Unit 5 Classes - Constructors

Introduction to Constructors in Java

What are Constructors?

In Java, a constructor is a special method used to initialize objects. The constructor is called when an object of a class is created. It has the same name as the class and does not return any value, not even void.

Key Points:

• Same name as class: The constructor must have the same name as the class.
• No return type: Constructors do not return any value.
• Used to initialize objects: They set initial values for object attributes.

Types of Constructors

1. Default Constructor:

A constructor that does not accept any parameters. Java provides a default constructor if you don’t define any.

class Car {
    String brand;
    
    // Default constructor
    Car() {
        brand = "Unknown";
    }
}

Car myCar = new Car();
System.out.println(myCar.brand);  // Outputs: Unknown

Unknown

2. Parameterized Constructor:

A constructor that takes parameters to initialize object attributes with custom values.

class Car {
    String brand;

    // Parameterized constructor
    Car(String b) {
        brand = b;
    }
}

Car myCar = new Car("Toyota");
System.out.println(myCar.brand);  // Outputs: Toyota

Toyota

3. Constructor Overloading

Constructor Overloading

In Java, you can have more than one constructor in a class. This is known as constructor overloading. Each constructor must have a different parameter list.

class Car {
    String brand;
    int year;

    // Default constructor
    Car() {
        brand = "Unknown";
        year = 0;
    }

    // Parameterized constructor
    Car(String b, int y) {
        brand = b;
        year = y;
    }
}
Car car1 = new Car();
Car car2 = new Car("Ford", 2020);
System.out.println(car1.brand + " " + car1.year);  // Outputs: Unknown 0
System.out.println(car2.brand + " " + car2.year);  // Outputs: Ford 2020

Unknown 0
Ford 2020

Escape Room: Constructor Puzzle

You’re trapped in a room and the only way out is to unlock a door using your Java constructor knowledge!

Rules:

• Each clue is a code snippet related to constructors. Solve the problems to reveal the next clue.
• Once you complete all tasks, you will receive the “escape” message.

Clue 1:

The constructor below is missing a key part. Complete the code so that it runs correctly.

// CLUE  1 

class Door {
    String lockCode;

    // Add the correct constructor here
    Door(_____ lock) {
        // Constructor body
        lockCode = _____;
    }
}


Door escapeDoor = new Door("1234");
System.out.println(escapeDoor.lockCode);  // Should print: 1234

//CLUE 2

class Room {
    String name;
    int area;

    // Constructor 1
    Room(String name) {
        this.name = name;
    }

    // Constructor 2 (missing)
    Room(String name, int area) {
        // Complete this constructor
        // Hint: Use the 'this' keyword
    }
}

Room escapeRoom = new Room("Puzzle Room", 500);
System.out.println("Room: " + escapeRoom.name + ", Area: " + escapeRoom.area);  
// Should print: Room: Puzzle Room, Area: 500

//CLUE 3
class Key {
    String keyType;

    // Default constructor (to be completed)
    Key() {
        // Hint: Initialize keyType to a default value, e.g., "Unknown"
    }

    // Parameterized constructor
    Key(String keyType) {
        this.keyType = keyType;
    }
}

Key defaultKey = new Key();
Key customKey = new Key("Golden Key");

System.out.println("Default Key: " + defaultKey.keyType);  // Should print: Unknown
System.out.println("Custom Key: " + customKey.keyType);    // Should print: Golden Key