Constructor

Constructor in Java

 

In Java, constructor is a block of codes similar to method. It is called when an instance of object is created and memory is allocated for the object.

It is a special type of method which is used to initialize the object.

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 doesn’t have any.

Rules for creating java constructor

There are basically two rules defined for the constructor.

  1. Constructor must  have the  same  name as its class name
  2. Constructor must not have return type

Types of java constructors

two types of constructors in java:

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

 

Java Default Constructor

A constructor is called “Default Constructor” when it doesn’t have any parameter.

Syntax of default constructor:

  1. <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.
  1. class Bike1{
  2. Bike1(){System.out.println(“Bike is created”);}
  3. public static void main(String args[]){
  4. Bike1 b=new Bike1();
  5. }
  6. }

Output:

Bike is created

If there is no constructor in a class, compiler automatically creates a default constructor.

Q) What is the purpose of default constructor?

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

  1. class Student3{
  2. int id;
  3. String name;
  4. void display(){System.out.println(id+” “+name);}
  5. public static void main(String args[]){
  6. Student3 s1=new Student3();
  7. Student3 s2=new Student3();
  8. s1.display();
  9. s2.display();
  10. }
  11. }

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 which has a specific number of parameters is called 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. We can have any number of parameters in the constructor.

  1. class Student4{
  2.     int id;
  3.     String name;
  4.     Student4(int i,String n){
  5.     id = i;
  6.     name = n;
  7.     }
  8.     void display(){System.out.println(id+” “+name);}
  9.     public static void main(String args[]){
  10.     Student4 s1 = new Student4(111,“Karan”);
  11.     Student4 s2 = new Student4(222,“Aryan”);
  12.     s1.display();
  13.     s2.display();
  14.    }
  15. }

 

Output:

111 Karan
222 Aryan

Constructor Overloading in Java

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

  1. class Student5{
  2.     int id;
  3.     String name;
  4.     int age;
  5.     Student5(int i,String n){
  6.     id = i;
  7.     name = n;
  8.     }
  9.     Student5(int i,String n,int a){
  10.     id = i;
  11.     name = n;
  12.     age=a;
  13.     }
  14.     void display(){System.out.println(id+” “+name+” “+age);}
  15.     public static void main(String args[]){
  16.     Student5 s1 = new Student5(111,“Karan”);
  17.     Student5 s2 = new Student5(222,“Aryan”,25);
  18.     s1.display();
  19.     s2.display();
  20.    }
  21. }

 

Output:

111 Karan 0
222 Aryan 25

Difference between constructor and method in java

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

Java Constructor Java Method
Constructor is used to initialize the state of an object. Method is used to expose behaviour 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 don’t have any constructor. Method is not provided by compiler in any case.
Constructor name must be same as the class name. Method name may or may not be same as class name.

Java Copy Constructor

There is no copy constructor in java. But, we can copy 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 in java. They are:

  • By constructor
  • By assigning the values of one object into another
  • By clone() method of Object class

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

  1. class Student6{
  2.     int id;
  3.     String name;
  4.     Student6(int i,String n){
  5.     id = i;
  6.     name = n;
  7.     }
  8.     Student6(Student6 s){
  9.     id = s.id;
  10.     name =s.name;
  11.     }
  12.     void display(){System.out.println(id+” “+name);}
  13.     public static void main(String args[]){
  14.     Student6 s1 = new Student6(111,“Karan”);
  15.     Student6 s2 = new Student6(s1);
  16.     s1.display();
  17.     s2.display();
  18.    }
  19. }

 

Output:

111 Karan
111 Karan

Copying values 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.

  1. class Student7{
  2.     int id;
  3.     String name;
  4.     Student7(int i,String n){
  5.     id = i;
  6.     name = n;
  7.     }
  8.     Student7(){}
  9.     void display(){System.out.println(id+” “+name);}
  10.     public static void main(String args[]){
  11.     Student7 s1 = new Student7(111,“Karan”);
  12.     Student7 s2 = new Student7();
  13.     s2.id=s1.id;
  14.     s2.name=s1.name;
  15.     s1.display();
  16.     s2.display();
  17.    }
  18. }

 

Output:

111 Karan
111 Karan

Q) Does constructor return any value?

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