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Java - Constructor

In simple words it is the Constructor which is helping the object to be created.

There are two types of Constructor :

1. Default Constructor or the no argument Constructor.

2. Constructor with arguments or Parameterized Constructor.


Constructor in detail

Say we are going to create two Human objects out of the above Human class. i.e. 'John' from America and Wang Chu from China.



Human Class

class Human{

  String name;
  String food;
  String language;

  public void eat(){
    System.out.println("Eats "+food);
  }

  public void speak(){
    System.out.println("Speaks "+language);
  }
}

We have seen how to create an object of 'Human' class using 'new' keyword with the below statement :


Human human1 = new Human();


What is 'Human()' in the above statement?

Although 'Human()' looks like a normal java method, but it has the same name as that of the class name(i.e Human). And that is what makes is different from a method and is called as 'Constructor' which is helping the compiler to create a java object.


Where is 'Human()' in the 'Human' class?

But the strange fact is 'Human()' is not present inside the above class. Then how is it helping the compiler to create an object?

The answer is, a constructor with no arguments is called as a default constructor. Even if you don't place it in the class, the compiler will automatically place it for you. Just wait a second, I just said a default constructor is a constructor with no arguments. So, that does mean there can be constructors with arguments. Yes, there are and we will have a detail look at it.

Lets modify the 'Human' class with constructors:


Class Human{

  String name;
  String food;
  String language;

  Human(){

  }

  Human(String nme, String fd, String lang){

    this.name = nme;
    this.food = fd;
    this.language = lang;
  }

  public void eat(){
    System.out.println("Eats "+food);
  }

  public void speak(){
    System.out.println("Speaks "+language);
  }
}

So, in the above class we have two contructors. One with no arguments(default constructor), which the compiler inserted by default in the previous 'Human' class.


Human(){

}

Note :If you are using the Constructor with arguments, then you must specify the default constructor.

And the other is the constructor with three arguments.


Human(String nme, String fd, String lang){

  this.name = nme;
  this.food = fd;
  this.language = lang;

}


How to create an object with the constructor with arguments?

Class CreateHuman{

  public static void main(String[] args){
    Human human1 = new Human(); //Calls the no-argument/default Constructor
    Human human2 = new Human(); //Calls the no-argument/default Constructor
    Human argumentHuman1 = new Human("John","Burger","English"); //Calls the constructor with arguments
    Human argumentHuman2 = new Human("Wang Chu","Chowmein","Chinese"); //Calls the constructor with arguments
  }
}

Lets see whats happens in the above code:


java_object

So, in the above image! name, food and language variables of human1 and human2 are not assigned any values. It is because 'human1' and 'human2' are created using the default constructor :


Human human1 = new Human();
Human human2 = new Human();

and no values are assigned to it.


Again let us relook at the below Image:


java_object

So, in the above image argumentHuman1 and argumentHuman2 contains the values we have put in the constructor with arguments.

Let's have a look at the below code :


Human argumentHuman1 = new Human("John","Burger","English");

As we have already seen argumentHuman1 is the name of the object. And on the right hand side we have 'new' which creates a new object named 'argumentHuman1'.

Now, let's see what 'Human("John","Burger","English")' does, relating with the below code(i.e constructor with arguments).


Human(String nme, String fd, String lang){

  this.name = nme;
  this.food = fd;
  this.language = lang;
}

So, "John" gets assigned to 'nme', "Burger" to 'fd' and "English" to 'lang'

java_constructor

Now, if we see the code inside the Constructor.


this.name = nme;
this.food = fd;
this.language = lang;

The value inside nme (i.e "John") gets assigned to 'this.name', value inside 'fd' (i.e. "Burger") inside 'this.food' and the value inside 'lang' (i.e. "English") inside 'this.language'.


Note :'this.name' is referring to the 'name' property of 'Human' class. Where 'this' refers to the current object.

Similarly, 'Burger' is assigned to 'this.food', which is the 'food' property of 'Human' class and 'English' is assigned to 'language' property of 'Human' class.



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