CODEWITHSUNDEEP
javamethods
Mike Khan
Mike Khan

Reputation: 595

How do Java methods using who passed them in them

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?

Upvotes: 1

Views: 66

Answers (2)

Rick Mangi
Rick Mangi

Reputation: 3769

This is the difference between static and instance methods. In your example, calling Piece.changeToWhite() is a static method, but you are trying to access instance variables (the type of piece).

You have two choices:

  1. Do as you describe and pass in the piece you want to change.
  2. Make it an instance method and then do something like Piece p1 = new Piece(); p1.changeToWhite();

Upvotes: 3

hvgotcodes
hvgotcodes

Reputation: 120268

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.

Upvotes: 9

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