And, I have an container, that duplicates items when moving items out of it using shift click, but only when player hasn't got itemstack to merge with.

I think error is in this function. Please, help me. Only small reply. Every reply will help. I have solving this for 5 months but without result.

 

Function where is the bug.

@Override
        public ItemStack transferStackInSlot(EntityPlayer player, int slot) {
                ItemStack stack = null;
                Slot slotObject = (Slot)inventorySlots.get(slot);
                
                if (slotObject != null && slotObject.getHasStack())
                {
                        ItemStack stackInSlot = slotObject.getStack();
                        stack = stackInSlot.copy();
                        if(slot < 9)
                        {
                        	if (!this.mergeItemStack(stackInSlot, 0, 35, true))
                        	{
                        		System.out.println("There are " + stackInSlot.stackSize + " of " + stackInSlot.getItem().getUnlocalizedName());
                        		return null;
                        	}
                        }
                        else
                        {
                        	if(!this.mergeItemStack(stackInSlot, 0, 9, false))
                        	{
                        		System.out.println("There are " + stackInSlot.stackSize + " of " + stackInSlot.getItem().getUnlocalizedName());
                        		return null;
                        	}
                		}

                        if (stackInSlot.stackSize == 0)
                        {
                                slotObject.putStack(null);
                        }
                        else
                        {
                                slotObject.onSlotChanged();
                        }

                        if (stackInSlot.stackSize == stack.stackSize)
                        {
                                return null;
                        }
                        slotObject.onPickupFromSlot(player, stackInSlot);
                }
                return stack;
        }

I assume that you are modifying the container of player.

Then, What is this? Why 0~35?

this.mergeItemStack(stackInSlot, 0, 35, true);

It seems that it should be 9~44.

 

Sorry, I did't understood the code, just followed tutorial. I didn't know why is it 0, 35.

And, thank you very much for answering me. :) :)

It worked fine. And, can you explain me this method, if you can? Please...

Sure, I'll explain the code to you, line by line.

@Override
        public ItemStack transferStackInSlot(EntityPlayer player, int slot) {
                ItemStack stack = null;
                Slot slotObject = (Slot)inventorySlots.get(slot); //get the shift-clicked slot.
                
                if (slotObject != null && slotObject.getHasStack()) //If the slot is valid and has ItemStack
                {
                        ItemStack stackInSlot = slotObject.getStack(); //Get the Stack on the slot.
                        stack = stackInSlot.copy(); //Copy stack.
                        if(slot < 9) //If the slot is on hotbar
                        {
                        	if (!this.mergeItemStack(stackInSlot, 9, 44, true)) //Try merging the stack to the rest of the slot.
                        	{
                        		System.out.println("There are " + stackInSlot.stackSize + " of " + stackInSlot.getItem().getUnlocalizedName()); //The print statement when it is not able to merge the stack.
                        		return null; // Drop nothing.
                        	}
                        }
                        else
                        {
                        	if(!this.mergeItemStack(stackInSlot, 0, 9, false))
                        	{
                        		System.out.println("There are " + stackInSlot.stackSize + " of " + stackInSlot.getItem().getUnlocalizedName()); //The print statement when it is not able to merge the stack.
                        		return null; // Drop nothing.
                        	}
                	}

                        //When it is successfully merged,
                        if (stackInSlot.stackSize == 0) //If the slot stack size is 0
                        {
                                slotObject.putStack(null); //Set the stack to null.
                        }
                        else //Yes, da else
                        {
                                slotObject.onSlotChanged(); // Mark the slot as changed.
                        }

                        if (stackInSlot.stackSize == stack.stackSize) //When Slot stacksize is same, (So every stack has merged)
                        { 
                                return null; // Drop Nothing.
                        }
                        slotObject.onPickupFromSlot(player, stackInSlot); // Let the slot to pick up rest of the stack
                                                                                                       //    (which is not merged).
                }

                return stack; //IDK, it just returns null; So it drops nothing when no slot is found.
        }

Well, I understood the code. Thank you very much.