Constructors in Java

Constructors in Java

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

When is a constructor called

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

Rules for creating Java constructor

There are two rules defined for the constructor.

1. Constructor name must be the same as its class name
2. A Constructor must have no explicit return type
3. A Java constructor cannot be abstract, static, final, and synchronized

Types of Java constructors

There are 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:

<class_name>(){}  

Example: -

1.    class Bike1{  

2.    //creating a default constructor  

3.    Bike1(){System.out.println("Bike is created");}  

4.    //main method  

5.    public static void main(String args[]){  

6.    //calling a default constructor  

7.    Bike1 b=new Bike1();  

8.    }  

9.    }  

Java Parameterized Constructor

A constructor which has a specific number of parameters is called a parameterized constructor.

Why use the parameterized constructor?

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

1.    //Java Program to demonstrate the use of parameterized constructor  

2.    class Student4{  

3.        int id;  

4.        String name;  

5.        //creating a parameterized constructor  

6.        Student4(int i,String n){  

7.        id = i;  

8.        name = n;  

9.        }  

10.     //method to display the values  

11.     void display(){System.out.println(id+" "+name);}  

12.    

13.     public static void main(String args[]){  

14.     //creating objects and passing values  

15.     Student4 s1 = new Student4(111,"Karan");  

16.     Student4 s2 = new Student4(222,"Aryan");  

17.     //calling method to display the values of object  

18.     s1.display();  

19.     s2.display();  

20.    }  

21. }  

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.    //Java program to overload constructors in java  

2.    class Student5{  

3.        int id;  

4.        String name;  

5.        int age;  

6.        //creating two arg constructor  

7.        Student5(int i,String n){  

8.        id = i;  

9.        name = n;  

10.     }  

11.     //creating three arg constructor  

12.     Student5(int i,String n,int a){  

13.     id = i;  

14.     name = n;  

15.     age=a;  

16.     }  

17.     void display(){System.out.println(id+" "+name+" "+age);}  

18.    

19.     public static void main(String args[]){  

20.     Student5 s1 = new Student5(111,"Karan");  

21.     Student5 s2 = new Student5(222,"Aryan",25);  

22.     s1.display();  

23.     s2.display();  

24.    }  

25. }  

Difference between constructor and method in Java

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

Java Constructor	Java Method
A constructor is used to initialize the state of an object.	A method is used to expose the behaviour of an object.
A constructor must not have a return type.	A method must have a return type.
The constructor is invoked implicitly.	The method is invoked explicitly.
The Java compiler provides a default constructor if you don't have any constructor in a class.	The method is not provided by the compiler in any case.
The constructor name must be same as the class name.	The method name may or may not be same as class name.