静态内部类中的静态成员

Question

为什么我们需要在静态内部类中将成员声明为静态？我认为在静态内部类中不可能有非静态成员，因此我们不能简单地在不使用静态关键字的情况下声明成员。

class MakeACaptain {
  private static MakeACaptain _captain;

  private MakeACaptain() {}

  //Bill Pugh solution
  private static class SingletonHelper {
    //Nested class is referenced after getCaptain() is called

    private static final MakeACaptain _captain = new MakeACaptain();
  }

  public static MakeACaptain getCaptain() {
    return SingletonHelper._captain;
  }
}

这里SingletonHelper是一个静态内部类