I'm not sure whether it's not saving properly or not loading properly. I can put items into the slots, but when I close and reopen, it isn't there.

 

Inventory Class:

package ttm.inventory;

import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.inventory.IInventory;
import net.minecraft.item.ItemStack;
import net.minecraft.nbt.NBTTagCompound;
import net.minecraft.nbt.NBTTagList;

public class InventoryPlayerExtended implements IInventory {
public EntityPlayer player;
public ItemStack[] inventory = new ItemStack[9];

public InventoryPlayerExtended(EntityPlayer player) {
	this.player = player;
}

@Override
public ItemStack decrStackSize(int par1, int par2) {
	ItemStack[] aitemstack = this.inventory;

	if(aitemstack[par1] != null) {
		ItemStack itemstack;

		if(aitemstack[par1].stackSize <= par2) {
			itemstack = aitemstack[par1];
			aitemstack[par1] = null;
			return itemstack;
		}
		else {
			itemstack = aitemstack[par1].splitStack(par2);

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

			return itemstack;
		}
	}
	else {
		return null;
	}
}

@Override
public ItemStack getStackInSlotOnClosing(int par1) {
	ItemStack[] aitemstack = this.inventory;

	if(aitemstack[par1] != null) {
		ItemStack itemstack = aitemstack[par1];
		aitemstack[par1] = null;
		return itemstack;
	}
	else {
		return null;
	}
}

@Override
public void setInventorySlotContents(int par1, ItemStack par2ItemStack) {
	inventory[par1] = par2ItemStack;
}

@Override
public int getSizeInventory() {
	return this.inventory.length;
}

@Override
public ItemStack getStackInSlot(int par1) {
	return inventory[par1];
}

public void dropAllItems() {
	int i;

	for(i = 0; i < this.inventory.length; ++i) {
		if(this.inventory[i] != null) {
			this.player.func_146097_a(this.inventory[i], true, false);
			this.inventory[i] = null;
		}
	}
}

public void copyInventory(InventoryPlayerExtended inventory) {
	int i;

	for(i = 0; i < this.inventory.length; ++i) {
		this.inventory[i] = ItemStack.copyItemStack(inventory.inventory[i]);
	}
}

public NBTTagList writeToNBT(NBTTagList par1NBTTagList) {
	System.err.println("WRITE");
	int i;
	NBTTagCompound nbttagcompound;

	for(i = 0; i < this.inventory.length; ++i) {
		if(this.inventory[i] != null) {
			nbttagcompound = new NBTTagCompound();
			nbttagcompound.setByte("Slot", (byte) (i));
			this.inventory[i].writeToNBT(nbttagcompound);
			par1NBTTagList.appendTag(nbttagcompound);
		}
	}

	return par1NBTTagList;
}

public void readFromNBT(NBTTagList par1NBTTagList) {
	System.err.println("READ");
	this.inventory = new ItemStack[9];

	for(int i = 0; i < par1NBTTagList.tagCount(); ++i) {
		NBTTagCompound nbttagcompound = par1NBTTagList.getCompoundTagAt(i);
		int j = nbttagcompound.getByte("Slot") & 255;
		ItemStack itemstack = ItemStack.loadItemStackFromNBT(nbttagcompound);

		if(itemstack != null) {
			if(j >= 0 && j < this.inventory.length) {
				this.inventory[j] = itemstack;
			}
		}
	}
}

@Override
public String getInventoryName() {
	return "container.inventory.extended";
}

@Override
public boolean hasCustomInventoryName() {
	return false;
}

@Override
public int getInventoryStackLimit() {
	return 64;
}

@Override
public void markDirty() {
}

@Override
public boolean isUseableByPlayer(EntityPlayer var1) {
	return true;
}

@Override
public void openInventory() {
	if(!player.worldObj.isRemote) {
		NBTTagList nbttaglist = player.getEntityData().getTagList("InventoryExtended", 10);
		this.readFromNBT(nbttaglist);
	}
}

@Override
public void closeInventory() {
	if(!player.worldObj.isRemote) {
		NBTTagList nbttaglist = player.getEntityData().getTagList("InventoryExtended", 10);
		this.writeToNBT(nbttaglist);
	}
}

@Override
public boolean isItemValidForSlot(int var1, ItemStack var2) {
	return true;
}

public static InventoryPlayerExtended forPlayer(EntityPlayer player) {
	InventoryPlayerExtended res = new InventoryPlayerExtended(player);
	res.openInventory();
	return res;
}
}

 

The prints in the read and write methods are being called correctly.

This code does not do what you think it does:

 

NBTTagCompound nbttagcompound;

for(i = 0; i < this.inventory.length; ++i) {
     if(this.inventory[i] != null) {
          nbttagcompound = new NBTTagCompound();
  nbttagcompound.setByte("Slot", (byte) (i));
  this.inventory[i].writeToNBT(nbttagcompound);
  par1NBTTagList.appendTag(nbttagcompound);
  }
}

 

Your problem is that nbttagcompound is a refrence to a NBtTag, so when you set it to a new NBTCompountTag, you are overwriting the previous one.  Also, there light already be Nbt tags in the TagList, so you need to save a starting point to read from.

 

Also, I think the method you want to override is writeToNBT(NBTTagCompound), not writeToNBT(NBTTagList), though it might have changed (I'm in 1.6.4).

 

Here is what I use (It works):

 

 

 

@Override
public void writeToNBT(NBTTagCompound compound) {
	super.writeToNBT(compound);

	NBTTagList items = new NBTTagList();

	for (int i = 0; i < getSizeInventory(); i++) {		
		ItemStack stack = getStackInSlot(i);

		if (stack != null) {
			NBTTagCompound item = new NBTTagCompound();
			item.setByte("Slot", (byte)i);
			stack.writeToNBT(item);
			items.appendTag(item);
		}
	}

	compound.setTag("Items", items);
}

@Override
public void readFromNBT(NBTTagCompound compound) {
	super.readFromNBT(compound);

	NBTTagList items = compound.getTagList("Items");

	for (int i = 0; i < items.tagCount(); i++) {
		NBTTagCompound item = (NBTTagCompound)items.tagAt(i);
		int slot = item.getByte("Slot");

		if (slot >= 0 && slot < getSizeInventory()) {
			setInventorySlotContents(slot, ItemStack.loadItemStackFromNBT(item));
		}
	}
}