Constructor
- Constructor is special kind of method which gets called when object is created
- Constructor don’t have return type
- Constructor cannot be inherited
- Constructor name is same has the class name
- Every class should have a constructor, if we don’t write a constructor compiler will
write a default constructor.
- Compiler will not write a default constructor, if we are going to write any of the
constructor explicitly
- Constructor with no parameters or a zero parameter constructor is called has default
constructor
- Constructor with parameter is called has parameterized constructor. These
constructor is used to initialise the instance variables at the time of creating object
Syntax
<accesses-specifier> ClassName ( )
{
//some code…
}
Eg :- class Cow
{
//zero parameter constructor of Cow Class
public Cow()
{
System.out.println("I am in Cow class constructor.....!");
}
}
Example:
class Cow
{
//zero parameter constructor of Cow Class
public Cow()
{
System.out.println("I am in Cow class constructor.....!");
}
}
class Test1
{
public static void main(String[] args)
{
System.out.println("Before creating object");
Cow ref = new Cow(); //when this line is executed the Cow() constructor is called
System.out.println("After creating 1st object");
new Cow(); // when this line is executed again the Cow() constructor is called
System.out.println("After creating 2nd object");
}
}
Output :
Before creating object
I am in Cow class constructor.....!
After creating 1st object
I am in Cow class constructor.....!
After creating 2nd object
Constructor Over Loading
A class can contain more than one constructor inside it, but they should vary with number of
parameters or type of parameters. This concept is called has contractor overloading.
class Cow
{
public Cow()
{
System.out.println("I am in Cow() constructor.....!");
}
public Cow(int leg)
{
System.out.println("I am in Cow(int leg) constructor.....!");
}
public Cow(double height)
{
System.out.println("I am in Cow(double hight) constructor.....!");
}
public Cow(int ears , int mouth)
{
System.out.println("I am in Cow(int ears , int mouth) constructor.....!");
}
}
These constructors can be called by creating a object like
new Cow(); // Cow() constructor is called
new Cow(4.5); // Cow(double height) constructor is called
new Cow(2,1); // Cow(int ears , int mouth)constructor is called
new Cow(4); // Cow(int leg) constructor is called
Note :
- The parameterized constructor is used to initialize the instance variables of the class
- If you don't define a constructor, the default constructor (which has no parameters)
will be generated by default.
- If you have any user-defined constructor, the default constructor will not be generated
at all.
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