Well Hi there. Recently, I've been getting a NullPointerException whenever I press a button in my gui and I'm not entirely sure why.

Here's what comes out in the console:

 

2012-08-24 18:42:49 [iNFO] [sTDERR] java.lang.NullPointerException
2012-08-24 18:42:49 [iNFO] [sTDERR] 	at net.minecraft.src.sorcery.TileEntityWandCrafting.craftWand(TileEntityWandCrafting.java:64)
2012-08-24 18:42:49 [iNFO] [sTDERR] 	at net.minecraft.src.sorcery.PacketHandler.onPacketData(PacketHandler.java:40)
2012-08-24 18:42:49 [iNFO] [sTDERR] 	at cpw.mods.fml.common.network.NetworkRegistry.handlePacket(NetworkRegistry.java:224)
2012-08-24 18:42:49 [iNFO] [sTDERR] 	at cpw.mods.fml.common.network.NetworkRegistry.handleCustomPacket(NetworkRegistry.java:212)
2012-08-24 18:42:49 [iNFO] [sTDERR] 	at cpw.mods.fml.common.network.FMLNetworkHandler.handlePacket250Packet(FMLNetworkHandler.java:63)
2012-08-24 18:42:49 [iNFO] [sTDERR] 	at net.minecraft.src.NetServerHandler.handleCustomPayload(NetServerHandler.java:1022)
2012-08-24 18:42:49 [iNFO] [sTDERR] 	at net.minecraft.src.Packet250CustomPayload.processPacket(Packet250CustomPayload.java:70)
2012-08-24 18:42:49 [iNFO] [sTDERR] 	at net.minecraft.src.MemoryConnection.processReadPackets(MemoryConnection.java:72)
2012-08-24 18:42:49 [iNFO] [sTDERR] 	at net.minecraft.src.NetServerHandler.networkTick(NetServerHandler.java:72)
2012-08-24 18:42:49 [iNFO] [sTDERR] 	at net.minecraft.src.NetworkListenThread.networkTick(NetworkListenThread.java:55)
2012-08-24 18:42:49 [iNFO] [sTDERR] 	at net.minecraft.src.IntegratedServerListenThread.networkTick(IntegratedServerListenThread.java:98)
2012-08-24 18:42:49 [iNFO] [sTDERR] 	at net.minecraft.server.MinecraftServer.updateTimeLightAndEntities(MinecraftServer.java:630)
2012-08-24 18:42:49 [iNFO] [sTDERR] 	at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:547)
2012-08-24 18:42:49 [iNFO] [sTDERR] 	at net.minecraft.src.IntegratedServer.tick(IntegratedServer.java:102)
2012-08-24 18:42:49 [iNFO] [sTDERR] 	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:453)
2012-08-24 18:42:49 [iNFO] [sTDERR] 	at net.minecraft.src.ThreadServerApplication.run(ThreadServerApplication.java:17)

 

 

My tile entity:

 

package net.minecraft.src.sorcery;

import net.minecraft.src.*;

import java.util.Random;

public class TileEntityWandCrafting extends TileEntity implements IInventory
{
    private ItemStack[] items = new ItemStack[4];

    public int getSizeInventory()
    {
        return 4;
    }
    
    public void craftWand()
    {
    	EnumWandMods[] wandMods = new EnumWandMods[3];
    	ItemStack wand = items[3];
    	ItemStack crystal = items[2];
    	ItemStack core1 = items[0];
    	ItemStack core2 = items[1];
    	
    	if(wand != null && crystal !=null)
    	{
    		if(wand.itemID == (new ItemStack(Sorcery.wand)).itemID && crystal.itemID == (new ItemStack(Sorcery.battery, 1, 0)).itemID && crystal.getItemDamage() == 0)
    		{
    			int j = 0;
    			
    			for(EnumWandMods mods : EnumWandMods.values())
    			{
    				if(core1 != null)
    				{
    					if(core1.itemID == EnumWandMods.values()[j].item.itemID)
    					{
    						wandMods[0] = SorceryHelper.getInstance().addWandMod(EnumWandMods.values()[j].item);
    						decrStackSize(0, 1);
   
    					}
    				}
    				
    				if(core2 != null)
    				{
    					if(core2.itemID == EnumWandMods.values()[j].item.itemID)
    					{
    						wandMods[1] = SorceryHelper.getInstance().addWandMod(EnumWandMods.values()[j].item);
    	        	    	decrStackSize(1, 1);
   
