null pointer exception with hashmap

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;
}
}

Answer 2

Firstly, I would use an ArrayList for indexed from 0 access.

In your case, the problem you have is that Player doesn't implement equals and hashCode so that means that new Player(0) and new Player(0) are two different objects which happen to have a value which is the same.

Answer 3

The problem is that two different instances of your Player class won't compare equal.

Player player1 = new Player(0);
Player player2 = new Player(0);
System.out.println(player1.equals(player2));

But I think that a hashmap isn't an appropriate structure here. The solution is to store the players in an array.

for (int i = 0; i < numberOfPlayers; i++) {
    Player player = players[i];
    // etc...
}

Answer 4

Root cause is that Player does not implement hashCode and equals. So when hashMapForPositionOfPlayer.get(player) searches the map, it will look for an object with another hashCode then the object that was put in.

Add the following code to Player, it should do the trick

@Override public int hashCode() { return 0; }

@Override
public boolean equals(Object obj) {
    Player other = (Player) obj;
    return this.playerId == other.playerId;
}

Answer 5

You create a series of players in the constructor of Game and put them in your HasMap, but when you do your look up in the play method, you create new players and try using those as the look up.

These are two different references, thus the HashMap can't find the player (you just created) and returns null

Create & populate HasMap

hashMapForPositionOfPlayer.put(new Player(i), new Position(0));

Create & find ... Null

Player player = new Player(i); // this reference does not exist in the HasMap
currentPosition = hashMapForPositionOfPlayer.get(player);

Answer 6

Or forget about using Player as the key and use the player ID instead.

Map<Integer, Position> 

would require a lookup for position using the player ID.