HashMap.get() works properly with inherited hashCode() method, doesn't recognize key with user-defined hashCode() method

Question

I have the following method in one of my classes. It's just a public wrapper around a HashMap (named teamOfPlayer, with keys of Player objects and values of Integer objects), nothing more.

public int getTeamOfPlayer(Player p)
{    
    return teamOfPlayer.get(p);
}

As long as my Player objects inherited the default hashCode() method from Object, this worked just fine. However, in order to save my Player objects to files, I implemented my own hashCode(). Suddenly, the method began throwing a NullPointerException.

I expanded the method as follows to print out some debugging information, but it's left me even more confused than before.

public int getTeamOfPlayer(Player p)
{
    Object[] o = teamOfPlayer.keySet().toArray();
    Player p2 = (Player) o[0];

    System.out.println("getTeamOfPlayer(" + p + ")"
        + "\n\thash of argument is " + p.hashCode()
        + "\n\tkeySet() of hashmap is " + teamOfPlayer.keySet() 
        + "\n\tcontainsKey() of hashmap is " + teamOfPlayer.containsKey(p) 
        + "\n\tplayer extracted from keySet() is " + p2 
        + "\n\tplayer extracted from keySet() has hash of" + p2.hashCode() 
        + "\n\targument.equals(key) returns " + p.equals(p2) 
        + "\n\tkey.equals(argument) returns " + p2.equals(p));

    int i = teamOfPlayer.get(p);
    return i;
}

The output of the above method is here:

getTeamOfPlayer(main.data.entities.Player@89f632df)
    hash of argument is -1980353825
    keySet() of hashmap is [main.data.entities.Player@89f632df]
    containsKey() of hashmap is false
    player extracted from keySet() is main.data.entities.Player@89f632df
    player extracted from keySet() has hash of-1980353825
    argument.equals(key) returns true
    key.equals(argument) returns true

The exception is thrown on the "int i = teamOfPlayer.get(p);" line, meaning that the map is returning null (because it doesn't think it contains the key). I know that's why the exception is being thrown. However, I think I've proven that the key DOES exist in the map. What's going on?

--

Update: Here are the equals() and hashCode() methods.

@Override
public boolean equals(Object obj)
{
    if (this == obj)
        return true;

    Player player;

    if (obj != null && obj instanceof Player)
        player = (Player) obj;
    else
        return false;

    if (status != player.status || !name.equals(player.name) || race != player.race || weeksOut != player.weeksOut || injuryType != player.injuryType
        || XP != player.XP)
        return false;

    for (int i = 0; i < 8; i++)
    {
        if (attributes[i] != player.attributes[i])
            return false;

        if (injuries[i] != player.injuries[i])
            return false;
    }

    for (int i = 0; i < 28; i++)
    {
        if (hasSkill[i] != player.hasSkill[i])
            return false;
    }

    for (int i = 0; i < 4; i++)
    {
        if (equipment[i] != player.equipment[i])
            return false;
    }

    return true;
}

@Override
public int hashCode()
{
    int hash = 11;

    hash = 31 * hash + status;
    hash = 31 * hash + name.hashCode();
    hash = 31 * hash + race;
    hash = 31 * hash + weeksOut;
    hash = 31 * hash + injuryType;
    hash = 31 * hash + XP;

    for (int i = 0; i < 8; i++)
    {
        hash = 31 * hash + attributes[i];
        hash = 31 * hash + injuries[i];
    }

    for (int i = 0; i < 28; i++)
        hash = hash + (hasSkill[i] ? 1 : 0);

    for (int i = 0; i < 4; i++)
        hash = 31 * hash + equipment[i];

    return hash;
}

please post the code for your hashCode() and equals() method — NickJ
@rgettman You can see from the debugging output that not only do both objects have identical hash codes, but they also equal each other. — Gamer_2k4
Your hashCode() method has some problems that may bite you down the road. For example hasSkill seems to be an array of 28 booleans. As coded, any two players with the same number of hasSkill elements true will have the same hashCode - even if none of the skills of one player match any of the others. For example, player A could have "good fastball" and "good curve ball" and player B could have "power hitter" and "great outfield speed" and both would have same hash code. But this is not the cause of your problem. Get something like Eclipse. It will generate a hashCode for you. Study it. — Steve Cohen
@SteveCohen Thanks for both tips. I never knew Eclipse could do that, and you're definitely right about the boolean array. Much appreciated. — Gamer_2k4

Steve Cohen Steve Cohen · Accepted Answer · 2013-11-14T20:38:36

Both these answers are right. More specifically

teamOfPlayer.get(p) is returning null (maybe because equals() and hashcode() are not in synch) and then getTeamOfPlayer() is trying to convert null into an int.

If that method returned Integer instead of int, you wouldn't have this problem, or if you coded it as

public int getTeamOfPlayer(Player p)
{    
    Integer t = teamOfPlayer.get(p);
    if (t == null) {
        return -1;
    } 
    return t;
}

you'd be okay. Probably you need to do both. By all means, fix hashCode(), but also, you probably need to consider the legitimate case of a player who is not on any team.

HashMap.get() works properly with inherited hashCode() method, doesn't recognize key with user-defined hashCode() method

3 Answers