Question

I’m sorry if this title doesn’t describe the problem properly but I wasn’t sure how to describe it.

I have a method called changeToWhite() that can be called on a Piece
e.g. Piece.changeToWhite()

but for me to be able to change the piece to white I need access to the piece so I decided to pass it in as an argument
e.g. Piece.changeToWhite(Piece)

The passing in as an argument seems unnecessary.

The toUpperCase() functions does it somehow e.g. someString.toUpperCase()

How can I do this?

Answer 1

If you define changeToWhite on a Piece class, then on an instance you can use this to get the reference to the current object. Something like:

class Piece {
    private String color;
    ...
    public void changeToWhite() {
       this.color = 'white';
    }
}

Note that when you define a method on a class, you need to have an instance on which to call the methods.

So

Piece piece1 = new Piece();
piece1.changeToWhite();

Note that the Java standard is to use uppercase to define a class (e.g. Piece), and camel case (e.g. changeToWhite) for instance methods and fields.

The OTHER way to do it would be to use a static method. In this case, the method belongs to the class, it doesn’t have a this context like instance methods do

class Piece {
   private String color; // instance field


   /**
       Takes a piece instance as an argument, and operates on that.
    */
   private static void changeToWhite(Piece piece) {
      piece.setColor('white'); // assume setColor exists
   }

}

but the first way is preferable.