Bob";

Exercise: Create a Book Class

Click Run to execute your code

public class Book {
    String title;
    String author;
    int pages;
    
    void displayInfo() {
        System.out.println("Title: " + title);
        System.out.println("Author: " + author);
        System.out.println("Pages: " + pages);
    }
}

public class Exercise {
    public static void main(String[] args) {
        Book book1 = new Book();
        book1.title = "The Java Handbook";
        book1.author = "John Doe";
        book1.pages = 350;
        
        Book book2 = new Book();
        book2.title = "Learning Java";
        book2.author = "Jane Smith";
        book2.pages = 280;
        
        System.out.println("Book 1:");
        book1.displayInfo();
        
        System.out.println("\nBook 2:");
        book2.displayInfo();
    }
}

What's Next?

Now that you understand classes and objects, the next lesson covers Constructors, which are special methods used to initialize objects when they're created. Constructors allow you to set initial values for your objects efficiently.