CODEWITHSUNDEEP
java
joy
joy

Reputation: 741

why cannot use "this" in the main method?

   public class example {

    int a = 0;

    public void m() {
        int b = this.a;
    }

    public static void main(String[] args) {
        int c = this.a;
    }

}

I am new in java. Why I cannot use "this" in the main method?

Upvotes: 4

Views: 5090

Answers (7)

Narendra Pathai
Narendra Pathai

Reputation: 41935

public class Example {

    int a = 0;

    // This is a non-static method, so it is attached with an instance 
    // of class Example and allows use of "this".
    public void m() {            
        int b = this.a;
    }

    // This is a static method, so it is attached with the class - Example
    // (not instance) and so it does not allow use of "this".
    public static void main(String[] args) {
        int c = this.a; // Cannot use "this" in a static context.
    }
}

Upvotes: 1

Eddy
Eddy

Reputation: 3723

Additional of Andreas Fester answer:

/****GRAMMER:
 * 
 *  Why cannot use "this" in the main method?
 * 
 *  -----------------------------------------------------------------------------------
 * 
 *      "this" refers to the current object. However, the main method is static, 
 *      which means that it is attached to the class, not to an object instance.
 *  
 *      Which means that the static method can run without instantiate an object,
 *      Hence, there is no current "this" object inside of main().
 * 
 * 
 */

public class Test{
    int a ;
    public static void main(String[] args) {
        int c = this.a;   // Cannot use "this" in a static context, 
                          // because there isn't a "this".
    }
}

Upvotes: 0

Andreas Fester
Andreas Fester

Reputation: 36630

this refers to the current object. However, the main method is static, which means that it is attached to the class, not to an object instance, hence there is no current object inside main().

In order to use this, you need to create an instance of your class (actually, in this example, you do not use this since you have a separate object reference. But you could use this inside your m() method, for example, because m() is an instance method which lives in the context of an object):

public static void main(String[] args){
    example e = new example();
    int c=e.a;
}

By the way: You should get familiar with the Java naming conventions - Class names usually start with a capital letter.

Upvotes: 14

Thorkil Holm-Jacobsen
Thorkil Holm-Jacobsen

Reputation: 7676

The main method is static, meaning it can be called without instantiating example class (static methods can also be referred to as Class methods). This means that in the context of main, the variable a may not exist (because example is not an instantiated object). Likewise, you can't call m from main without first instantiating example:

public class example {

    int a=0;

    public void m(){
       // Do stuff
    }

    public static void main(String[] args){
       example x = new example();
       x.m(); // This works
       m(); // This doesn't work
    }
}

Upvotes: 0

java_dude
java_dude

Reputation: 4088

In addition to comments from Andreas

If you would like to use 'a'. Then you will have to instantiate new example [better make example class name as capital Eaxmple].

Something like 

public static void main(String[] args) {
  Example e = new Example();
  int c = e.a;
}

HTH

Upvotes: 0

Paolo
Paolo

Reputation: 2472

Becasue main is static. To access a you should declare it as static as well. Static variables exist without any instance of the class. Non-static variables exist only if an instance exist, and each instance has its own attribute named a.

Upvotes: 1

ollins
ollins

Reputation: 1849

You must create an instance of example

example e = new example()
e.m()

Upvotes: 2

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