有关Java构造函数（用于类及其超类）的问题

Question

I'm trying to figure out how the constructors work in the following program. Basically there is a Seeker class that extends Player class, and a Game class that contains Seeker object, as shown in the code below.

My question is, when the Game class instantiates mySeeker using mySeeker = new Seeker( "Sally", this ); does this invoke the player's constructor? If so, where did it pass on the name of the game? I saw a Game g in the constructor for Player , but I can't figure out how the game myGame (in the main method) has been passed to the Seeker .

//Game class

class Game {

   Seeker mySeeker;
   int gridSize;

   Game( int gs ) {
     gridSize = gs;
     mySeeker = new Seeker( "Sally", this ); //what does 'this' do here? why it does not include the name of the game?
   }

   void play() {
     mySeeker.seek();
   }
}

//Player class

  abstract class Player { 

     private Point location;
     private String name;
     Game game;

Player( Point p, String n, Game g ) {
    location = p; 
    name = n;
    game = g;
}

//Seeker class

class Seeker extends Player {

   Seeker( String n, Game g ) {
       super( new Point( 0, 0 ), n, g ); // seeker starts at 0,0
}

//Main Program

public static void main(String[] args) {

    int gridSize = 3; // default value
    Game myGame = new Game( gridSize );
    myGame.play();
}

Answer 1

The "this" reference is referring to the instance of the Game class. The game class is lending itself to the seeker/player class so the seeker/player class can access data from the game class.

edit: The "seeking" is being handed "this" instance of "Game" and using the "super" command to pass data to it's inherited "player"

Answer 2

My question is, when the Game class instantiate mySeeker using mySeeker = new Seeker( "Sally", this ); does ``this invoke the player's constructor?

No.

The Player constructor >>is<< being invoked, but not because of this . In the context where that statements comes from, this will denote a Game instance. It is "just a parameter" that is being passed to the Seeker constructor.

The Player constructor is being invoked in the Seeker` constructor; specifically this line:

    super( new Point( 0, 0 ), n, g );

What happens is that when you new the Seeker , the Seeker constructor is called, and it immediately calls the Player constructor, via the super call.

---

If so, where did it pass on the name of the game?

The line above is where the Game object is being passed to Player . See the g parameter? It the same Game object that you passed as this .

(The code you have shown us doesn't support a "name" for the game. What is being passed is a reference. And when you assign the new Game instance to myGame , it isn't the "name of the game". The myGame variable is simply that ... a variable ... that can contain a reference to some Game object.)

Answer 3

Seeker gets instantiated with a name and a Game object. Passing this is just a reference to the current object. The Seeker will be created with the already created Game object meaning the Seeker can access data from the Game object.

Answer 4

mySeeker = new Seeker( "Sally", this )

The above code allocate a Seeker object, and then execute its constructor to initiate its value, and then assign the variable mySeeker with a reference on the new Seeker object.

The constructor is called with two arguments: a string and this . this refers to the current object that is executing the above code, in this case it refers to the Game object that is being constructed.