迭代器

• Iterator

• public interface Iterator<E> {
      boolean hasNext();
      E next();
  }
  public interface Collection<E> {
      Iterator<E> iterator();
  }
  class List<E> implements Collection<E> {
      private class Itr<E> implements Iterator<E> {
          @Override
          public boolean hasNext(){}
          @Override
          public E next(){}
      }
      @Override
      public Iterator iterator(){
          return new Itr();
      }
  }