Java – Constructor

Constructor in Java

Constructor 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.

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 constructor


 

Java Default Constructor

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

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

    public static void main(String args[]) {
        Bike1 b = new Bike1();
    }
}

Test it NowOutput:

Bike is created

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

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

public class Student3 {
    int id;
    String name;

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

    public static void main(String args[]) {
        Student3 s1 = new Student3();
        s1.display();

    }
}

Test it NowOutput:

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. We can have any number of parameters in the constructor.
public 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();
    }
}

Test it NowOutput:

111 Karan
222 Aryan

 

Constructor Overloading in Java

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

package constructor;

public class Student5 {
    int id;
    String name;
    int age;

    Student5(int i, String n) {
        id = i;
        name = n;
    }

    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();
    }
}

Test it NowOutput:

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.

 

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