How did the author foreshadow in chapter 2 that Johnny would use his knife in chapter 4 of The Outsiders?

Ponyboy tells Cherry the story of when Johnny was beaten badly in the park. Johnny was practicing kicks when a group a soc's jumped him. He was almost beaten to death by one of the boys who wore a lot of rings. Ponyboy explained that the beating didn't bother Johnny that much but he hated that he was scared. He vowed if he was ever jumped again, he would kill them. This foreshadows the outcome of the fight in chapter four, when Johnny kills Bob to save Ponyboy's fight.