0

I have used Java for about six months but I am somewhat at a loss as to exactly how the this statement works. I know that it is used to refer to an calling instance. But why does it not need to be used in the main method to reference the object that is created in the main method?

I think that the this is always referring to the current object that has priority over the other objects. In the constructor the this is for the new object being created, correct. In the main method the this.x.method is referring to the object that is in the main statement.

Question: The this keyword refers to the current object that is being created? or does it refer to the object that is being made into a new object?

class:

public class DDHThisTest {
    public int x = 0;
    public int y = 0;

    public DDHThisTest(int a, int b) {
        this.x = a;
        this.y = b;
    }

    public static void main(String[] args) {
        DDHThisTest i = new DDHThisTest();
        this.i.x = 10;

    }
}

Error:

Cannot use this in a static context

Doug Hauf
  • 3,025
  • 8
  • 46
  • 70
  • 2
    As error message says, you cannot use `this` in static method. Just `i.x` should work. – Pradeep Simha Jan 20 '14 at 02:45
  • 1
    `this` is a speical variable which is automatically supplied to all *instances* of a class. `static` methods have no instance, thus they can not reference `this`....You might [Using the `this` keyword](http://docs.oracle.com/javase/tutorial/java/javaOO/thiskey.html) of some use... – MadProgrammer Jan 20 '14 at 02:45
  • i **is not** a member field, it's just a variable in a local scope. And main is a static method which do not reference a context object (not concerning how static is implemented under the hood). – TwilightSun Jan 20 '14 at 02:50
  • can someone comment on *why* `main` is `static`? – bcr Jan 20 '14 at 02:55

6 Answers6

3

You cannot use this.i.x since you cannot refer to instance variables inside a static method, in this case main(). To solve this just remove the this before i.x.

public static void main(String[] args)
{
    DDHThisTest i = new DDHThisTest(1, 2);
    i.x = 10;
}

Note: Don't forget to pass arguments to the constructor while creating an instance of DDHThisTest, because you have not declared a constructor without parameters.

Christian Tapia
  • 33,620
  • 7
  • 56
  • 73
3

main method is a static method. keyword this is to be used in object/instance scope not in a static block.

Simply replace this.i.x = 10; with i.x = 10; and you should be fine.

Nithya
  • 136
  • 6
2

When you are in a static block, you have no object scope. That means that there is not a particular instance of the class you are working with, so there is no reference to "this instance of the class".

Edwin Buck
  • 69,361
  • 7
  • 100
  • 138
2

Main is a static method and it is called before the object of DDHThisTest class is made. So ypu are trying ro refer to an instance of a class which is yet to be instantiated. So you are getting the error. In other words you know that only static variables can be accessed from static methods. But 'this' is an instance variable so to say; because of which you can't access it from static method.

Vipin Singh
  • 21
  • 3
1
i.x = 10;

In static function, there is NO this.

ikh
  • 10,119
  • 1
  • 31
  • 70
1

The keyword "this" is reserved to be used as part of an instance. If you remove the "this.", you should be fine.

Faran
  • 11
  • 1