A constructor in Java is a special method that is used to initialize objects. It is called when an instance of a class is created. Constructors have the same name as the class and do not have a return type. There are two types of constructors in Java:

  1. Default Constructor (no-arg constructor)
  2. Parameterized Constructor

##Default Constructor

A default constructor is a no-argument constructor that the Java compiler inserts on your behalf if no constructor is defined in the class.

Example 

  public class Dog {
    String name;
    int age;

    // Default constructor
    public Dog() {
        this.name = "Unknown";
        this.age = 0;
    }

    public void display() {
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
    }

    public static void main(String[] args) {
        Dog myDog = new Dog();
        myDog.display();
    }
}

Explanation

  • public Dog() is the default constructor that initializes the name and age attributes.
  • When Dog myDog = new Dog(); is executed, the default constructor is called.
  • myDog.display(); prints the default values “Unknown” and 0.

Parameterized Constructor

A parameterized constructor is one that accepts arguments to initialize the object’s attributes with specific values.

Example 

  public class Dog {
    String name;
    int age;

    // Parameterized constructor
    public Dog(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public void display() {
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
    }

    public static void main(String[] args) {
        Dog myDog = new Dog("Buddy", 3);
        myDog.display();
    }
}

Explanation

  • public Dog(String name, int age) is the parameterized constructor that initializes the name and age attributes with the provided values.
  • When Dog myDog = new Dog("Buddy", 3); is executed, the parameterized constructor is called with “Buddy” and 3 as arguments.
  • myDog.display(); prints “Buddy” and 3.

Constructor Overloading

Constructor overloading is the process of defining multiple constructors in a class, each with a different parameter list.

Example 

  public class Dog {
    String name;
    int age;

    // Default constructor
    public Dog() {
        this.name = "Unknown";
        this.age = 0;
    }

    // Parameterized constructor
    public Dog(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public void display() {
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
    }

    public static void main(String[] args) {
        Dog defaultDog = new Dog();
        defaultDog.display();

        Dog myDog = new Dog("Buddy", 3);
        myDog.display();
    }
}

Explanation

  • The Dog class has both a default constructor and a parameterized constructor.
  • Dog defaultDog = new Dog(); calls the default constructor.
  • Dog myDog = new Dog("Buddy", 3); calls the parameterized constructor.
  • Both objects display their respective values when display() is called.

Example Program

Here is a complete example program demonstrating the use of both default and parameterized constructors:

  public class Person {
    String name;
    int age;

    // Default constructor
    public Person() {
        this.name = "Unknown";
        this.age = 0;
    }

    // Parameterized constructor
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public void display() {
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
    }

    public static void main(String[] args) {
        // Using default constructor
        Person person1 = new Person();
        person1.display();

        // Using parameterized constructor
        Person person2 = new Person("Alice", 25);
        person2.display();
    }
}

Explanation

  • The Person class has both a default constructor and a parameterized constructor.
  • Person person1 = new Person(); calls the default constructor and initializes name to “Unknown” and age to 0.
  • Person person2 = new Person("Alice", 25); calls the parameterized constructor and initializes name to “Alice” and age to 25.
  • Both objects display their respective values when display() is called.