Vigneshwaran V

Posted on Aug 26

Constructor in Java

#java #backend #coding #programming

What Is Constructor

A constructor in Java is a special method that is automatically called when an object of a class is created. Its main purpose is to initialize the object's data.

Rules:

Constructor name must be the same as the class name.
It does not have a return type (not even void).
It is called automatically when you create an object using the new keyword.
A class can have multiple constructors (constructor overloading).

Example:

class Student {

    String name;
    int age;

    Student() {
        name = "Vignesh";
        age = 22;
    }

    void display() {
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
    }

    public static void main(String[] args) {

        Student s1 = new Student();

        s1.display();
    }
}

Output:

Name: Vignesh
Age: 22

When we write Student s1 = new Student(); the constructor Student() is automatically called.

So the flow is

new Student() => Student() constructor => name = "Vignesh"
age = 22 => s1 object is initialized

Constructor Overloading

Constructor overloading means having multiple constructors in the same class with different parameter lists.

For example

class Student {

    String name;
    int age;

    // Constructor 1
    Student() {
        name = "Unknown";
        age = 0;
    }

    // Constructor 2
    Student(String n) {
        name = n;
        age = 0;
    }

    // Constructor 3
    Student(String n, int a) {
        name = n;
        age = a;
    }

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

    public static void main(String[] args) {

        Student s1 = new Student();
        Student s2 = new Student("Vignesh");
        Student s3 = new Student("Arun", 21);

        s1.display();
        s2.display();
        s3.display();
    }
}

Output

Unknown 0
Vignesh 0
Arun 21

Here we have three constructors

Student()
Student(String n)
Student(String n, int a)

They have the same name, but their parameter lists are different,

That's constructor overloading.

this Keyword

Once you understand constructors, this becomes much easier.
The this keyword refers to the current object.
The most common use is when the instance variable and constructor parameter have the same name.

Without this

class Student {

    String name;
    int age;

    Student(String name, int age) {

        name = name;
        age = age;
    }
}

This is a problem because Java cannot distinguish the instance variable from the parameter in the way we intend. Both sides refer to the parameter.

So we use this

this with Constructor

class Student {

    String name;
    int age;

    Student(String name, int age) {

        this.name = name;
        this.age = age;
    }

    void display() {
        System.out.println(this.name);
        System.out.println(this.age);
    }

    public static void main(String[] args) {

        Student s1 = new Student("Vignesh", 22);

        s1.display();
    }
}

Output

Vignesh
22

Understand this line

this.name = name;

There are two names.

this.name       =       name
   ↑                       ↑
instance variable       parameter
of current object

So here

this.name = name;

Store the parameter name into the current object's name variable.

this Means Current Object

Consider these Two Objects

Student s1 = new Student("Vignesh", 22);
Student s2 = new Student("Arun", 21);

For s1:

this → s1

For s2:

this → s2

So Therefore here:

this.name

name of the current object

this is a reference variable in Java that refers to the current object. It is commonly used to distinguish instance variables from parameters

References

https://www.geeksforgeeks.org/java/constructors-in-java/
https://www.w3schools.com/java/java_constructors.asp
https://www.programiz.com/java-programming/constructors

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