So, I went to go and shift-click my custom item into my custom slot, and I get this error:

Caused by: java.lang.IndexOutOfBoundsException: Index: 40, Size: 37
at java.util.ArrayList.rangeCheck(Unknown Source) ~[?:1.8.0_65]
at java.util.ArrayList.get(Unknown Source) ~[?:1.8.0_65]
at net.minecraft.inventory.Container.mergeItemStack(Container.java:638) ~[Container.class:?]
at com.happykiller.weightlimit.player.inventory.container.ContainerWeightedPlayer.transferStackInSlot(ContainerWeightedPlayer.java:81) ~[ContainerWeightedPlayer.class:?]
at net.minecraft.inventory.Container.slotClick(Container.java:291) ~[Container.class:?]
at net.minecraft.client.multiplayer.PlayerControllerMP.windowClick(PlayerControllerMP.java:540) ~[PlayerControllerMP.class:?]
at net.minecraft.client.gui.inventory.GuiContainer.handleMouseClick(GuiContainer.java:690) ~[GuiContainer.class:?]
at net.minecraft.client.gui.inventory.GuiContainer.mouseClicked(GuiContainer.java:422) ~[GuiContainer.class:?]
at net.minecraft.client.gui.GuiScreen.handleMouseInput(GuiScreen.java:606) ~[GuiScreen.class:?]
at net.minecraft.client.gui.GuiScreen.handleInput(GuiScreen.java:572) ~[GuiScreen.class:?]
at net.minecraft.client.Minecraft.runTick(Minecraft.java:1760) ~[Minecraft.class:?]
... 15 more

 

Here is the line that gives the error:

if(!this.mergeItemStack(stack1, INV_START, HOTBAR_END + 1, true)) {
return null;
}

 

Here are those variables (NOTE: I do not have armor slots in my Inventory. This might be causing the problem, but I am not sure):

private static final int ARMOR_START = InventoryWeightLimit.INV_SIZE, ARMOR_END = ARMOR_START + 3, 
		INV_START = ARMOR_END + 1, INV_END = INV_START + 26, 
		HOTBAR_START = INV_END + 1, HOTBAR_END = HOTBAR_START + 8;

 

Any ideas?

Pretty sure that the 4 armor slots do not have inventory slot numbers.

 

Or at least, in this context they don't appear to.  I see 37: 1-9 (hotbar), 10-36 (inventory), 37 (your backpack).

Draco is probably right - did you add the armor slots to your container? If so, your inventory size should be 40 and you can merge those slots; if not, you need to remove the ARMOR_START and ARMOR_END indexes and set INV_START to the inventory size.