Java Constructors:

Java constructors or constructors in Java is a terminology used to construct something in our programs. A constructor in Java is a special method that is used to initialize objects. The constructor is called when an object of a class is created. It can be used to set initial values for object attributes.

What are Constructors in Java?:

In Java, a Constructor is a block of codes similar to the method. It is called when an instance of the class is created. At the time of calling the constructor, memory for the object is allocated in the memory. It is a special type of method that is used to initialize the object. Every time an object is created using the new() keyword, at least one constructor is called.

Understanding how to effectively use constructors can significantly improve your Java programming skills, especially when you’re dealing with complex applications. It’s crucial to grasp the nuances of constructors to build scalable and maintainable software.

Example of Java Constructor:

// Driver Class
class Geeks {

    // Constructor
    Geeks()
    {
        super();
        System.out.println("Constructor Called");
    }

    // main function
    public static void main(String[] args)
    {
        Geeks geek = new Geeks();
    }
}

** How Java Constructors are Different From Java Methods?**

1.Constructors must have the same name as the class within which it is defined it is not necessary for the method in Java.
2.Constructors do not return any type while method(s) have the return type or void if does not return any value.
3.Constructors are called only once at the time of Object creation while method(s) can be called any number of times.

When Java Constructor is called?

Each time an object is created using a new() keyword, at least one constructor (it could be the default constructor) is invoked to assign initial values to the data members of the same class. Rules for writing constructors are as follows:

1.The constructor(s) of a class must have the same name as the class name in which it resides.
2.A constructor in Java can not be abstract, final, static, or Synchronized.
3.Access modifiers can be used in constructor declaration to control its access i.e which other class can call the constructor.

Types of Constructors in Java(TBD)

Now is the correct time to discuss the types of the constructor, so primarily there are three types of constructors in Java are mentioned below:

3.copy constructor

1. Default Constructor in Java

A constructor that has no parameters is known as default constructor. A default constructor is invisible. And if we write a constructor with no arguments, the compiler does not create a default constructor. It is taken out. It is being overloaded and called a parameterized constructor. The default constructor changed into the parameterized constructor. But Parameterized constructor can’t change the default constructor. The default constructor can be implicit or explicit.

Implicit Default Constructor: If no constructor is defined in a class, the Java compiler automatically provides a default constructor. This constructor doesn’t take any parameters and initializes the object with default values, such as 0 for numbers, null for objects.

2. Parameterized Constructor in Java

A constructor that has parameters is known as parameterized constructor. If we want to initialize fields of the class with our own values, then use a parameterized constructor.

** 3. Copy Constructor in Java**(TBD)

Unlike other constructors copy constructor is passed with another object which copies the data available from the passed object to the newly created object.

Reference:https://www.geeksforgeeks.org/constructors-in-java/

Constructor overloading in Java:

In Java, we can overload constructors like methods. The constructor overloading can be defined as the concept of having more than one constructor with different parameters so that every constructor can perform a different task.

Here, we need to understand the purpose of constructor overloading. Sometimes, we need to use multiple constructors to initialize the different values of the class.

We must also notice that the java compiler invokes a default constructor when we do not use any constructor in the class. However, the default constructor is not invoked if we have used any constructor in the class, whether it is default or parameterized. In this case, the java compiler throws an exception saying the constructor is undefined.

this keyword in java

In Java, ‘this’ is a reference variable that refers to the current object, or can be said “this” in Java is a keyword that refers to the current object instance. It can be used to call current class methods and fields, to pass an instance of the current class as a parameter, and to differentiate between the local and instance variables. Using “this” reference can improve code readability and reduce naming conflicts.

Methods to use ‘this’ in Java(TBD)

Following are the ways to use the ‘this’ keyword in Java mentioned below:

1.Using the ‘this’ keyword to refer to current class instance variables.
2.Using this() to invoke the current class constructor
3.Using ‘this’ keyword to return the current class instance
4.Using ‘this’ keyword as the method parameter
5.Using ‘this’ keyword to invoke the current class method
6.Using ‘this’ keyword as an argument in the constructor call

Use of this () in constructor overloading:

However, we can use this keyword inside the constructor, which can be used to invoke the other constructor of the same class.

Example

    public class Student {  
    //instance variables of the class  
    int id,passoutYear;  
    String name,contactNo,collegeName;  

    Student(String contactNo, String collegeName, int passoutYear){  
    this.contactNo = contactNo;  
    this.collegeName = collegeName;  
    this.passoutYear = passoutYear;  
    }  

    Student(int id, String name){  
    this("9899234455", "IIT Kanpur", 2018);  
    this.id = id;  
    this.name = name;  
    }  

    public static void main(String[] args) {  
    //object creation  
    Student s = new Student(101, "John");  
    System.out.println("Printing Student Information: \n");  
    System.out.println("Name: "+s.name+"\nId: "+s.id+"\nContact No.: "+s.contactNo+"\nCollege Name: "+s.contactNo+"\nPassing Year: "+s.passoutYear);  
    }  
    }  

Reference:https://www.javatpoint.com/constructor-overloading-in-java

Program:

public class SuperMarket
{
//class specific
static String name = "SB SuperMarket"; 
static int doorNo = 10; 
static boolean open = true; 
//non-static ---> Instance specific
String product_name; 
int price, discount; 

SuperMarket(String product_name, int price, int discount)
{
this.product_name = product_name; 
this.price = price; 
this.discount = discount; 
}

public static void main(String[] args)
{
SuperMarket product1 = new SuperMarket("cinthol", 22,2); 
SuperMarket product2 = new SuperMarket("biscuits",30,5);
SuperMarket product3 = new SuperMarket("cake",10,1); 
product1.sell();
product2.sell(); 
product3.sell();
product2.return_product(); 

}
public void return_product()
{
System.out.println("returning "+product_name);
}
public void sell()
{
System.out.println(product_name); 
System.out.println(price);
System.out.println(discount);
}
}

Output:

neelakandan@neelakandan-HP-Laptop-15s-eq2xxx:~/Documents/B14$ java SuperMarket 
cinthol
22
2
biscuits
30
5
cake
10
1
returning biscuits
neelakandan@neelakandan-HP-Laptop-15s-eq2xxx:~/Documents/B14$ ^C