So, I managed to create a half slab that works nicely, but when I try to put another one on top of it, it doesn't create a double slab; instead it creates a new slab above it.  It inherits from BlockHalfSlab, and I used that class as well as BlockWoodSlab as examples, but I can't seem to figure this out.  Here's what I have so far:

 

public class SlabHay extends BlockHalfSlab
{
protected static Icon iconSide;

public SlabHay(int id, boolean doubleslab)
{
	super(id, doubleslab, Material.wood);

	setHardness(1.5f);
	setResistance(4.0f);
	setStepSound(Block.soundGrassFootstep);
	setUnlocalizedName("haySlab");

	useNeighborBrightness[id] = true;
}

public int idDropped(int par1, Random par2Random, int par3)
{
	return HayMod.haySlab.blockID;
}

public String getFullSlabName(int i)
{
	if (!isDoubleSlab)
		return super.getUnlocalizedName() + "haySlab";
	else
		return super.getUnlocalizedName() + "hayDoubleSlab";
}

@Override
public void func_94332_a(IconRegister par1IconRegister)
{
	iconSide = par1IconRegister.func_94245_a("hvh:hay_side");
	field_94336_cN = par1IconRegister.func_94245_a("hvh:hay_top");
}

@SideOnly(Side.CLIENT)
public Icon getBlockTextureFromSideAndMetadata(int par1, int par2)
{
	if (par1 == 0 || par1 == 1)
		return field_94336_cN;
	else
		return iconSide;
}
} 

 

Is there a method that I missed?  Am I supposed to register it with something?

It's because of the slab is an itemblock,you need a custom itemblock for it,but a more simple thing is to put a line in the FMLPostInitializationEvent :

 

Item.itemsList[YourSingleSlab.blockID] = (new ItemSlab(YourSingleSlab.blockID - 256, (BlockHalfSlab)YourSingleSlab, (BlockHalfSlab)YourDoubleSlab, false)).setItemName("YourSlabName");

 

It works great (I had to use setUnlocalizedName() instead of setItemName()), but now I have working double slabs.  Problem is, now my double slab has a name, but my single slabs have a blank tooltip.

 

Here's some bits of code that might be relevant:

public static BlockHalfSlab haySlab = (BlockHalfSlab) new SlabHay(505, false) .setUnlocalizedName("haySlab") .setCreativeTab(CreativeTabs.tabBlock);
public static BlockHalfSlab hayDoubleSlab = (BlockHalfSlab) new SlabHay(506, true) .setUnlocalizedName("hayDoubleSlab");

@Init
public void load(FMLInitializationEvent event)
{
	proxy.registerRenderers();

	//// Register the block, it's name, and it's harvest level
	// Hay Double Slab
	GameRegistry.registerBlock(hayDoubleSlab, "hayDoubleSlab");
	LanguageRegistry.addName(hayDoubleSlab, "Hay Double Slab");
	MinecraftForge.setBlockHarvestLevel(hayDoubleSlab, "hatchet", 0);
	// Hay Slab
	GameRegistry.registerBlock(haySlab, "haySlab");
	LanguageRegistry.addName(haySlab, "Hay Slab");
	MinecraftForge.setBlockHarvestLevel(haySlab, "hatchet", 0);

	//// Register the recipes
	// Hay Slab
	GameRegistry.addRecipe(new ItemStack(haySlab, 6), "xxx",
							'x', new ItemStack(hayBlock));
}

@PostInit
public void postInit(FMLPostInitializationEvent event)
{
	Item.itemsList[haySlab.blockID] = new ItemSlab(haySlab.blockID - 256, (BlockHalfSlab)haySlab, (BlockHalfSlab)hayDoubleSlab, false) .setUnlocalizedName("haySlab");
}

 

Any idea why my single slab has no name?  I tried using

LanguageRegistry.addName(Item.itemsList[haySlab.blockID], "Hay Slab");

to no avail.

For registering name,i have used:

 

LanguageRegistry.instance().addStringLocalization(((BlockStoneSlabs)YourSingleSlab).getFullSlabName(0)+".name", "Your Slab");

It works! Thank you thank you sooo much!