为什么java给我这个NullPointerException？

时间：2013-01-20 02:22:47

标签： java plugins nullpointerexception minecraft bukkit

好的，所以我一直在使用Eclipse为Minecraft制作Bukkit插件。出于某种原因，我得到了许多看似不合理的NullPointerExceptions。我发现当您尝试使用值为null的varriable类型的值和函数时，会发生NullPointerExceptions。我在我的代码中找不到它，所以我想知道是否有其他人可以找到它。

错误：

20:16:58 [SEVERE] java.lang.NullPointerException
20:16:58 [SEVERE]       at com.tommy3244.plugins.MakeYourOwnBlocks.MakeYourOwnBl
ocks.onDisable(MakeYourOwnBlocks.java:73)
20:16:58 [SEVERE]       at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlug
in.java:219)
20:16:58 [SEVERE]       at org.bukkit.plugin.java.JavaPluginLoader.disablePlugin
(JavaPluginLoader.java:481)
20:16:58 [SEVERE]       at org.bukkit.plugin.SimplePluginManager.disablePlugin(S
implePluginManager.java:400)
20:16:58 [SEVERE]       at org.bukkit.plugin.SimplePluginManager.disablePlugins(
SimplePluginManager.java:393)
20:16:58 [SEVERE]       at org.bukkit.plugin.SimplePluginManager.clearPlugins(Si
mplePluginManager.java:434)
20:16:58 [SEVERE]       at org.bukkit.craftbukkit.v1_4_6.CraftServer.reload(Craf
tServer.java:563)
20:16:58 [SEVERE]       at org.bukkit.Bukkit.reload(Bukkit.java:184)
20:16:58 [SEVERE]       at org.bukkit.command.defaults.ReloadCommand.execute(Rel
oadCommand.java:23)
20:16:58 [SEVERE]       at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCo
mmandMap.java:186)
20:16:58 [SEVERE]       at org.bukkit.craftbukkit.v1_4_6.CraftServer.dispatchCom
mand(CraftServer.java:514)
20:16:58 [SEVERE]       at org.bukkit.craftbukkit.v1_4_6.CraftServer.dispatchSer
verCommand(CraftServer.java:506)
20:16:58 [SEVERE]       at net.minecraft.server.v1_4_6.DedicatedServer.al(Dedica
tedServer.java:260)
20:16:58 [SEVERE]       at net.minecraft.server.v1_4_6.DedicatedServer.r(Dedicat
edServer.java:225)
20:16:58 [SEVERE]       at net.minecraft.server.v1_4_6.MinecraftServer.q(Minecra
ftServer.java:494)
20:16:58 [SEVERE]       at net.minecraft.server.v1_4_6.MinecraftServer.run(Minec
raftServer.java:427)
20:16:58 [SEVERE]       at net.minecraft.server.v1_4_6.ThreadServerApplication.r
un(SourceFile:849)

这是我的实际插件代码：

package com.tommy3244.plugins.MakeYourOwnBlocks;

import java.io.File;
import java.util.*;

import org.bukkit.block.*;
import org.bukkit.command.*;
import org.bukkit.configuration.ConfigurationSection;
import org.bukkit.entity.Player;
import org.bukkit.event.*;
import org.bukkit.event.block.Action;
import org.bukkit.event.player.PlayerInteractEvent;
import org.bukkit.inventory.ItemStack;
import org.bukkit.inventory.meta.ItemMeta;
import org.bukkit.plugin.java.JavaPlugin;

public class MakeYourOwnBlocks extends JavaPlugin implements Listener
{
    public Map<NewItem, List<ItemStack>> items;
    public File configfile;
    @SuppressWarnings("unchecked")
   public void onEnable()
 {
      try
        {
          Map<List<Map<String, Object>>, NewItem> smap;
            smap = SaveLoadAPI.load(this.getDataFolder().getPath()+"\\Items.data");
          for(List<Map<String, Object>> maps : smap.keySet())
            {
              NewItem item = smap.get(maps);
             List<ItemStack> stacks = new ArrayList<ItemStack>();
               for(Map<String, Object> map : maps)
              {
                  stacks.add(ItemStack.deserialize(map));
                }
              items.put(item, stacks);
           }
      }
      catch(Exception e)
     {
          items = new HashMap<NewItem, List<ItemStack>>();
       }
      try
        {
          this.getDataFolder().mkdirs();
     }
      catch(Exception e)
     {
          
