public static class A {
        @JsonAdapter(NullTolerantStringAdapter.class)
        String nonNullString = "";

        public static class NullTolerantStringAdapter extends TypeAdapter<String> {

            @Override
            public void write(JsonWriter out, String value) throws IOException {
                out.value(value);
            }

            @Override
            public String read(JsonReader in) throws IOException {
                String val = in.nextString();
                return val != null ? val : "";
            }
        }
    }

    @Test
    public void testNullDeserialization () {
        A a = new A();
        a.nonNullString = null;

        Gson gson = new GsonBuilder().create();
        String json = gson.toJson(a);
        A deserialized = gson.fromJson(json, A.class);

        assertThat(deserialized.nonNullString).isNotNull();

        Gson gsonWithNull = new GsonBuilder().serializeNulls().create();
        String jsonWithNull = gson.toJson(a);
        A deserializedWithNull = gsonWithNull.fromJson(jsonWithNull, A.class);

        assertThat(deserializedWithNull.nonNullString).isNotNull();
    }