Constructor Overloading

Constructor Overloading

Constructor overloading is a technique in Java in which a class can have any number of constructors that differ in parameter lists.The compiler differentiates these constructors by taking into account the number of parameters in the list and their type.

Example of Constructor Overloading

    class Student{ 
        int id; 
        String name; 
        int age; 
        Student(int i,String n){ 
        id = i; 
        name = n; 
        } 
        Student(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[]){ 
        Student s1 = new Student(111,"Karan"); 
        Student s2 = new Student(222,"Aryan",25); 
        s1.display(); 
        s2.display(); 
       } 
    } 

Output:111 Karan 0
           222 Aryan 25


What is the difference between constructor and method ?

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

Constructor                                                                      Method

Constructor is used to initialize the state of an object.         Method is used to expose behavior of an object.

Constructor must not have return type.                               Method must have return type.

Constructor is invoked implicitly.                                        Method is invoked explicitly.

The java compiler provides a default constructor if you        Method is not provided by compiler in any
don't have any constructor.                                                   case.

Constructor name must be same as the class name.              Method name may or may not be same
                                                                                           as class name.


Copying the values of one object to another like copy constructor in C++

There are many ways to copy the values of one object into another. They are:

    1. By constructor
    2. By assigning the values of one object into another
    3. By clone() method of Object class

In this example, we are going to copy the values of one object into another using constructor.

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

Output:111 Karan
           111 Karan

Copying the values of one object to another without constructor

We can copy the values of one object into another by assigning the objects values to another object.
In this case, there is no need to create the constructor.

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

Output:111 Karan
           111 Karan

Que)Does constructor return any value?

Ans:yes,that is current class instance (You cannot use return type yet it returns a value).

Que)Can constructor perform other tasks instead of initialization?

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