I have created a custom slab in my mod and when I place one down on top of another slab, It doesn't form a double slab, but instead places a slab on top of the block. This is my Generic Slab code: 

package com.kpzip.SushiMod.blocks.shapes.slab;

import java.util.Random;

import net.minecraft.block.BlockSlab;
import net.minecraft.block.SoundType;
import net.minecraft.block.material.Material;
import net.minecraft.block.properties.IProperty;
import net.minecraft.block.state.BlockStateContainer;
import net.minecraft.block.state.IBlockState;
import net.minecraft.item.Item;
import net.minecraft.item.ItemBlock;
import net.minecraft.item.ItemStack;

import com.kpzip.SushiMod.Main;
import com.kpzip.SushiMod.init.ModBlocks;
import com.kpzip.SushiMod.init.ModItems;
import com.kpzip.SushiMod.util.IHasModel;

public abstract class BambooSlab extends BlockSlab implements IHasModel{

	public BambooSlab(String name) {
		super(Material.WOOD);
		this.setUnlocalizedName(name);
		this.setRegistryName(name);
		this.setSoundType(SoundType.WOOD);
		if (this instanceof BambooHalfSlab) {
			this.setCreativeTab(Main.SUSHI_BLOCKS);
		}
		setHardness(2);
		setResistance(15);
		setHarvestLevel("axe", 0);
		
		IBlockState state = this.blockState.getBaseState();
		
		if (!this.isDouble()) {
			state = state.withProperty(HALF, EnumBlockHalf.BOTTOM);
			
		}
		
		this.setDefaultState(state);
		this.useNeighborBrightness = true;
		
		
		ModBlocks.BLOCKS.add(this);
		ModItems.ITEMS.add(new ItemBlock(this).setRegistryName(this.getRegistryName()));
		
	}

	@Override
	public void registerModels() {
		Main.proxy.registerItemRenderer(Item.getItemFromBlock(this), 0, "inventory");
		
	}
	
	@Override
	public String getUnlocalizedName(int meta) {
		return this.getUnlocalizedName();
		
	}
	
	@Override
	public IProperty<?> getVariantProperty() {
		
		return HALF;
	}
	
	@Override
	public Comparable<?> getTypeForItem(ItemStack stack) {
		
		return EnumBlockHalf.BOTTOM;
		
	}
	
	@Override
	public int damageDropped(IBlockState state) {
		return 0;
	}
	
	@Override
	public IBlockState getStateFromMeta(int meta) {
		
		if (!this.isDouble()) {
			return this.getDefaultState().withProperty(HALF, EnumBlockHalf.values()[meta % EnumBlockHalf.values().length]);
		}
		return this.getDefaultState();
			
		
	}
	
	@Override
	public int getMetaFromState(IBlockState state) {
		if (!this.isDouble()) {
			return 0;
		}
		
		return ((EnumBlockHalf)state.getValue(HALF)).ordinal() + 1;
		
	}
	
	@Override
	public Item getItemDropped(IBlockState state, Random rand, int fortune) {
		return Item.getItemFromBlock(ModBlocks.BAMBOO_SLAB_HALF);
		
	}
	
	@Override
	protected BlockStateContainer createBlockState() {
		return new BlockStateContainer(this, new IProperty[] {HALF});
	}


}

This is my Double Slab code:

 

 

package com.kpzip.SushiMod.blocks.shapes.slab;

public class BambooDoubleSlab extends BambooSlab {
	
	public BambooDoubleSlab(String name) {
		super(name);
		
		

	}
	
	@Override
	public boolean isDouble() {
		return true;
	}

}

And this is my half slab code:

 

 

package com.kpzip.SushiMod.blocks.shapes.slab;

import net.minecraft.block.state.IBlockState;
import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.item.Item;
import net.minecraft.util.EnumFacing;
import net.minecraft.util.EnumHand;
import net.minecraft.util.math.BlockPos;
import net.minecraft.world.World;

import com.kpzip.SushiMod.blocks.shapes.IHasCustomShape;
import com.kpzip.SushiMod.init.ModBlocks;

public class BambooHalfSlab extends BambooSlab implements IHasCustomShape {

	public BambooHalfSlab(String name) {
		super(name);
	}
	
	@Override
	public boolean isDouble() {
		return false;
	}
	
	@Override
	public IBlockState getStateFromMeta(int meta) {
		if (meta == 0) {
			return this.getDefaultState().withProperty(HALF, EnumBlockHalf.BOTTOM);
		}
		else {
			return this.getDefaultState().withProperty(HALF, EnumBlockHalf.TOP);
		}
	}
	
	@Override
	public int getMetaFromState(IBlockState state) {
		if (state.getValue(HALF) == EnumBlockHalf.BOTTOM) {
			return 0;
		}
		else {
			return 1;
		}
	}

}

Edited July 20, 20196 yr by kpzip

I assume I need to use the custom ItemBlock when registering the block, but what code do I put in the itemblock?

 

Thanks, By looking at the ItemBlockSpecial class, I was able to figure out what to do