Please explain prompter.java. Do not understand him passing the Game object into the Prompter object.

Question

Ok so here is the code:

Prompter.java

import java.io.Console;

//class
public class Prompter {
  private Game mGame;

  //constructor
  public Prompter(Game game){
  mGame = game;
  }

  public boolean promptForGuess(){
  Console.console = System.console();
    String guessAsString - console.readLine("Enter a letter:  ");
    char guess = guessAsString.charAt(0);
    return mGame.applyGuess(guess);
  }
}

Hangman.java

public class Hangman {

    public static void main(String[] args) {
        // Enter amazing code here:
        Game game = new Game("treehouse");
      Prompter = prompter = new Prompter(game);

      if(isHit){
      System.out.println("We got a hit!");
      } else {

      System.out.println("Whoops that was a miss");
      }
    }

}

So I really do not understand in Prompter.java this part:

//class
public class Prompter {
  private Game mGame;

  //constructor
  public Prompter(Game game){
  mGame = game;
  }

I get what is being passed but I do not understand. It looks like he first called a variable of mGame and said it was an object of Game. Then in the constructor he called another variable of the Game object and set it to the variable game. Then he set that variable to mGame.

Now why did he do that? please explain. Iv already went thought this course and this is the only part that I do not understand. I see him using the mGames through out the Prompter along with methods of Game.java which means he some how shared the methods of another class. I think this is how but I do not understand this part. Can someone please explain how this works.

I also do not understand in Hangman.java why he did

Prompter = prompter = new Prompter(game);

I understand Prompter is the class assigned to the instance variable of prompter as a new object of the class Prompter but why did he pass the variable "game" in the parameter?

I can see all this is to share methods of different classes but I do not understand how this is working or why.

Answer 1 · 2015-02-03T21:55:21Z

February 3, 2015 9:55pm

Yes, in the line:

    Prompter prompter = new Prompter(game);

The game variable is a reference to the Game object that was created on the previous line:

    Game game = new Game("treehouse");

What's going on in these lines can be broken down in to two pieces, a declaration and an initialization. In other words, we can rewrite Prompter prompter = new Prompter(game); as follows:

    Prompter prompter;              // Declare an instance of the class Prompter that is named prompter.
    prompter = new Prompter(game);  // Initialize the variable prompter to be a reference to the specific
                                    //   instance of Prompter returned by the Prompter constructor.
                                    //   More on 'game' below.

Because Java is a statically-typed, compiled language you need to declare the type of your variables before you use them. Optionally, at the class level, you can provide a scope keyword: private, protected, or public, to control the visibility of the thing you are declaring. Functions are declared with a slightly different format where you specify the return type first, the name of the function, and then the parameter list, which is itself a sequence of declarations that will be used inside the function block to define the function's implementation. Note, in addition to the visibility modifiers there are other keywords which can precede a declaration such as static and final, but that doesn't have much bearing in this conversation.

To recap:

Given the class declaration (which Java requires to be in it's own file named Prompter.java):

    class Prompter {
        private Game mGame;

        public Prompter(Game game){
            mGame = game;
        }
    }

What this is saying is that there is a class named Prompter with a private variable of type Game named mGame and a single constructor for objects of type Prompter that takes as an argument a variable of the type Game named game and inside the constructor it sets the value of the instance's private variable mGame to be the value of game. (I'm intentionally glossing over the reference vs. value discussion.)

Now inside the Hangman class we have a main method as follows:

public class Hangman {                                                                                                                                                                                         

    public static void main(String[] args) {                                                                                                                                                                   
        Game game = new Game("treehouse");                                                                                                                                                                     
        Prompter = prompter = new Prompter(game);                                                                                                                                                              

        // Rest of method elided for brevity.                                                                                                                                                                  
    }                                                                                                                                                                                                          

}

The body of the main method is the entry point of your program so this is the first code that will run, starting from the first line of main executing each statement consecutively. So the first line declares and initializes the variable game (what is its type?) and the second line uses the variable game to initialize the variable prompter that it just declared to have the type Prompter.

Answer 2 · 2015-02-03T08:01:20Z

February 3, 2015 8:01am

I haven't watched this video so I can't answer your question completely, but I can try to explain some of the parts that seem to be confusing you. The difficulty I think you are having (please correct me if I'm wrong) is due to the overlapping of some of the names assigned and the overloading of some words such as "prompt" and "game".

So at a high level, everywhere you see the word new in Java you are creating a new instance of a class, or instantiating a new instance of that class. When this happens, the constructor for that class is run with whatever arguments are required. The constructor is denoted in a class by declaring a public method with the same name as the class and without a return value. Its return value is implicit: a new instance of the class. So the Prompter class has one constructor with the parameter list: (Game game). In other words, in order to create a new instance of the Prompter class you need to give it an instance of Game to work with and inside the constructor, it will refer to that instance as game. In the particular case of this constructor, all it does is to assign the private instance member (more specifically property, in this case) mGame to game.

So in the main method of Hangman, you first create a Game, passing in the literal string argument "treehouse", then you create a new Prompter that keeps a reference to that Game which it privately refers to as mGame.

I hope that my explanation is helpful and I am happy to clarify any further confusion you have. My advice would be to review the scoping rules for Java and to focus on the distinctions between classes and instances, arguments and parameters. Often, the only way to know what a particular name is, is by knowing the context in which that name is declared.

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Lucas Santos

Lucas Santos

Please explain prompter.java. Do not understand him passing the Game object into the Prompter object.

2 Answers

Raymond Wach

Raymond Wach

Lucas Santos

Lucas Santos

Raymond Wach

Raymond Wach

Lucas Santos

Lucas Santos