Constructors

A constructor is a member method of a class that is used to initialise the instance variables. It has the same name as that of its defining class and does not have a return type.

A constructor is needed as it instructs the computer to perform the start-up tasks of a new object. These can be like assigning default values to instance variables, aligning all the buttons on a screen when a new window object is created, or moving the cursor to a specific data entry field for the user input.

A default constructor does not have any statement inside it. It initializes default values to the instance variables.

If we define our own constructor in a class, we can initialise the instance variables according to our requirements. A parameterised constructor allows the programmer to initialise objects with different values. This is achieved by passing the required values as arguments to the constructor method.

A constructor cannot be explicitly invoked like a method. It is automatically invoked via the new operator at the time of object creation. On the other hand, a method can be called directly by an object that has already been created. Normal method calls are specified by the programmer.

The below program highlights the difference in the way the constructor is invoked versus a normal method call:

class Test  {
    int a, b;
    public Test(int x, int y)   {
        a = x;
        b = y;
    }
    void add()  {
        System.out.println(a + b);
    }
}

class invokeConst   {
    public static void main(String args[])  {
        Test obj1 = new Test(10, 20);   // constructor invoked
        obj1.add();    // calling add() method
    }
}

Parameterized constructors are important because they provide a way to initialize objects with different values. For example, if we have a class representing a bank account, we could use a parameterized constructor to initialize a bank account with a specific balance, rather than always starting with a balance of zero. This makes the class more flexible and reusable, as it can be used to create objects with a variety of different initial states.

The constructor can only have the name of the class in which it is defined. Thus, all the constructors of class DemoClass will be named DemoClass.

The process of creating more than one constructor with the same name but with different parameter declarations is called constructor overloading.

The below example shows constructor overloading being used to create a class having three constructors:

public class Rectangle
{
    int length;
    int width;

    //Constructor without any parameters
    public Rectangle()   {
        System.out.println("Invoking constructor with no parameters"); 
        length = 20;
        width = 15;
    }

    // Constructor with one parameter
    public Rectangle(int len)   {
        System.out.println("Invoking constructor with one parameter"); 
        length = len;
        width = 20;
    }

    // Constructor with two parameters
    public Rectangle(int  len, int  wd)    {
        System.out.println("Invoking constructor with two parameters"); 
        length = len;
        width = wd;
    }

    public void Area()    {
        int area = length * width;
        System.out.println("Length = " + length);
        System.out.println("Width = " + width);
        System.out.println("Area = " + area);
    }

    public static void main(String args[])  {
        Rectangle rect1 = new Rectangle();
        rect1.Area();

        Rectangle rect2 = new Rectangle(30);
        rect2.Area();

        Rectangle rect3 = new Rectangle(30, 10);
        rect3.Area();
    }
}

Within a constructor or a method, this is a reference to the current object — the object whose constructor or method is being invoked. The keyword this stores the address of the currently-calling object.

For example, the below program makes use of the this keyword to resolve the conflict between method parameters and instance variables.

class  Area
{
    int  length,  breadth;

    public Area(int length, int breadth) {
        this.length = length;
        this.breadth =  breadth;
    }
    
    public void Display() {
    int	area = length * breadth;
    System.out.println("Area is " + area);
    }

    public static void main(String args[J) {
    Area area = new Area(2, 3);
    area.Display();
    }
}

A no-argument constructor is a constructor that accepts no arguments and has no statements in its body. It is also called the default constructor.

If the programmer has not explicitly defined a constructor for the class then Java compiler automatically adds a no-argument constructor/default constructor. However, if the constructor is defined explicitly then no-argument constructor is not added.

a constructor

Reason — A member method having the same name as that of the class is called constructor.

no

Reason — A constructor has no return type.

created

Reason — A constructor is used when an object is created.

no parameters

Reason — A default constructor has no parameters.

All of the above

Reason — All the statements are correct.

All constructors must have the same name as that of the class.

Reason — In constructor overloading, all constructors must have the same name as that of the class.