Definition

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Constructors is a special type of function or member method of a class used to intialize default values to instance variables.It is same as that of class name

Let us understand,this defination by an example:

The following characteristics of constructors are:

1. The name of the constructor is same as that of class name,that is from above example we can see that class Abc has same name constructor Abc().
2. In a class there can be more than one constructors.
3. The main purpose of constructor is to intialize the objects of the class
4. Every java program has a constructor i.e default constructors
5. There is no return type in a constructor, not even void also

Invoke a Constructor

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When we create a main() function of xyz classname,then we create object of the class that is:

So during creation of object of a class each time,at least one constructor(that is default constructor) is invoked to assign initial value to data members of a class.
Here in the above object creation,we first created class object as obj using new() keyword.After that default constructor of a same class name Abc() is invoked.


Example:

Write a program a and print employee name and id of xyz company.

Types of constructors

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There are basically 2 types of constructors:

1. default Constructors
2. Parametrized Constructors

Default Constructors

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Default constructors are also called as non-parameterized or no-arguments constructor.These constructors does not contain any parameters.Whenever we define any default constructors,we dont pass any parameters inside the member method.



As shown in above example,to invoke a default constructors we can simply invoke while creating class object.

test ob=new test(); // default

If any constructor is not defined for the class,then the java compiler will automatically includes a default constructor.That is a constructor with no argument passed.

Parameterized Constructors

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Parameterized constructors,are those constructors in which we pass parameters inside member functions.It allow programmers to intialize instance variable with our own values.



From the above example,to invoke a Parameterized constructor,we will provide values while creating object.If not then compiler will show error.

test ob=new test(7,3);//Parameterized

Example for both Default and Parameterized Constructors:

Write a program to print student name and their roll number using default and Parameterized constructor concept.