Firebase实时数据库不保存包含列表或映射属性的POJO

public class User {
    private Map<String, Boolean> friends;
    private Map<String, String> preferences;

    // Getters and setters
    // ...
}

User user = new User();
user.setFriends(new HashMap<>());
user.getFriends().put("friend1", true);
user.getFriends().put("friend2", true);

user.setPreferences(new HashMap<>());
user.getPreferences().put("preference1", "value1");
user.getPreferences().put("preference2", "value2");

FirebaseDatabase database = FirebaseDatabase.getInstance();
DatabaseReference usersRef = database.getReference("users");
usersRef.child("user1").setValue(user);