I'm learning modding minecraft with a "Super cheaty block" hehe. Basically what this block does is when right clicked with a dirt it should give a diamond or spawn it on the player's feet.

 

I've already managed to add the block, spawn it with the /give command and give the diamond to the inventory if otherwise i'm spawning the diamond on the floor (code bellow) but it shows 2 diamonds on the floor and if relog or open a free slot it only show one diamond.

 

public boolean onBlockActivated(World worldIn, BlockPos pos, IBlockState state, EntityPlayer playerIn, EnumHand hand, @Nullable ItemStack heldItem, EnumFacing side, float hitX, float hitY, float hitZ)
{
    ItemStack reward = new ItemStack(Items.DIAMOND, 1);
    Item dirtItem = ItemBlock.getItemFromBlock(Blocks.DIRT);

    if (playerIn != null) {
        heldItem = playerIn.getHeldItem(EnumHand.MAIN_HAND);
        if (heldItem != null && dirtItem.equals(heldItem.getItem())) {
            heldItem.stackSize -= 1;
            if (!playerIn.inventory.addItemStackToInventory(reward)) {
                worldIn.spawnEntityInWorld(new EntityItem(worldIn, playerIn.posX, playerIn.posY, playerIn.posZ, reward));
            }
        }
    }

    return true;
}

 

1. If you ever manipulate data - you do it on server (!world.isRemote). That includes giving items and spawning entities.

http://mcforge.readthedocs.org/en/latest/concepts/sides/

 

2. You can compare Items with "==". equals is usually discouraged (they are singletons).

Thank you, it worked perfectly here.