Example GuiHandler

package iitest.gui;

import iitest.capability.ITest;
import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.world.World;
import net.minecraftforge.fml.common.network.IGuiHandler;

import javax.annotation.Nullable;

public class TestHandler implements IGuiHandler {
    public Object getServerGuiElement(int ID, EntityPlayer player, World world, int x, int y, int z) {
        // I am aware this code makes a few potentially incorrect assumptions.
        return new TestContainer(player.getHeldItemMainhand().getCapability(ITest.instance, null), player);
    }

    public Object getClientGuiElement(int ID, EntityPlayer player, World world, int x, int y, int z) {
        return new TestGui(player.getHeldItemMainhand().getCapability(ITest.instance, null), player);
    }
}