Serialize nested class null objects with gson?

Question

I know how to serialize null objects. but what if I have an objects and inside another object that is null?

e.g.

My 1st class:

public class Invoice {

   private Adresse adresse;
   private Double betrag;
   private Double Ust;
   private String zweck;

}

My 2nd class:

public class Adresse {
   private String name;
   private Ort ort;
}

And more sub-classes...

And if I do:

Gson gson = new GsonBuilder().serializeNulls().create();

I only get this:

{"adresse":null,"betrag":null,"Ust":null,"zweck":null}

instead of this:

{"adresse":{"name":null,"ort"{"plz":null,"name":null}},"betrag":null,"Ust":null,"zweck":null}

There should be an automated approach to it. There will always be other sub-classes. So a manual approach is a no-go.

Answer 1

I got it working! With Java Reflection!

Here the code:

1. userDefined() checks if a class is user defined or primitive, can be customized as you wish.

   public boolean isUserDefined(Class o) {
     if (o.isAssignableFrom(String.class)) {
       return false;
     }
     if (o.isAssignableFrom(Double.class)) {
       return false;
     }
     if (o.isAssignableFrom(Integer.class)) {
       return false;
     }
     if (o.isAssignableFrom(Boolean.class)) {
       return false;
     }
     if (o.isAssignableFrom(Short.class)) {
       return false;
     }
     if (o.isAssignableFrom(Float.class)) {
       return false;
     }
     if (o.isAssignableFrom(Long.class)) {
       return false;
     }
     return true;
   }
   

2. getSetterMethod() returns the setter Method of a field.

   private static Method getSetterMethod(Object o, Field f) {       
       for (Method method : o.getClass().getMethods()) {
           String fieldname = "set" + f.getName();
           String methodname = method.getName().toLowerCase();
           if (fieldname.equals(methodname)) {
               return method;
           }
       }
       return null;
   }
   

3. And finally, invokeSetters() invoke user defined object setter Methods and those of the subclasses! yea.

       private void invokeSetters(Object o) throws IllegalArgumentException, IllegalAccessException, ClassNotFoundException, InstantiationException, NoSuchMethodException, InvocationTargetException {
       System.out.println("INVOKING THE OBJECT " + o.getClass().toString());
         for (Field f : o.getClass().getDeclaredFields()) {
           System.out.println("NOW IN THE FIELD: " + f.getName());
           f.setAccessible(true);
           Object obj = null;
           if (isUserDefined(f.getType()) && f.get(o) == null) {
               System.out.println("FIELD is USER DEFINED AND NULL");
               Class c = Class.forName(f.getType().getName());
               System.out.println("CLASS IS " + c.getName());
               obj = c.getConstructor().newInstance();
               System.out.println("GOING INSIDE " + obj.toString() + " NOW.");
               invokeSetters(obj);
           }
           if (obj != null) {
               Method setter = getSetterMethod(o, f);
               System.out.println("IM IN PARENT OBJECT " + o + " AND INVOKING METHOD " + setter.getName() + " WITH PARAMETER " + obj.toString());
               setter.invoke(o, obj);
               System.out.println("METHOD WAS INVOKED");
           }
         }
       }