Please Check my comments for this code

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Please notify with any improvement if required . or any explanation regarding this.

public class Test{
	//this is instance or class varible or field that is declared as static
	static int total = 10;
	public static void main(String[]args){
	//Object of class Test created
	new Test();
	}
	//Method test Declared
	public Test(){
	System.out.println("In Test");
	//Here this is calling current obect of the class Test.
	System.out.println(this);
	//Here we are calling static variable total of the current class using this keyword.
	int temp = this.total;
	if(temp>5){
	System.out.println(temp);
	}
  }
}

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Gellert Varga

Level 23 , Szekesfehervar, Hungary

15 January 2021, 18:58

Here are good explanations about Java basic elements: https://www.tutorialspoint.com/java/java_object_classes.htm

Gellert Varga

Level 23 , Szekesfehervar, Hungary

12 January 2021, 23:44

Line 3: You declared a static variable. It means this is a class variable. It means, this never will be a part of any Test object. In other words, it means the static variable is not an instance-variable. (Test type object = an instance of the Test class) Line 5., Line 6. : OK. But you did not place the reference of the new object into any variable, so you can't work with it later in the main method. Line 8. , Line 9.: You declared not an ordinary method but a special one. Its name is constructor. This constructor is called in every case when You create a new Test object. The constructor is written generally for the purpose to set the instance variables of the newly created object. You did write other codes inside the constructor, not to set the instance variable of the newly created object. But this is no problem, and your codes in the constructor will be executed every times you create a new Test object. The command of creating a new object (line 4) calls this constructor, and all of the codes inside the constructor will be executed. Line 11., 12.: Yes, and the println command will display the reference (the memory-address) of this new object. Line 13., 14: You can here use the static variable "total" without using "this" keyword, too. The "total" variable is not the own of this object, because of the total variable is not an instance variable. The "total" variable is not part of the object. (A variable will be instance-variable only if it's non-static.) The "this" keyword is needed when you want use an instance variable of the current object. If you have any question, feel free to ask. (by the "Reply" button.)

Dinesh

Level 7 , Delhi, India

13 January 2021, 07:37

Ok. Thank you so much Stuffs that I learnt from your fruitful explanation are as follows:- 1. static variable can never be a instance variable 2. static variable be ONLY called as class variable. 3. static variable cannot be called as field of class. 4. Constructors are used to set the instance variable of a newly created object. 5. Constructor will execute every time you create a new object . 6. this is used to access instance variable of the current object. If Static variables can never be instance variable (ref point 1), and this keyword is use to access the instance variable (ref point 6). Than the system must give error at line 14 of the above code i.e

int temp = this.total;

because

static int total =10;

is a class variable. Please clarify. Thank You Now moving to Second Snippet of code below

public class DogEx{
		   String name;
           public static void main(String[] args) {
           DogEx dog1 = new DogEx();

 	       dog1.bark();
	       dog1.name = "Bart";

	}
	public void bark(){
		System.out.println(dog1.name +" Barks Bow! Bow!");
	}
  }

In this case String name is a instance variable or say field of class It can and should be accessed by object of that very particular class but when tried to execute this code its error why so If I am trying to access instance variable name by dog1 object of Dog type

Gellert Varga

Level 23 , Szekesfehervar, Hungary

13 January 2021, 21:04

1) Yes 2) Yes 3) I'm sorry, i'm not sure of the exact meaning of "field of class" I can imagine this expression is not so exact. Or just me don't know exactly. But the very important decidable question is: static or not static? static = class-variable non-static= instance-variable 4) Yes 5) Yes 6) Yes The question is good, but system not have to give error if write this:

this.total

because of: a static variable is a common variable. Each objects of the class may use the static variables. You may write this way (inside in an instance-method):

this.total

but inside its own class, you may write this way, too:

total

Gellert Varga

Level 23 , Szekesfehervar, Hungary

13 January 2021, 21:31

about class DogEx: Yes, String name is an instance-variable. Yes, public void bark() is an instance-method. = all the DogEx objects will have an own bark() method! So, this method is a blueprint of all the future objects. So, you can't write inside this method, a name of one of the later objects! (dog1). This gives error. But you can write so:

this.name

(in line 11.) and the program will work well.

Dinesh

Level 7 , Delhi, India

14 January 2021, 08:53

It means a class variable can also be accessed by using this keyword. Is it so?

Gellert Varga

Level 23 , Szekesfehervar, Hungary

14 January 2021, 17:52

Yes! (But you have to know, the "this" keyword can be used only within the instance-methods.)

public class Bird {

        static String classVariable = "Bird";

        public Bird () {  // Constructor. = a special instance-method
           System.out.println(this.classVariable);  // it works well
           System.out.println(classVariable);  // it works well
           System.out.println(Bird.classVariable);  // it works well
        }

        public static void main(String args[]) {

        new Bird(); // calling the constructor
    }
}

Output: Bird Bird Bird