Object-oriented programming explained for students

Classes and objects

A class is a blueprint. An object is one thing built from it. A Car class says every car has a colour and can drive; myCar and yourCar are two objects with their own colours.

public class Car {
    private String colour;

    public Car(String colour) {
        this.colour = colour;
    }

    public void drive() {
        System.out.println("The " + colour + " car drives");
    }
}

// elsewhere
Car myCar = new Car("red");
myCar.drive();   // The red car drives

Pillar 1: encapsulation

Keep fields private and expose safe methods. The class controls its own data, so it can reject bad values.

public class BankAccount {
    private double balance;

    public void deposit(double amount) {
        if (amount <= 0) throw new IllegalArgumentException("Amount must be positive");
        balance += amount;
    }

    public double getBalance() {
        return balance;
    }
}

Pillar 2: inheritance

A subclass reuses and extends a parent class with extends. A Dog is an Animal, so it gets the animal's fields and methods.

Pillar 3: polymorphism

The same method call behaves differently depending on the actual object. A subclass overrides a method, and Java picks the right version at run time.

class Animal {
    void speak() { System.out.println("..."); }
}

class Dog extends Animal {
    @Override
    void speak() { System.out.println("Woof"); }
}

// elsewhere
Animal a = new Dog();
a.speak();   // Woof

Pillar 4: abstraction

Show what an object does and hide how it does it. Abstract classes and interfaces describe the actions; concrete classes fill in the details.

interface Shape {
    double area();
}

class Circle implements Shape {
    private final double r;

    Circle(double r) { this.r = r; }

    public double area() { return Math.PI * r * r; }
}

Common misunderstandings

• A class is not an object; new creates the object.
• Inheritance is not the only way to reuse code; composition (has-a) is often better.
• Overloading (same name, different parameters) is not the same as overriding.
• Making every field public breaks encapsulation even if you add getters.

• What is OOP
• OOP interview questions in Java
• Object-oriented programming course
• Java programming course