Which of the following classes in java library do not implement a design pattern?

Today, I came across this interesting question. Which of the following classes in java library do not implement a design pattern? Honestly, I don't know the answer as they are all design patterns, and no idea on the story behind.

1. Singleton
2. Observer
3. Iterator
4. Proxy

Does anyone has a clue? Please, if you don't know the answer, don't down vote. This is a legitimate question.