Update according to suggestions

src/main/java/net/minecraftforge/items/wrapper/PlayerArmorInvWrapper.java

@@ -4,29 +4,14 @@ import net.minecraft.entity.player.InventoryPlayer;
 import net.minecraft.inventory.EntityEquipmentSlot;
 import net.minecraft.item.ItemStack;
 
-public class PlayerArmorInvWrapper extends InvWrapper
+public class PlayerArmorInvWrapper extends RangedWrapper
 {
     public final InventoryPlayer inventoryPlayer;
-    private final int offset;
 
     public PlayerArmorInvWrapper(InventoryPlayer inv)
     {
-        super(inv);
-
+        super(new InvWrapper(inv), inv.mainInventory.length, inv.mainInventory.length + inv.armorInventory.length);
         inventoryPlayer = inv;
-        offset = inventoryPlayer.mainInventory.length;
-    }
-
-    @Override
-    public int getSlots()
-    {
-        return inventoryPlayer.armorInventory.length;
-    }
-
-    @Override
-    public ItemStack getStackInSlot(int slot)
-    {
-        return super.getStackInSlot(slot + offset);
     }
 
     @Override
@@ -44,20 +29,8 @@ public class PlayerArmorInvWrapper extends InvWrapper
         // check if it's valid for the armor slot
         if (slot < 4 && stack != null && stack.getItem().isValidArmor(stack, equ, inventoryPlayer.player))
         {
-            return super.insertItem(slot + offset, stack, simulate);
+            return super.insertItem(slot, stack, simulate);
         }
         return stack;
     }
-
-    @Override
-    public void setStackInSlot(int slot, ItemStack stack)
-    {
-        super.setStackInSlot(slot + offset, stack);
-    }
-
-    @Override
-    public ItemStack extractItem(int slot, int amount, boolean simulate)
-    {
-        return super.extractItem(slot + offset, amount, simulate);
-    }
 }

src/main/java/net/minecraftforge/items/wrapper/PlayerMainInvWrapper.java

@@ -8,54 +8,19 @@ import net.minecraft.item.ItemStack;
  * Exposes the player inventory WITHOUT the armor inventory as IItemHandler.
  * Also takes core of inserting/extracting having the same logic as picking up items.
  */
-public class PlayerMainInvWrapper extends InvWrapper
+public class PlayerMainInvWrapper extends RangedWrapper
 {
     public final InventoryPlayer inventoryPlayer;
 
     public PlayerMainInvWrapper(InventoryPlayer inv)
     {
-        super(inv);
-
+        super(new InvWrapper(inv), 0, inv.mainInventory.length);
         inventoryPlayer = inv;
     }
 
-    @Override
-    public int getSlots()
-    {
-        return inventoryPlayer.mainInventory.length;
-    }
-
-    @Override
-    public void setStackInSlot(int slot, ItemStack stack)
-    {
-        // prevent setting of armor and offhand
-        if (slot > getSlots())
-        {
-            return;
-        }
-        super.setStackInSlot(slot, stack);
-    }
-
-    @Override
-    public ItemStack getStackInSlot(int slot)
-    {
-        // prevent getting of armor and offhand
-        if (slot > getSlots())
-        {
-            return null;
-        }
-        return super.getStackInSlot(slot);
-    }
-
     @Override
     public ItemStack insertItem(int slot, ItemStack stack, boolean simulate)
     {
-        // prevent inserting into armor and offhand
-        if (slot > getSlots())
-        {
-            return stack;
-        }
-
         ItemStack rest = super.insertItem(slot, stack, simulate);
         if (rest == null || rest.stackSize != stack.stackSize)
         {
@@ -74,15 +39,4 @@ public class PlayerMainInvWrapper extends InvWrapper
         }
         return rest;
     }
-
-    @Override
-    public ItemStack extractItem(int slot, int amount, boolean simulate)
-    {
-        // prevent extraction from armor and offhand
-        if (slot > getSlots())
-        {
-            return null;
-        }
-        return super.extractItem(slot, amount, simulate);
-    }
 }

src/main/java/net/minecraftforge/items/wrapper/PlayerOffhandInvWrapper.java

@@ -3,47 +3,11 @@ package net.minecraftforge.items.wrapper;
 import net.minecraft.entity.player.InventoryPlayer;
 import net.minecraft.item.ItemStack;
 
