Gson -> importing a JSON with objects

I'm trying to use Gson to import an Json file with an Structure of classes I created. But, although the method toJson works, the structure of objects not.

I have a class called 'Tienda' which have two attributes: (String) Name, (List<Venta>) ventas. Where 'Venta' is other class, but when I call the method toJson, the result is a List of objects 'Tienda' where the attribute List<Venta> is null. Should I do something different?

This is how I get the file to the List<Tienda>:

Type listOfMyClassObject = new TypeToken<ArrayList<Tienda>>() {}.getType();
        List<Tienda> tiendas= new GsonBuilder().setDateFormat("dd/MM/yyy").create().fromJson(new FileReader("C:\\Archivos\\Ejemplo JSON Ventas.JSON"), listOfMyClassObject);

And when I see the content of the result it is like this:

I think you have the wrong type field in class. The field must be of type Date (java.util.Date, java.sql.Timestamp, java.sql.Date)

this is a sample task to get a date object

//simple Object
Tienda demo = new GsonBuilder().setDateFormat("dd/MM/yyy").create().fromJson(json, Tienda.class);
//array
List<Tienda> demo = new GsonBuilder().setDateFormat("dd/MM/yyy").create().fromJson(json, type);

this is my example of the class "Tienda"

import java.util.Date;
public class Tienda {
public Date ventas;
}

I've written some more tests maybe they'll help you to solve this problem more

import com.google.gson.GsonBuilder;
import com.google.gson.reflect.TypeToken;
import org.junit.Test;
import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import static org.junit.Assert.assertEquals;
public class FormatData {
@Test
public void testDeserializeArray() {
//given
String json = "[{\"ventas\":\"08/04/2020\"}, {\"ventas\":\"08/03/2020\"}]";
Type type = new TypeToken<ArrayList<Tienda>>() {
}.getType();
//when
List<Tienda> demo = new GsonBuilder().setDateFormat("dd/MM/yyy").create().fromJson(json, type);
//then
assertEquals(2, demo.size());
assertEquals(1586296800000L, demo.get(0).ventas.getTime());
assertEquals(1583622000000L, demo.get(1).ventas.getTime());
}
@Test
public void testSimpleDeserialize() {
//given
String json = "{\"ventas\":\"08/04/2020\"}";
//when
Tienda demo = new GsonBuilder().setDateFormat("dd/MM/yyy").create().fromJson(json, Tienda.class);
//then
assertEquals(1586296800000L, demo.ventas.getTime());
}
@Test
public void testSerialize() {
//given
Tienda demo = new Tienda();
demo.ventas = new Date(1586296800000L);
//when
String json = new GsonBuilder().setDateFormat("dd/MM/yyy").create().toJson(demo);
//then
assertEquals("{\"ventas\":\"08/04/2020\"}", json);
}
}

I found that the error was so fool. I have Tienda with capital letter and it should be with lower case. It does't return an error and just let the array as null because it doesn't find the exactly name.