Constructors

What is a Constructor? A constructor is a special method that is called when an object is created. Its primary purpose is to initialize the fields of the object.

Rules for Constructors:

• A constructor must have the same name as the class.

• A constructor cannot have a return type (not even void).

Default Constructor: If you do not define any constructor in your class, the Java compiler provides a default, no-argument constructor for you. This constructor initializes fields to their default values (0 for numbers, false for booleans, null for objects).

Parameterized Constructor: You can define your own constructor that accepts parameters. This is the best way to ensure that an object is created in a valid state.

Constructor Overloading: Just like methods, constructors can be overloaded. You can have multiple constructors with the same name but different parameter lists.

The this Keyword: Inside a constructor, the this keyword is a reference to the current object being created. It is often used to disambiguate between instance variables and parameters with the same name.

class User {
    String username;
    int level;

    // Default (no-arg) constructor
    public User() {
        this.username = "Guest";
        this.level = 1;
    }

    // Parameterized constructor
    public User(String username, int level) {
        this.username = username;
        this.level = level;
    }
}

User guest = new User(); // Calls the default constructor
User proUser = new User("Alice", 10); // Calls the parameterized constructor