HashMap总是返回null？

Question

出于某种奇怪的原因，我的 hashmap 总是返回一个空值，即使它不应该为空。该行在 mysql 数据库中不为空。我没有收到任何错误我进行了调试测试以查看它打印出来的内容并且它正在返回 null。

auth.java

private String uuid;
private String name;
private int bits;
private String gang;
private String rank;



public auth(String uuid, String name, int bits, String gang, String rank)
{
    this.uuid = uuid;
    this.name = name;
    this.bits = bits;
    this.gang = gang;
    this.rank = rank;
}

authmanager.java

public HashMap<String, auth> auth = new HashMap<>();



public void saveUser(String uuid, String name, int bits, String gang, String rank)
{

    this.auth.put(uuid, new auth(uuid, name, bits, gang, rank));

}

建立连接.java

public void establishProfile(Player p){

    String UUID = p.getUniqueId().toString();
    String NAME = p.getName();

    try
    {


        ResultSet query = sql.querySQL("SELECT * FROM `profiles` WHERE `UUID`= '" + UUID + "';");

        PreparedStatement CREATE = c.prepareStatement("INSERT INTO `profiles` (`UUID`,`Name`, `Bits`, `Bans`, `Gang`, `Rank`) VALUES (?, ?, ?, ?, ?, ?) ");
        PreparedStatement LOAD = c.prepareStatement("SELECT * FROM `profiles` WHERE `UUID`= ?");

        if ( query.next() )
        {
            LOAD.setString(1, UUID);
            plugin.authmanager.saveUser(query.getString("UUID"), query.getString("Name"), query.getInt("Bits"), query.getString("Gang"), query.getString("Rank"));
            LOAD.close();
            query.close();
            p.sendMessage(tables.PROFILE_LOADED);
            Bukkit.getConsoleSender().sendMessage(ChatColor.GREEN + "[SQL]" +
                    " has loaded profile for " + NAME + "(" + UUID + ")"  );


        } else

        {
            CREATE.setString(1, UUID);
            CREATE.setString(2, NAME);
            CREATE.setInt(3, 0);
            CREATE.setInt(4, 0);
            CREATE.setString(5, null);
            CREATE.setString(6, "Default");

            CREATE.executeUpdate();
            CREATE.close();
            p.sendMessage(tables.PROFILE_CREATED);
            Bukkit.getConsoleSender().sendMessage(ChatColor.YELLOW + "[SQL]" +
                    " executed a new query for " + NAME + "(" + UUID + "}" +  plugin.authmanager.auth.get(0) );

        }


    }   catch (Exception e)

    {
        e.printStackTrace();

            Bukkit.broadcastMessage("Someone's profile has failed to load!\n ERROR: " + e);
    }

}

Answer 1

如果您要将 auth 对象粘贴到 HashMap 中，那么您需要在 auth 类中添加 properly override equals and hashCode。

另外，你应该将你的班级命名为Auth 而不是auth 来关注standard Java naming conventions。所以将auth.java重命名为Auth.java，并定义如下（需要Java 7或更高版本）：

Auth.java

import java.util.Objects;

public class Auth {
    private String uuid;
    private String name;
    private int bits;
    private String gang;
    private String rank;

    public Auth() {
    }

    public Auth(String uuid, String name, int bits, String gang, String rank) {
        this.uuid = uuid;
        this.name = name;
        this.bits = bits;
        this.gang = gang;
        this.rank = rank;
    }

    public String getRank() {
        return rank;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }
        if (!(o instanceof Auth)) {
            return false;
        }
        Auth auth = (Auth) o;
        return Objects.equals(uuid, auth.uuid);
    }

    @Override
    public int hashCode() {
        return Objects.hash(uuid);
    }

    @Override
    public String toString() {
        return "Auth{" +
            "uuid='" + uuid + '\'' +
            ", name='" + name + '\'' +
            ", bits=" + bits +
            ", gang='" + gang + '\'' +
            ", rank='" + rank + '\'' +
            '}';
    }
}

我只包括了uuid 而不是equals 和hashCode 中的其他字段，这意味着如果两个Auths 具有相同的uuid，它们将被视为相等。如果在对象已将 put 更改为 HashMap 之后修改字段，则包含其他字段会使您出错。我认为这对您来说是最简单的方法，但要知道，对于持久性对象，equals 和 hashCode 中确切包含的内容是 subject of debate for some time。