Rules for creating Java constructor

Every time an object is created using new() keyword, at least one constructor is called. It calls a default constructor.

Note: It is called constructor because it constructs the values at the time of object creation. It is not necessary to write a constructor for a class. It is because java compiler creates a default constructor if your class does not have any.

There are two types of constructors in Java:

1. Default constructor (no-arg constructor): A constructor is called Default Constructor when it does not have any parameter.
2. Parameterized constructor: A constructor which has a specific number of parameters is called a parameterized constructor.

The default constructor is used to provide the default values to the object like 0, null, etc., depending on the type.

Example of default constructor that displays the default values

//Let us see another example of default constructor  
//which displays the default values  
class Student3{  
int id;  
String name;  
//method to display the value of id and name  
void display(){System.out.println(id+" "+name);}  
  
public static void main(String args[]){  
//creating objects  
Student3 s1=new Student3();  
Student3 s2=new Student3();  
//displaying values of the object  
s1.display();  
s2.display();  
}  
}  

0 null
0 null

The parameterized constructor is used to provide different values to the distinct objects. However, you can provide the same values also.

Example of parameterized constructor

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

//Java Program to demonstrate the use of parameterized constructor  
class Student4{  
    int id;  
    String name;  
    //creating a parameterized constructor  
    Student4(int i,String n){  
    id = i;  
    name = n;  
    }  
    //method to display the values  
    void display(){System.out.println(id+" "+name);}  
   
    public static void main(String args[]){  
    //creating objects and passing values  
    Student4 s1 = new Student4(111,"Karan");  
    Student4 s2 = new Student4(222,"Aryan");  
    //calling method to display the values of object  
    s1.display();  
    s2.display();  
   }  
}  

111 Karan
222 Aryan

In Java, a constructor is just like a method but without return type. It can also be overloaded like Java methods.

Constructor overloading in Java is a technique of having more than one constructor with different parameter lists. They are arranged in a way that each constructor performs a different task. They are differentiated by the compiler by the number of parameters in the list and their types.

Example of Constructor Overloading

//Java program to overload constructors in java  
class Student5{  
    int id;  
    String name;  
    int age;  
    //creating two arg constructor  
    Student5(int i,String n){  
    id = i;  
    name = n;  
    }  
    //creating three arg constructor  
    Student5(int i,String n,int a){  
    id = i;  
    name = n;  
    age=a;  
    }  
    void display(){System.out.println(id+" "+name+" "+age);}  
   
    public static void main(String args[]){  
    Student5 s1 = new Student5(111,"Karan");  
    Student5 s2 = new Student5(222,"Aryan",25);  
    s1.display();  
    s2.display();  
   }  
}  

Output

111 Karan 0
222 Aryan 25

There are many differences between constructors and methods. They are given below.

Yes, like object creation, starting a thread, calling a method, etc. You can perform any operation in the constructor as you perform in the method.