       }
      configfile = new File(getDataFolder(), "config.yml");
        if(!configfile.exists())
       {
          saveResource("config.yml", false);
       }
      try
        {
          getConfig().load(configfile);
      }
      catch(Exception error)
     {
          getLogger().severe("Could not load config!");
            error.printStackTrace();
       }
      getServer().getPluginManager().registerEvents(this, this);
     getLogger().info("MakeYourOwnBlocks successfully enabled!");
 }
  public void onDisable()
    {
      try
        {
          Map<List<Map<String, Object>>, NewItem> smap = new HashMap<List<Map<String, Object>>, NewItem>();
          for(NewItem item : items.keySet())
         {
              List<ItemStack> stacks = items.get(item);
                List<Map<String, Object>> stacks2 = new ArrayList<Map<String, Object>>();
              for(ItemStack stack : stacks)
              {
                  stacks2.add(stack.serialize());
                }
              smap.put(stacks2, item);
           }
          SaveLoadAPI.save(smap, this.getDataFolder().getPath()+"\\Items.data");
       }
      catch (Exception e)
        {
          getLogger().severe("Was unable to save the items file to the default path of: "+this.getDataFolder().getPath()+"\\Items.data");
            e.printStackTrace();
       }
  }
  @EventHandler(priority = EventPriority.HIGHEST)
    public void onItemStackRightClick(PlayerInteractEvent e)
   {
      Player player = e.getPlayer();
     //getLogger().info("Player "+player.getName()+" has right clicked.");
      for(NewItem item : items.keySet())
     {
          for(ItemStack curstack : items.get(item))
          {
              ItemMeta lore = ((ItemStack)curstack).getItemMeta();
               List<String> lore2 = player.getItemInHand().getItemMeta().getLore();
             if(lore == null || lore2 == null)
              {
                  getLogger().info("Pointer null!");
                   if(lore == null)
                   {
                      getLogger().info("Pointer 1 null");
                  }
                  else
                   {
                      getLogger().info("Pointer 2 null");
                  }
              }
              if(lore2.equals(lore))
             {
                  getLogger().info(player.getDisplayName()+" right clicked with the same item!");
                  boolean right = e.getAction() == Action.RIGHT_CLICK_AIR || e.getAction() == Action.RIGHT_CLICK_BLOCK;
                  boolean block = e.getAction() == Action.RIGHT_CLICK_BLOCK || e.getAction() == Action.LEFT_CLICK_BLOCK;
                 boolean worked;
                    if(right)
                  {
                      worked = item.onRightClick(player, right, block);
                  }
                  else
                   {
                      worked = item.onLeftClick(player, right, block);
                   }
                  if(!worked)
                    {
                      player.sendMessage("Script fault! Error raised.");
                   }
              }
          }
      }
  }
  public boolean onCommand(CommandSender sender, Command cmd, String label, String[] args)
   {
      if(sender instanceof Player)
       {
          
