Question about abstract, extends & polymorphism

Question

Hello there! I'm trying to build a simple game in Java called Nim, where you are supposed to pick sticks from a pile, you can't pick more than half the pile and and the person who picks the last stick loses.

My problem is that I don't know how to do this in proper object oriented programming. I want it to be something like:

my superClass(Nim) to be abstract
User & Computer to extend from Nim.
Game to handle the game-logic and have the 'void main()'.

But I don't know how I should instantiate the User & Computer objects in the Game class. Either my thinking is wrong in all classes or I don't know the best to intantiate User & Computer in the Game class. I feel like if there were a map over the correlation between the classes it would look like:

          >    User         
      /                \
Nim                       >> Game       
      \                /
          > Computer

Does this make sense?

Here's some example code:

abstract public class Nim {

   //Fields
    private int pile = 10;

    //Getters
    public int GetPile(){
        return pile;
    }

    //Setters
    public void SetPile(int num){
        pile = num;
    }

    public boolean CheckPile(){
        boolean ok;
        if ( pile >= 1 )
            ok = true;
        else 
            ok = false;
        return ok;
    }
}

// User
public class User extends Nim {

    public void UserRemoveFromPile(){
        if ( CheckPile() ){
            int x = GetPile();
            System.out.println( "User tar bort från hög");
            x--;
            SetPile(x);
            System.out.println( "Antalet stickor kvar: " +  x );
        }
        else
            System.out.println( "Det är bara en sticka kvar");
    }
}

// Computer

public class Computer extends Nim {

    public int GenerateRandomNumber(){
        int min = 1;
        int max = GetPile()/2;
        int randomNum = ThreadLocalRandom.current().nextInt(min, max + 1);

        return randomNum;
    }
    public void ComputerRemoveFromPile( int y){
        if ( CheckPile () ){
            int x = GetPile();
            System.out.println( "Computer tar bort " + y + " från hög");
            x = x - y;
            SetPile(x);
            System.out.println( "Antalet stickor kvar: " +  x );
        }
        else 
            System.out.println( "Det är bara en sticka kvar");
    }

}

// Game 

public class Prompter {

    Upp2Test2 u = new User();
    Upp2Test2 c = new Computer();


    public void Play(){
        while ( u.GetPile() > 1 ){
            u.UserRemoveFromPile();
            c.ComputerRemoveFromPile(c.GenerateRandomNumber());
        }
    }

    public static void main(String[] args) {
        Prompter p = new Prompter();
        p.Play();
    }
}

Any help is apreciated. Thanks!

Answer 1 · 2017-02-07T16:01:25Z

February 7, 2017 4:01pm

Why do you want your Nim class to be abstract? You are not defining any abstract methods in it. Anyway, composition would still work if you pass in a concrete subclass of the abstract Nim class like this one:

public class NimGame extends Nim {
    ...
}

Then in a game class you could pass it in like this:

Nim nimGame = new NimGame();
Computer computer = new Computer(nimGame);

Florian

Answer 2 · 2017-02-07T14:26:17Z

February 7, 2017 2:26pm

Hi Björn,

I would not make User and Computer inherit from Nim. You want to ask yourself: Is a user a Nim? Is a computer a Nim? I would say it's not.

You can use composition and pass in the Nim object to the User and Computer constructor. Example:

public class Computer {
    Nim nim;

    public Computer(Nim nim) {
        this.nim = nim;
    }
}

Then you can call nim.checkPile() or nim.getPile() on the nim field.

Kind Regards, Florian

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Björn Norén

Björn Norén

Question about abstract, extends & polymorphism

2 Answers

Florian Tönjes

Florian Tönjes

Björn Norén

Björn Norén

Florian Tönjes

Florian Tönjes

Björn Norén

Björn Norén