Java Constructor

What is Constructor

The constructor is special member function which initializes object when it is created.

The constructor is needed when we want to assign default values if user has not given.

Characteristics of Constructor

• Constructor has same name as a class name
• Constructor does not have any return type
• Constructor does not return any value
• Constructor can be overloaded as normal function
• Constructor can accept any number of parameters
• Constructor is called automatically when object is created
• Constructor allocated the memory to object and garbage-collection deallocates the memory

Types of Constructors

There are two types of constructor

• Default Constructor
• Parameterized Constructor

Default Constructor

Default Constructor is called when object of class is created. When there is no constructor has been defined explicitly (by user), then java calls default constructor automatically.

We can define only one default constructor in the class. Java initializes all instance variables to zero after creation.

Default Constructor is also known as Zero Parameterized Constructor because it does not take any argument

Syntax

class classname
{
	classname ()	// default constructor
	{
		// body of constructor function
	}
}

Example of Default Constructor

Calculating area of Rectangle using Default constructor

class Rectangle
{
     int length,width;
     Rectangle()
     {
	 length = 12;
	 width = 8;
	 System.out.println("Values initialized");
     }
     int area()
     {
	 return  length * width;
     }
     public static void main(String args[])
     {
	 Rectangle rect = new Rectangle();
	 System.out.println("Area of Rectangle : "+rect.area());
     }
}
	

Output

In above example we have created class names as Rectangle which is having Default Constructor as Rectangle(). This constructor is executed when the object is created.

Parameterized Constructor

The Constructor which accepts the parameters known as Parameterized Constructor.

Sometimes it necesary to pass arguments to constructor while creating an object. It can have any number of parameters separated by commas.

Syntax

class className
{
   className( parameter1, parameter2, ... parameterN)
   {
	  body of Parameterized Constructor
	  ...
	  ...
   }
}

Example

Program to print the given string upto N times using Parameterized Constructor

class ParamDemo
{
  int number;
  String message;
  ParamDemo(String msg, int num)
  {
      message = msg;
      number = ntimes;
  }
	
  void print()
  {
      for (int i=1;i<=number;i++)
	System.out.println("msg : "+message);
  }
  public static void main(String args[])
  {
	ParamDemo pd = new ParamDemo("AndroidBerry", 7);
	pd.print();
  }
}

Output

In above example we have printed the message AndroidBerry 7 times using Parameterized Constructor