For my mod comeCloser i edit RenderPlayer class in order to change a few lines of code to make seeing a player's name tag harder. This works find if my mod is the only one doing so but if another mod editing this file is installed both mods fail to work. So what i'm asking is there a way i can change those lines without editing the file? I've heard there is a way but have no clue what it is or how to do it. 

Pull a hook to Forge or FML, then make your mod use that hook.

Pull a hook to Forge or FML, then make your mod use that hook.

actual tried to once but couldn't figure out how to make a good working hook. Any ideals?

Depends on the lines.  If it is possible, you could use setPrivateValue.

not sure if i can use that but i'll toy with the ideal

Here is the code section i want to edit

   protected void renderName(EntityPlayer par1EntityPlayer, double par2, double par4, double par6)
    {
        if (Minecraft.isGuiEnabled() && par1EntityPlayer != this.renderManager.livingPlayer)
        {
            float var8 = 1.6F;
            float var9 = 0.016666668F * var8;
            double var10 = par1EntityPlayer.getDistanceSqToEntity(this.renderManager.livingPlayer);
            float var12 = par1EntityPlayer.isSneaking() ? 32.0F : 64.0F;

            if (var10 < (double)(var12 * var12))
            {
Cut.................................Cut

var12 is what i want changed to my own numbers