Question

this is the player class

package day6;

public class Player {
int playerId;

public Player(int playerId) {
    super();
    this.playerId = playerId;
}

public int getPlayerId() {
    return playerId;
}

}

this is the position class

package day6;

public class Position {


    private int positionNumber = 0;

    public Position(int positionNumber) throws Exception {
        super();
        this.positionNumber = positionNumber;

    }

    public int getPositionNumber() {
        return positionNumber;
    }

    }

this is the game class

 package day6;

    import java.util.ArrayList;
    import java.util.HashMap;
    import java.util.List;
    import java.util.Random;

    public class Game {


        private int numberOfPlayers;


        HashMap<Player, Position> hashMapForPositionOfPlayer = new HashMap<Player, Position>(numberOfPlayers);

        @SuppressWarnings("unchecked")
        public Game(int numberOfPlayers) throws Exception {
            super();

            this.numberOfPlayers = numberOfPlayers;
            for (int i = 0; i <numberOfPlayers; i++) {
                hashMapForPositionOfPlayer.put(new Player(i), new Position(0));
            }
        }

        public void play() throws Exception {

            Position currentPosition;




                for (int i = 0; i < numberOfPlayers; i++) {

                    Player player = new Player(i);
                    currentPosition = hashMapForPositionOfPlayer.get(player);
                    int value = currentPosition.getPositionNumber();

                    }
                }
            }

I tried to run the program Game class and game.play() but it is showing null value to the 0th value of the hashmap.

it means currentPosition=null for hashmap of key value 0.

Help me..

Answer 1

Since you’re using Player instances as keys of the HashMap, it should implement hashCode() and equals() consistently, so that two Player instances are considered to represent the same player when their playerId‘s are equal.

Without implementing equals() properly, the following snippet would return false:

Player p1 = new Player(1);
Player p2 = new Player(1);
return p1.equals(p2);

Also, given the nature of the Map implementation you’ve chosen, HashMap, the check for existing keys in the Map will be associated to their hashCode, so it is necessary to also implement hashCode to reflect that two Player instances are equal if their playerId‘s are.

The following question might be useful to you: Understanding the workings of equals and hashCode in a HashMap.

Sample code:

package day6;

public class Player {
    int playerId;

public Player(int playerId) {
    super();
    this.playerId = playerId;
}

public int getPlayerId() {
    return playerId;
}

public int hashCode(){
    return playerId.hashCode();
}

public boolean equals(Object o){
    if (o instanceOf Player) { 
        return playerId.equals( ((Player)o).getPlayerId());
    }
    return false;
}
}