Please could someone explain why the program outputs Point(10, 20);Point(0,20)

[Clemonte Johnstone]

I have added the below comments to the program in the constructor, but I still misunderstood what the program is doing. Thanks for any help that you can give.


/* Given the following code
  What will be the result of compiling and executing Test class?
  A) Point(10, 20);Point(0,20)         CORRECT
  B) Point(0, 20);Point(10,20)
  C) Point(10, 20);Point(10,20)        WRONG ANSWER
  D) None of the other options                                             */


class Point {
   int x;
   int y;
   void assign(int x, int y) {
       x = this.x;                // Redundant as the current value of class instance variable which is 0, is assigned to x
       this.y = y;                // The standard way to initialise param using this keyword
   }

   public String toString() {
       return "Point(" + x + ", " + y + ")";
   }
}

public class _31_Question_This_Keyword_And_Constructors {
   public static void main(String[] args) {
       Point p1 = new Point();
       p1.x = 10;
       p1.y = 20;
       Point p2 = new Point();
       p2.assign(p1.x, p1.y);
       System.out.println(p1.toString() + ";" + p2.toString());
   }
}

[Piet Souris]

Have a look at the assign method.

[Clemonte Johnstone]

Piet Souris wrote:Have a look at the assign method.

Please could you kindly elaborate.

I confused it for a constructor, but now I can see as you rightly say that its a method. However, this difference does not change the behaviour of the program.

[Campbell Ritchie]

How on earth could it be a constructor? Its name is nothing like the name of the class, for a start. You even marked the problem in your first post.

[Dave Tolls]

x = this.x;                // Redundant as the current value of class instance variable which is 0, is assigned to x

This is the line in question.
That comment explains why you get the result you see.

[Campbell Ritchie]

Dave Tolls wrote:. . . That comment explains . . .

The comment is a bit mild; that code isn't “redundant”, but it hides a logic error.

[Clemonte Johnstone]

Campbell Ritchie wrote:

Dave Tolls wrote:. . . That comment explains . . .

The comment is a bit mild; that code isn't “redundant”, but it hides a logic error.

Thanks Campbell. Please could you elaborate

[Campbell Ritchie]

Look what that line is assigning to what. What are the values of the different xs after that assignment? That sort of line is a classic mistake.