Constructors in Java

#java #programming #learning #ai

Constructors:

A constructor in Java is a special member that is called when an object is created. It initializes the new object’s state. It is used to set default or user-defined values for the object's attributes
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.

A constructor has the same name as the class.
It does not have a return type, not even void.
It can accept parameters to initialize object properties.

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 = "Kamalesh";
        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:

Kamalesh
22

Constructor Overloading

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

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("Kamalesh");
        Student s3 = new Student("Raju", 21);

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

Output

Unknown 0
Kamalesh 0
Raju 21

Here have a three constructors

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

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 with constructors

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("Kamalesh", 22);

        s1.display();
    }
}

Output

Kamalesh
22

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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