Classes and Objects

Learn the foundations of OOP in Java - define classes as blueprints and create objects (instances) from them.

Overview

Object-Oriented Programming (OOP) organises code around objects - bundles of data (fields) and behaviour (methods). A class is the blueprint that defines what data and behaviour an object will have. An object is a specific instance of a class. For example, Dog is a class. Each individual dog (Rex, Bella, Max) is an object of that class. All dogs have the same fields (name, age, breed) and behaviours (bark, eat), but each has its own specific values. Classes are the core building block of Java programs. Understanding them is the gateway to all advanced Java topics.

Key Points

•A class defines the structure (fields) and behaviour (methods) of a type
•An object is an instance of a class created with the new keyword
•Fields (instance variables) hold the state of each object
•Methods define what an object can do
•Each object has its own independent copy of instance fields
•Access with the dot operator: object.field or object.method()
•Constructors initialise new objects - covered in depth in J035

Examples

Defining a class and creating objects

// Usually in its own file: Dog.java
class Dog {
    // Fields (instance variables)
    String name;
    String breed;
    int age;

    // Method
    void bark() {
        System.out.println(name + " says: Woof!");
    }

    void info() {
        System.out.println(name + " is a " + age + "-year-old " + breed);
    }
}

public class Main {
    public static void main(String[] args) {
        // Create two Dog objects
        Dog rex = new Dog();
        rex.name = "Rex";
        rex.breed = "German Shepherd";
        rex.age = 3;

        Dog bella = new Dog();
        bella.name = "Bella";
        bella.breed = "Labrador";
        bella.age = 5;

        rex.bark();   // Rex says: Woof!
        bella.bark(); // Bella says: Woof!
        rex.info();   // Rex is a 3-year-old German Shepherd
    }
}

rex and bella are separate objects - each has its own name, breed, and age.

A more complete class with methods that use fields

class BankAccount {
    String owner;
    double balance;

    void deposit(double amount) {
        balance += amount;
        System.out.printf("Deposited $%.2f. Balance: $%.2f%n", amount, balance);
    }

    void withdraw(double amount) {
        if (amount > balance) {
            System.out.println("Insufficient funds!");
        } else {
            balance -= amount;
            System.out.printf("Withdrew $%.2f. Balance: $%.2f%n", amount, balance);
        }
    }

    void showBalance() {
        System.out.printf("%s's balance: $%.2f%n", owner, balance);
    }
}

public class Main {
    public static void main(String[] args) {
        BankAccount account = new BankAccount();
        account.owner = "Alice";
        account.balance = 1000.00;

        account.deposit(250.0);
        account.withdraw(100.0);
        account.showBalance();
    }
}

Methods can read and modify the object's own fields directly without parameters.

null - an object reference with no object

public class Main {
    public static void main(String[] args) {
        Dog myDog = null; // reference, no object yet

        myDog = new Dog(); // now it points to a Dog object
        myDog.name = "Buddy";
        myDog.bark();

        // myDog = null; // if you uncomment and call myDog.bark(), NullPointerException!
    }
}

A reference variable set to null points to nothing. Calling methods on null throws NullPointerException.

Common Mistakes

Calling methods or accessing fields on a null reference - always ensure the object is created (new) before use
Thinking all objects share one set of fields - each object has its own independent field values
Forgetting that class names should be PascalCase (Dog, BankAccount) while variable names use camelCase (myDog)

Note

Design classes around real-world concepts or logical entities in your domain. A good class name is a noun (Dog, Account, Product, Order). Methods are usually verbs (bark(), deposit(), calculate()).

Classes and Objects

Overview

Key Points

Examples

Common Mistakes

Note

Exercises

Knowledge Check

What is the relationship between a class and an object?