Why This Keyword Cannot Be Used in the Main Method of Java Class

The static methods belong to the class and they will be loaded into the memory along with the class. You can invoke them without creating an object. (using the class name as reference).

Example

public class Sample{
   static int num = 50;
   public static void demo(){
      System.out.println("Contents of the static method");
   }
   public static void main(String args[]){
      Sample.demo();
   }
}

Output

Contents of the static method

The "this" keyword is used as a reference to an instance. Since the static methods doesn’t have (belong to) any instance you cannot use the "this" reference within a static method. If you still, try to do so a compile time error is generated.

And main method is static therefore, you cannot use the "this" reference in main method.

Example

public class Sample{
   int num = 50;
   public static void main(String args[]){
      System.out.println("Contents of the main method"+this.num);
   }
}

Compile time error

Sample.java:4: error: non-static variable this cannot be referenced from a static context
   System.out.println("Contents of the main method"+this.num);
                                                    ^
1 error