-public class PlayerOffhandInvWrapper extends InvWrapper
+public class PlayerOffhandInvWrapper extends RangedWrapper
 {
-    public final InventoryPlayer inventoryPlayer;
-    private final int offset;
-
     public PlayerOffhandInvWrapper(InventoryPlayer inv)
     {
-        super(inv);
-
-        inventoryPlayer = inv;
-        offset = inventoryPlayer.mainInventory.length + inventoryPlayer.armorInventory.length;
+        super(new InvWrapper(inv), inv.mainInventory.length + inv.armorInventory.length,
+                inv.mainInventory.length + inv.armorInventory.length + inv.offHandInventory.length);
     }
-
-    @Override
-    public int getSlots()
-    {
-        return inventoryPlayer.offHandInventory.length;
-    }
-
-    @Override
-    public ItemStack getStackInSlot(int slot)
-    {
-        return super.getStackInSlot(slot + offset);
-    }
-
-    @Override
-    public ItemStack insertItem(int slot, ItemStack stack, boolean simulate)
-    {
-        return super.insertItem(slot + offset, stack, simulate);
-    }
-
-    @Override
-    public void setStackInSlot(int slot, ItemStack stack)
-    {
-        super.setStackInSlot(slot + offset, stack);
-    }
-
-    @Override
-    public ItemStack extractItem(int slot, int amount, boolean simulate)
-    {
-        return super.extractItem(slot + offset, amount, simulate);
-    }
-
 }

src/main/java/net/minecraftforge/items/wrapper/RangedWrapper.java

@@ -0,0 +1,78 @@
+package net.minecraftforge.items.wrapper;
+
+import com.google.common.base.Preconditions;
+import net.minecraft.item.ItemStack;
+import net.minecraftforge.items.IItemHandlerModifiable;
+
+/**
+ * A wrapper that composes another IItemHandlerModifiable, exposing only a range of the composed slots.
+ * Shifting of slot indices is handled automatically for you.
+ */
+public class RangedWrapper implements IItemHandlerModifiable {
+
+    private final IItemHandlerModifiable compose;
+    private final int minSlot;
+    private final int maxSlot;
+
+    public RangedWrapper(IItemHandlerModifiable compose, int minSlot, int maxSlotExclusive)
+    {
+        Preconditions.checkArgument(maxSlotExclusive > minSlot, "Max slot must be greater than min slot");
+        this.compose = compose;
+        this.minSlot = minSlot;
+        this.maxSlot = maxSlotExclusive;
+    }
+
+    @Override
+    public int getSlots()
+    {
+        return maxSlot - minSlot;
+    }
+
+    @Override
+    public ItemStack getStackInSlot(int slot)
+    {
+        if (checkSlot(slot))
+        {
+            return compose.getStackInSlot(slot + minSlot);
+        }
+
+        return null;
+    }
+
+    @Override
+    public ItemStack insertItem(int slot, ItemStack stack, boolean simulate)
+    {
+        if (checkSlot(slot))
+        {
+            return compose.insertItem(slot + minSlot, stack, simulate);
+        }
+
+        return stack;
+    }
+
+    @Override
+    public ItemStack extractItem(int slot, int amount, boolean simulate)
+    {
+        if (checkSlot(slot))
+        {
+            return compose.extractItem(slot + minSlot, amount, simulate);
+        }
+
+        return null;
+    }
+
+    @Override
+    public void setStackInSlot(int slot, ItemStack stack)
+    {
+        if (checkSlot(slot))
+        {
+            compose.setStackInSlot(slot + minSlot, stack);
+        }
+    }
+
+    private boolean checkSlot(int localSlot)
+    {
+        return localSlot + minSlot < maxSlot;
+    }
+
+}