Is it possible to access variable of subclass using object of superclass in polymorphism

问题

How can I access state variable of class KeyBoardPlayer with an object of class KalaPlayer?

/**
  * An abstract class representing a player in Kala.  Extend this class
  * to make your own players (e.g. human players entering moves at the keyboard
  * or computer players with programmed strategies for making moves).
  */
public abstract class KalaPlayer {

    /**
      * Method by which a player selects a move.
      * @param gs The current game state
      * @return A side pit number in the range 1-6
      * @throws NoMoveAvailableException if all side pits for the player are empty 
      * (i.e. the game is over)
      */
    public abstract int chooseMove(KalaGameState gs) throws NoMoveAvailableException;

}



public class KeyBoardPlayer extends KalaPlayer {
    /**
     * Method by which a player selects a move.
     * @param gs The current game state
     * @return A side pit number in the range 1-6
     * @throws NoMoveAvailableException if all side pits for the player are empty 
     * (i.e. the game is over)
     */
    public KalaGameState state;

    public KeyBoardPlayer() {
        System.out.println("Enter the number of stones to play with: ");
        try {
            BufferedReader br = new BufferedReader(new InputStreamReader(System.in));  
            int key = Integer.parseInt(br.readLine());  
            state=new KalaGameState(key);
            //key=player1.state.turn;
        } catch(IOException e) {
            System.out.println(e);
        }
    }

    public int chooseMove(KalaGameState gs) throws NoMoveAvailableException{
        return 0;
    }
}

import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;

public class KalaGame {
    KalaPlayer player1,player2;

    public KalaGame(KeyBoardPlayer Player1,KeyBoardPlayer Player2) {
        //super(0);
        player1=new KeyBoardPlayer();
        player2 = new KeyBoardPlayer(); 
        //player1=Player1;
        //player2=Player2;
        //player1.state  ****how can i access the stae variable from Keyboard CLass using object from KalaPlayer 
        key=player1.state.turn;
    }

    public void play() {
        System.out.println("Enter the number of stones to play with: ");
        try {
            BufferedReader br = new BufferedReader(new InputStreamReader(System.in));  
            int key = Integer.parseInt(br.readLine());  
            System.out.println(key);
            KalaGameState state=new KalaGameState(key);
            printGame();
        } catch(IOException  e) {
            System.out.println(e);
        }
    }
}

回答1:

You can't.

There is no ability access members of a subclass from its parent. If there were, the following would cause mass chaos:

public class Vehicle
{

}

public class Car extends Vehicle 
{
    SteeringWheel _wheel = new SteeringWheel();

    public SteeringWheel getSteeringWheel { return _wheel; }

    public SteeringWheel setSteeringWheel(SteeringWheel value)
    {
        _wheel = value;
    }
}

public class Bicycle extends Vehicle
{

}

Then:

Vehicle myVehicle = new Bicycle();

// This call couldn't possibly work since Bicylce has no steering wheel
// and Vehicle isn't aware of what the type of the is/isn't.
SteeringWheel myWheel = myVehicle.getSteeringWheel();

回答2:

It is impossible to do.

But you can take the line:

public KalaGameState state;

and put it in the definition of the class KalaPlayer. This assumes that each subclass of KalaPlayer should have a state of itself.

You can then still set the state in your constructor for each instance you are creating.

So this is more a design problem than a problem with the language because your superclass should contain all of the information which is common to all subclasses.

来源：https://stackoverflow.com/questions/2593258/is-it-possible-to-access-variable-of-subclass-using-object-of-superclass-in-poly

标签

polymorphism

subclass

superclass