Code

public class MinefusInventoryPlayer implements IInventory

{
    /** The name your custom inventory will display in the GUI, possibly just "Inventory" */
    private String name;

    /** The key used to store and retrieve the inventory from NBT */
    private final String tagName = "CustomInvTag";

    /** Define the inventory size here for easy reference */
    // This is also the place to define which slot is which if you have different types,
    // for example SLOT_SHIELD = 0, SLOT_AMULET = 1;
    public static final int INV_SIZE = 64;

    /** Inventory's size must be same as number of slots you add to the Container class */
    private ItemStack[] inventory = new ItemStack[INV_SIZE];

    public MinefusInventoryPlayer()
    {
    }

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

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

    @Override
    public ItemStack decrStackSize(int slot, int amount)
    {
    ItemStack stack = getStackInSlot(slot);
    if (stack != null)
    {
    if (stack.stackSize > amount)
    {
    stack = stack.splitStack(amount);
    if (stack.stackSize == 0)
    {
    setInventorySlotContents(slot, null);
    }
    }
    else
    {
    setInventorySlotContents(slot, null);
    }

    this.markDirty();;
    }
    return stack;
    }

    @Override
    public ItemStack getStackInSlotOnClosing(int slot)
    {
    ItemStack stack = getStackInSlot(slot);
    if (stack != null)
    {
    setInventorySlotContents(slot, null);
    }
    return stack;
    }

    @Override
    public void setInventorySlotContents(int slot, ItemStack itemstack)
    {
    this.inventory[slot] = itemstack;

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

    this.markDirty();
    }

    @Override
    public String getInventoryName()
    {
    return this.hasCustomInventoryName() ? this.name : "key.minefusinventory.desc";
    }

    public MinefusInventoryPlayer getMinefusInventory(){

    return this.getMinefusInventory();
    }

    @Override
    public boolean hasCustomInventoryName()
    {
    return this.name != null && this.name.length() > 0;
    }

    /**
    * Our custom slots are similar to armor - only one item per slot
    */
    @Override
    public int getInventoryStackLimit()
    {
    return 64;
    }

    @Override
    public void markDirty()
    {
    for (int i = 0; i < this.getSizeInventory(); ++i)
    {
    if (this.getStackInSlot(i) != null && this.getStackInSlot(i).stackSize == 0)
    this.setInventorySlotContents(i, null);
    }
    }

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

    /**
    * This method doesn't seem to do what it claims to do, as
    * items can still be left-clicked and placed in the inventory
    * even when this returns false
    */
    @Override
    public boolean isItemValidForSlot(int slot, ItemStack itemstack)
    {
    return true;
    }

    public void writeToNBT(NBTTagCompound compound)
    {
    NBTTagList items = new NBTTagList();

    for (int i = 0; i < getSizeInventory(); ++i)
    {
    if (getStackInSlot(i) != null)
    {
    NBTTagCompound item = new NBTTagCompound();
    item.setByte("Slot", (byte) i);
    getStackInSlot(i).writeToNBT(item);
    items.appendTag(item);
    }
    }

    // We're storing our items in a custom tag list using our 'tagName' from above
    // to prevent potential conflicts
    compound.setTag(tagName, items);
    }

    public void readFromNBT(NBTTagCompound compound) {

    NBTTagList items = compound.getTagList(tagName, compound.getId());
    for (int i = 0; i < items.tagCount(); ++i) {

    NBTTagCompound item = items.getCompoundTagAt(i);
    byte slot = item.getByte("Slot");

    if (slot >= 0 && slot < getSizeInventory()) {
    inventory[slot] = ItemStack.loadItemStackFromNBT(item);
    }
    }
    }

    @Override
    public void openInventory() {

    }

    @Override
    public void closeInventory() {

    }

    public void setCustomGuiName(String name)
        {
           this.name = name;
        }

    public int getFirstEmptyStack(){

            for (int i = 0; i < this.inventory.length; ++i)
            {
                if (this.inventory[i] == null)
                {
                    return i;
                }
            }

            return -1;
        }
}