Hello, I've been trying to make an item so that when you hold it in your hand it gives you blindness.

But, when I have the code set up like this (in the class of that item):

@Override
    public void onUpdate (ItemStack stack, World world, Entity entity, int par4, boolean par5)
    {
        super.onUpdate(stack, world, entity, par4, par5);
        if (entity instanceof EntityPlayer)
        {
            EntityPlayer player = (EntityPlayer) entity;
            ItemStack equipped = player.getCurrentEquippedItem();
            if (equipped == stack)
            {
                player.addPotionEffect(new PotionEffect(Potion.blindness.id, 1, 3));
            }
        }

It only gives me this in-game:

And the only effect it gives me is that I am unable to sprint...

 

Can someone help me?

The second parameter is in ticks I believe. Try something higher like 20 (which should correspond to ~1s).

player.addPotionEffect(new PotionEffect(Potion.blindness.id, 20, 3));//1 replaces with 20

I'm not sure, but it might be because in this line:

player.addPotionEffect(new PotionEffect(Potion.blindness.id, 1, 3));

In the second parameter you wrote 1. The second parameter is for the duration, so you wrote it gives the player a blindness effect for one second, which probably isn't even long enough to show the actual blindness effect. Again I'm not completely sure, but hopefully this helps!

The second parameter is in ticks I believe. Try something higher like 20 (which should correspond to ~1s).

player.addPotionEffect(new PotionEffect(Potion.blindness.id, 20, 3));//1 replaces with 20

Thanks, that did it

Always glad to help out.