Question

我试图将键和值放在另一个类的HashMap中。 NPE不应该发生。我可以正确设置实例设置，因为我可以毫无问题地访问RingCreation中的方法，但是当我尝试将对象放入Main中的HashMap时，它将返回空指针错误，但是当我生成HashMap的调试消息时，它检测到它不为空。我不想通过发布“又一个NPE”日志来造成麻烦。通常，我可以修复大多数出现的错误。

主班

HashMap<Integer,ItemStack> ringHolder = new HashMap<Integer,ItemStack>();
private RingCreation createRing;

private static Main instance;

public Main() {
    this.createRing = new RingCreation();

}

@Override
public void onEnable() {
    if(ringHolder.isEmpty()) System.out.println("ringholder is empty");
    instance = this;
    createRing.CreateLifeRing();
    createRing.CreateRegenerationRing();
    createRing.CreateStarterRing();
}

public static Main getInstance() {
    return instance;
}

}

RingCreation类：

private RingOfLife ringLife;
private StarterRing startRing;
private RingOfRegeneration regenRing;

private Main main = Main.getInstance();

public RingCreation() { 
    this.ringLife = new RingOfLife();
    this.startRing = new StarterRing();
    this.regenRing = new RingOfRegeneration();
}

ItemStack ringOfLife;
ItemMeta ringOfLifeMeta;

ItemStack starterRing;
ItemMeta starterRingMeta;

ItemStack ringOfRegeneration;
ItemMeta ringOfRegenerationMeta;

public void CreateLifeRing() {
    ringOfLife = new ItemStack(Material.RECORD_4);
    ringOfLifeMeta = ringOfLife.getItemMeta();
    ringOfLifeMeta.setDisplayName(ChatColor.GRAY+"Ring of"+ChatColor.GREEN+" Life");

    ringOfLifeMeta.addItemFlags(ItemFlag.values());
    ringOfLife.setItemMeta(ringOfLifeMeta);
    System.out.println("createlifering working");
    int loc = 2;
    main.ringHolder.put(loc, ringOfLife);
}

public void CreateStarterRing() {

    starterRing = new ItemStack(Material.RECORD_11);
    starterRingMeta = starterRing.getItemMeta();
    starterRingMeta.setDisplayName(ChatColor.GRAY+"Starter Ring");

    //removes the music disc lore V
    starterRingMeta.addItemFlags(ItemFlag.values());
    starterRing.setItemMeta(starterRingMeta);

    int loc = 1;
    main.ringHolder.put(loc, starterRing);
}

public void CreateRegenerationRing() {

    ringOfRegeneration = new ItemStack(Material.RECORD_5);
    ringOfRegenerationMeta = ringOfRegeneration.getItemMeta();
    ringOfRegenerationMeta.setDisplayName(ChatColor.GRAY+"Ring of"+ChatColor.GREEN+" Regeneration");
    ringOfRegenerationMeta.addItemFlags(ItemFlag.values());
    ringOfRegeneration.setItemMeta(ringOfRegenerationMeta);

    int loc = 3;
    main.ringHolder.put(loc, ringOfRegeneration);
}

错误：

[14:17:37 ERROR]: Error occurred while enabling VelocusRings v1.0 (Is it up to date?)
java.lang.NullPointerException: null

at me.redshadus.velocus.RingCreation.CreateLifeRing(RingCreation.java:44) ~[?:?]
at me.redshadus.velocus.Main.onEnable(Main.java:33) ~[?:?]
at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:264) ~[spigot-1.12.2.jar:git-Spigot-eb3d921-2b93d83]
at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader.java:337) [spigot-1.12.2.jar:git-Spigot-eb3d921-2b93d83]
at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:403) [spigot-1.12.2.jar:git-Spigot-eb3d921-2b93d83]
at org.bukkit.craftbukkit.v1_12_R1.CraftServer.enablePlugin(CraftServer.java:381) [spigot-1.12.2.jar:git-Spigot-eb3d921-2b93d83]
at org.bukkit.craftbukkit.v1_12_R1.CraftServer.enablePlugins(CraftServer.java:330) [spigot-1.12.2.jar:git-Spigot-eb3d921-2b93d83]
at net.minecraft.server.v1_12_R1.MinecraftServer.t(MinecraftServer.java:422) [spigot-1.12.2.jar:git-Spigot-eb3d921-2b93d83]
at net.minecraft.server.v1_12_R1.MinecraftServer.l(MinecraftServer.java:383) [spigot-1.12.2.jar:git-Spigot-eb3d921-2b93d83]
at net.minecraft.server.v1_12_R1.MinecraftServer.a(MinecraftServer.java:338) [spigot-1.12.2.jar:git-Spigot-eb3d921-2b93d83]
at net.minecraft.server.v1_12_R1.DedicatedServer.init(DedicatedServer.java:272) [spigot-1.12.2.jar:git-Spigot-eb3d921-2b93d83]
at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:545) [spigot-1.12.2.jar:git-Spigot-eb3d921-2b93d83]
at java.lang.Thread.run(Unknown Source) [?:?]

第33行：

createRing.CreateLifeRing();

RingCreation第44行：

main.ringHolder.put(loc, starterRing);

控制台还输出：createlifering工作

Answer 1

问题是您永远不会初始化instance的{{1}}：
更改：

Main

到

public static Main getInstance() {
    return instance;
}

这是“延迟加载”单例的典型模式。
另外，您应该将构造函数设为私有public static Main getInstance() { if(instance == null) { instance = new Main(); } return instance; }，因为我希望应该只有private Main()的一个实例。

Answer 2

我认为您忘记创建一个Main实例。

尝试在其他类中修改HashMap时使用NPE

2 个答案: