java - HashMap.get() 与继承的 hashCode() 方法一起正常工作，不能识别用户定义的 hashCode() 方法的键

Question

我的一个类(class)中有以下方法。它只是 HashMap 的公共(public)包装器(名为 teamOfPlayer，具有 Player 对象的键和 Integer 对象的值)，仅此而已。

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

只要我的 Player 对象从 Object 继承了默认的 hashCode() 方法，就可以正常工作。但是，为了将我的 Player 对象保存到文件中，我实现了自己的 hashCode()。突然，该方法开始抛出 NullPointerException。

我扩展了如下方法来打印一些调试信息，但它让我比以前更加困惑。

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;
}

上面方法的输出在这里:

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

“int i = teamOfPlayer.get(p);”抛出异常行，意味着 map 返回 null(因为它认为它不包含键)。我知道这就是抛出异常的原因。但是，我认为我已经证明 map 中确实存在 key 。怎么回事？

--

更新:下面是 equals() 和 hashCode() 方法。

@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;
}

Answer 1

这两个答案都是正确的。更具体地说

teamOfPlayer.get(p) 返回 null(可能是因为 equals() 和 hashcode() 不同步)然后 getTeamOfPlayer() 尝试将 null 转换为 int。

如果该方法返回 Integer 而不是 int，您就不会遇到这个问题，或者如果您将其编码为

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

你会没事的。可能你需要两者都做。一定要修复 hashCode()，而且，您可能还需要考虑不在任何球队的球员的合法情况。