       }
      else
       {
          sender.sendMessage("You must be a player to perform this command!");
         return false;
      }
      if(cmd.getName().equalsIgnoreCase("makeitem"))
       {
          if(args.length < 4)
         {
              return false;
          }
          String name = args[0];
         String type = args[1];
         String data = args[2];
         String damage = args[3];
           try
            {
              int dmg = Integer.parseInt(damage);
                byte dta = (byte)Integer.parseInt(data);
               int id = Integer.parseInt(type);
               if(getConfig().contains("items."+name))
              {
                  ConfigurationSection section = getConfig();
                    List<List<String>> lines = new ArrayList<List<String>>();
                  lines.add(null);
                   lines.add(null);
                   if(getConfig().contains("items."+name+".onrightclick"))
                    {
                      List<String> cur = getConfig().getStringList("items."+name+".onrightclick");
                     lines.set(0, cur);
                 }
                  else
                   {
                      lines.set(0, null);
                    }
                  if(getConfig().contains("items."+name+".onleftclick"))
                 {
                      List<String> cur = getConfig().getStringList("items."+name+".onleftclick");
                      lines.set(1, cur);
                 }
                  else
                   {
                      lines.set(1, null);
                    }
                  List<ItemStack> stacks = new ArrayList<ItemStack>();
                   ItemStack stack = new ItemStack(id, 1, dta);
                   ItemMeta meta = stack.getItemMeta();
                   meta.setDisplayName(name);
                 List<String> lore;
                   if(getConfig().contains("items."+name+".description"))
                 {
                      lore = getConfig().getStringList("items."+name+".description");
                    }
                  else
                   {
                      lore = new ArrayList<String>();
                  }
                  lore.add(name);
                    NewItem item = new NewItem(lore, name, id, dta, lines, dmg, false, section);
                   meta.setLore(lore);
                    stack.setItemMeta(meta);
                   getLogger().info(stack.toString());
                    stacks.add(stack);
                 if(items == null)
                  {
                      getLogger().info("unused");
                  }
                  items.put(item, stacks);
                   Player player = (Player)sender;
                    player.getLocation().getWorld().dropItem(player.getLocation(), stack);
                 player.sendMessage("Here you go!");
              }
              else
               {
                  sender.sendMessage("No configuration section at \""+"items."+name+"\"");
                   return true;
               }
          }
          catch(Exception error)
         {
              //sender.sendMessage("Error: "+error.getLocalizedMessage());
             //System.out.println(error);
               //throw error;
             //makeitem test 272 0 0
                error.printStackTrace();
               return true;
           }
          sender.sendMessage("Error: No command specified");
       }
      return true;
   }
}

更新：

nakib回答了我的问题，错误已修复！非常感谢！

4 个答案:

答案 0 :(得分：2)

在第73行，你有

for(NewItem item : items.keySet())

调用items时，属性onDisable()为空。

要解决此问题，您应该初始化声明中的items 

public Map<NewItem, List<ItemStack>> items = new HashMap<NewItem, List<ItemStack>>();

答案 1 :(得分：1)

items变量必须为null。您可以在课程顶部修复它：

public Map<NewItem, List<ItemStack>> items = new HashMap<NewItem, List<ItemStack>>();

你看起来有几个区域你认为它已经被初始化了，所以我要么在类的顶部，要么在你的构造函数中（它还不存在:)。

答案 2 :(得分：1)

从它的外观来看，我打赌items（第73行）为空，你要求它的值。添加一些内容以确保如果第28行的for块没有至少运行一次，则不会调用该方法。

答案 3 :(得分：0)

堆栈跟踪告诉您NullPointerException来自：

    20:16:58 [SEVERE] java.lang.NullPointerException
    20:16:58 [SEVERE]       at com.tommy3244.plugins.MakeYourOwnBlocks.MakeYourOwnBl
    ocks.onDisable(MakeYourOwnBlocks.java:73) <-- HELPFUL ADVICE FROM STACK TRACE


68     public void onDisable()
69        {
70          try
71            {
72              Map<List<Map<String, Object>>, NewItem> smap = new HashMap<List<Map<String, Object>>, NewItem>();
73              for(NewItem item : items.keySet()) <--- THIS LINE IS PUKING, SO items is probably null!

了解如何解释这些消息以及使用调试器在现代IDE中逐步执行代码至关重要。如果没有这些工具，您就没有足够的能力来导航大型代码库。

我给出的另一条建议是smap不是面向对象的。某种类型的封装在这里会有所帮助，因此您不需要Map List Map的{​​{1}} .....我们还在谈论什么？

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