    					}
    				}
    				j = j+1;
    			}	
    			
    			decrStackSize(2, 1);
    			
    			wand.setTagCompound(new NBTTagCompound());
    			wand.stackTagCompound.setTag("mods", new NBTTagList("mods"));
    			NBTTagList list = (NBTTagList)wand.stackTagCompound.getTag("mods");
    			try
    			{
    				for(int i = 0; i < wandMods.length; i++)
    				{
    					NBTTagCompound tag = new NBTTagCompound();
    					tag.setInteger("ID", wandMods[i].modID); //<<--- Line 64
    					tag.setBoolean("Positive", wandMods[i].pos);
    					list.appendTag(tag);
    				}
    			}
    			catch(Exception e)
    			{
    				e.printStackTrace();
    			}
    		}
    	}
    }
    
    public ItemStack getStackInSlot(int par1)
    {
        return this.items[par1];
    }

    public ItemStack decrStackSize(int par1, int par2)
    {
        if (this.items[par1] != null)
        {
            ItemStack var3;

            if (this.items[par1].stackSize <= par2)
            {
                var3 = this.items[par1];
                this.items[par1] = null;
                this.onInventoryChanged();
                return var3;
            }
            else
            {
                var3 = this.items[par1].splitStack(par2);

                if (this.items[par1].stackSize == 0)
                {
                    this.items[par1] = null;
                }

                this.onInventoryChanged();
                return var3;
            }
        }
        else
        {
            return null;
        }
    }

    public ItemStack getStackInSlotOnClosing(int par1)
    {
        if (this.items[par1] != null)
        {
            ItemStack var2 = this.items[par1];
            this.items[par1] = null;
            return var2;
        }
        else
        {
            return null;
        }
    }

    public void setInventorySlotContents(int par1, ItemStack par2ItemStack)
    {
        this.items[par1] = par2ItemStack;

        if (par2ItemStack != null && par2ItemStack.stackSize > this.getInventoryStackLimit())
        {
            par2ItemStack.stackSize = this.getInventoryStackLimit();
        }

        this.onInventoryChanged();
    }

    public int func_70360_a(ItemStack par1ItemStack)
    {
        for (int var2 = 0; var2 < this.items.length; ++var2)
        {
            if (this.items[var2] == null || this.items[var2].itemID == 0)
            {
                this.items[var2] = par1ItemStack;
                return var2;
            }
        }

        return -1;
    }

    public String getInvName()
    {
        return "container.wandcrafting";
    }

    public void readFromNBT(NBTTagCompound par1NBTTagCompound)
    {
        super.readFromNBT(par1NBTTagCompound);
        NBTTagList var2 = par1NBTTagCompound.getTagList("Items");
        this.items = new ItemStack[this.getSizeInventory()];

        for (int var3 = 0; var3 < var2.tagCount(); ++var3)
        {
            NBTTagCompound var4 = (NBTTagCompound)var2.tagAt(var3);
            int var5 = var4.getByte("Slot") & 255;

            if (var5 >= 0 && var5 < this.items.length)
            {
                this.items[var5] = ItemStack.loadItemStackFromNBT(var4);
            }
        }
    }

    public void writeToNBT(NBTTagCompound par1NBTTagCompound)
    {
        super.writeToNBT(par1NBTTagCompound);
        NBTTagList var2 = new NBTTagList();

        for (int var3 = 0; var3 < this.items.length; ++var3)
        {
            if (this.items[var3] != null)
            {
                NBTTagCompound var4 = new NBTTagCompound();
                var4.setByte("Slot", (byte)var3);
                this.items[var3].writeToNBT(var4);
                var2.appendTag(var4);
            }
        }

        par1NBTTagCompound.setTag("Items", var2);
    }

    public int getInventoryStackLimit()
    {
        return 64;
    }

    public boolean isUseableByPlayer(EntityPlayer par1EntityPlayer)
    {
        return this.worldObj.getBlockTileEntity(this.xCoord, this.yCoord, this.zCoord) != this ? false : par1EntityPlayer.getDistanceSq((double)this.xCoord + 0.5D, (double)this.yCoord + 0.5D, (double)this.zCoord + 0.5D) <= 64.0D;
    }

    public void openChest() {}

    public void closeChest() {}
    
    @Override
    public void onInventoryChanged()
    {
    	
    }
}

 

 

This exception doesn't seem to hinder gameplay at all, but it's probably not a good idea for my mod to be pumping out exceptions all the time, so I want to fix it. Anyone know what's happening here? If you need more info, just ask.

Seems fairly straight forward, you're trying to access something that is null.

For one thing, you're specifying wandMods is 3 elements, and on a good day you're only setting 2.

You need to lower the size of the array to a proper one, and also add in a null check before you try to access soemthing that is possibly null.

Silly me.

Added in a null check and all is working fine.

Thanks lex.