why ? to json return null

[YanQin-JAVA]

   public static void main(String[] args) throws JsonProcessingException {
	class User{
		private String a;

		public User() {
		}

		public User(String a) {
			this.a = a;
		}

		@Override
		public String toString() {
			return "User{" +
					"a='" + a + '\'' +
					'}';
		}

		public String getA() {
			return a;
		}

		public void setA(String a) {
			this.a = a;
		}
	}

	ArrayList<Object> users = Lists.newArrayList(new User("a"), new User("b"));
	Gson gson = new GsonBuilder().create();
	String s = gson.toJson(new User("c"));
	String s1 = (new ObjectMapper()).writeValueAsString(users);
	System.out.println("s = " + s);
	System.out.println("s1 = " + s1);


}

//return :
s = null
s1 = [{"a":"a"},{"a":"b"}]

[YanQin-JAVA]

gson version : 2.8.6

[Marcono1234]

Local classes are not supported. See #1510 and https://github.com/google/gson/blob/master/UserGuide.md#finer-points-with-objects. To solve this make User either a top-level class or a static nested class.

(comment copied from lyubomyr-shaydariv's comment here)