Constructor

Constructor

• Definition: The constructor is a specialized method for initializing objects.

• Name of the constructor is same as that of its class name. In other words, the name of the constructor and class name is same.

• Whenever an object of its associated class is created, the constructor is invoked automatically.

• The constructor is called constructor because it creates values for data fields of class.

• Example -

Java Program[Rectangle1.java]

public class Rectangle 1 {

int height;

int width;

Rectangle1()

{

System.out.println(" Simple constructor: values are initialised...");

height=10;

width=20;

}

Rectangle 1 (int h,int w)

{

System.out.println(" Parameterised constructor: values are initialised...");

height=h;

width=w;

}

void area()

{

System.out.println("Now, The function is called...");

int result height*width;

System.out.println("The area is "+result);

}

class Constr_Demo {

public static void main(String args[])

{

Rectangle1 obj = new Rectangle 1();

obj.area();//call the to method

Rectangle1 obj1 = new Rectangle 1(11,20);

obj1.area();//call the to method

}

}

Output

F:\test>javac Rectangle1.java

F:\test>java Constr_Demo

Simple constructor: values are initialised...

Now, The function is called...

The area is 200

Parameterised constructor: values are initialised...

Now, The function is called...

The area is 220

F:\test>

Program Explanation

In above program, there are two classes - Rectangle1 and Constr_Demo. In the Rectangle1 class, data fields, method and constructor is defined. In the Constr_Demo class the objects are created. When an object obj gets created, then the simple constructor Rectangle1() gets invoked and the data fields height and width gets initialized

Hence the area function will compute the area=10*20=200.

When an object obj1 gets created, then the parameterised constructor Rectangle1(11, 20) gets invoked and the data fields height and width gets initialised.

Hence the area function will compute the area = 11*20 = 220.

The class Constr_Demo is called main class because it consists of main function.

Properties of Constructor

1. Name of constructor must be the same as the name of the class for which it is being used.

2. The constructor must be declared in the public mode.

3. The constructor gets invoked automatically when an object gets created.

4. The constructor should not have any return type. Even a void data type should not be written for the constructor.

5. The constructor cannot be used as a member of union or structure.

6. The constructors can have default arguments.

7. The constructor cannot be inherited. But the derived class can invoke the constructor of base class.

8. Constructor can make use of new or delete operators for allocating or releasing memory respectively.

9. Constructor can not be virtual.

10. Multiple constructors can be used by the same class.

11. When we declare the constructor explicitly then we must declare the object of that class.