public abstract static class SomeAbstractClass {
    public static final SomeAbstractClass INSTANCE = new SomeAbstractClass() {};

    public SomeAbstractClass() {}
  }

  static class NonPublicClass {
    public NonPublicClass() {}
  }

  private static class WithPrivateConstructor {
    public static final WithPrivateConstructor INSTANCE = new WithPrivateConstructor();
  }