Lesson 12 – Constructors

A constructor is a special method that runs automatically when an object is created. It is used to set initial values for object variables.

1. Default Constructor (No Parameters)

class Car {
    Car() {
        System.out.println("A new car object has been created!");
    }
}

public class Main {
    public static void main(String[] args) {
        Car myCar = new Car();  
    }
}

The constructor runs immediately after new Car() is called.

2. Constructor With Parameters

class Student {
    String name;
    int age;

    Student(String n, int a) {
        name = n;
        age = a;
    }
}

public class Main {
    public static void main(String[] args) {
        Student s1 = new Student("Khalid", 25);
        Student s2 = new Student("Ayesha", 23);

        System.out.println(s1.name + " - " + s1.age);
        System.out.println(s2.name + " - " + s2.age);
    }
}

This constructor allows setting values when creating the object.

3. Using the `this` Keyword

class Book {
    String title;
    double price;

    Book(String title, double price) {
        this.title = title;
        this.price = price;
    }
}

public class Main {
    public static void main(String[] args) {
        Book b = new Book("Java Made Easy", 49.99);

        System.out.println(b.title + " - $" + b.price);
    }
}

this.title refers to the class variable, while title on the right side is the parameter.

4. Multiple Constructors (Constructor Overloading)

class Player {
    String name;
    int score;

    Player() {
        name = "Unknown";
        score = 0;
    }

    Player(String n) {
        name = n;
        score = 0;
    }

    Player(String n, int s) {
        name = n;
        score = s;
    }
}

public class Main {
    public static void main(String[] args) {

        Player p1 = new Player();
        Player p2 = new Player("Ali");
        Player p3 = new Player("Sara", 100);

        System.out.println(p1.name + " - " + p1.score);
        System.out.println(p2.name + " - " + p2.score);
        System.out.println(p3.name + " - " + p3.score);
    }
}

Java picks the constructor based on the number of arguments.

5. Constructor Initialising Complex Objects

class Engine {
    int horsepower;

    Engine(int hp) {
        horsepower = hp;
    }
}

class Car {
    String model;
    Engine engine;

    Car(String m, int hp) {
        model = m;
        engine = new Engine(hp);
    }
}

public class Main {
    public static void main(String[] args) {
        Car c = new Car("Corolla", 180);

        System.out.println(c.model);
        System.out.println("Engine HP: " + c.engine.horsepower);
    }
}

6. Constructor Calling a Method

class User {
    String username;

    User(String name) {
        username = name;
        welcome();
    }

    void welcome() {
        System.out.println("Welcome, " + username + "!");
    }
}

public class Main {
    public static void main(String[] args) {
        User u = new User("Khalid");
    }
}

The method welcome() is automatically called when the object is created.

7. Real-World Example: Bank Account Constructor

class BankAccount {
    String owner;
    double balance;

    BankAccount(String ownerName, double initialDeposit) {
        owner = ownerName;
        balance = initialDeposit;
    }

    void show() {
        System.out.println(owner + "'s balance: $" + balance);
    }
}

public class Main {
    public static void main(String[] args) {
        BankAccount acc = new BankAccount("Khalid", 2500);
        acc.show();
    }
}

Summary

Constructors run automatically when objects are created.
They initialise variables inside the class.
Constructors can take parameters.
You can have multiple constructors (overloading).
The this keyword helps avoid naming conflicts.

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