Container.transferStackInSlot(EntityPlayer, int)

Container.java

public ItemStack transferStackInSlot(EntityPlayer par1EntityPlayer, int par2) {
        ItemStack itemstack = null;
        Slot slot = (Slot)this.inventorySlots.get(par2);
        if (slot != null && slot.getHasStack()) {
            ItemStack itemstack1 = slot.getStack();
            itemstack = itemstack1.copy();
            if (par2 < this.tile.getSizeInventory())
                if (!this.mergeItemStack(itemstack1, this.tile.getSizeInventory(), this.inventorySlots.size(), true))
                    return null;
            else if (!this.mergeItemStack(itemstack1, 0, this.tile.getSizeInventory(), false))
                return null;
            if (itemstack1.stackSize == 0)
                slot.putStack((ItemStack)null);
            else
                slot.onSlotChanged();
        }
        return itemstack;
    }