Constructor in java Example

Constructor in Java

onstructor in java is a special type of method that is used to initialize the object.

Java constructor is invoked at the time of object creation. It constructs the values i.e. provides data for the object that is why it is known as constructor.

Rules for creating java constructor

There are basically two rules defined for the constructor.

1. Constructor name must be same as its class name
2. Constructor must have no explicit return type

Types of java constructors

There are two types of constructors:

1. Default constructor (no-arg constructor)
2. Parameterized constructor

Java Default Constructor

A constructor that have no parameter is known as default constructor.

Syntax of default constructor:

<class_name>(){}  

Example of default constructor

In this example, we are creating the no-arg constructor in the Bike class. It will be invoked at the time of object creation. class Bike1{   Bike1(){System.out.println("Bike is created");}   public static void main(String args[]){   Bike1 b=new Bike1();   } } Output: Bike is created Note:- Rule: If there is no constructor in a class, compiler automatically creates a default constructor.

Q) What is the purpose of default constructor?

Default constructor provides the default values to the object like 0, null etc. depending on the type.

Example of default constructor that displays the default values

class Student3{  
int id;  
String name;  
  
void display(){System.out.println(id+" "+name);}  
  
public static void main(String args[]){  
Student3 s1=new Student3();  
Student3 s2=new Student3();  
s1.display();  
s2.display();  
}  
}  

Output:

0 null
0 null

Explanation:In the above class,you are not creating any constructor so compiler provides you a default constructor.Here 0 and null values are provided by default constructor.

Java parameterized constructor

A constructor that have parameters is known as parameterized constructor.


Why use parameterized constructor?


Parameterized constructor is used to provide different values to the distinct objects.



Example of parameterized constructor

In this example, we have created the constructor of Student class that have two 
parameters. Wecan have any number of parameters in the constructor.

class Student4{  
    int id;  
    String name;  
      
    Student4(int i,String n){  
    id = i;  
    name = n;  
    }  
    void display(){System.out.println(id+" "+name);}  
   
    public static void main(String args[]){  
    Student4 s1 = new Student4(111,"Karan");  
    Student4 s2 = new Student4(222,"Aryan");  
    s1.display();  
    s2.display();  
   }  
}
output:


111 Karan 
222 Aryan