Add support to set a setting to serialize null values

src/main/java/com/microsoft/graph/serializer/DefaultSerializer.java

@@ -65,10 +65,21 @@ public class DefaultSerializer implements ISerializer {
 	 * @param logger the logger
 	 */
 	public DefaultSerializer(@Nonnull final ILogger logger) {
-		this.logger = Objects.requireNonNull(logger, "parameter logger cannot be null");
-		this.gson = GsonFactory.getGsonInstance(logger);
+		this(logger, false);
 	}
 
+
+    /**
+     * Creates a DefaultSerializer with an option to enable serializing of the null values.
+     *
+     * @param logger         the logger
+     * @param serializeNulls the setting of whether or not to serialize the null values in the JSON object
+     */
+    public DefaultSerializer(@Nonnull final ILogger logger, @Nonnull final boolean serializeNulls) {
+        this.logger = Objects.requireNonNull(logger, "parameter logger cannot be null");
+        this.gson = GsonFactory.getGsonInstance(logger, serializeNulls);
+    }
+
 	@Override
 	@Nullable
 	public <T> T deserializeObject(@Nonnull final String inputString, @Nonnull final Class<T> clazz, @Nullable final Map<String, List<String>> responseHeaders) {

src/main/java/com/microsoft/graph/serializer/GsonFactory.java

@@ -44,8 +44,10 @@
 import java.util.Arrays;
 import java.util.EnumSet;
 import java.util.GregorianCalendar;
+import java.util.Objects;
 import java.util.UUID;
 
+import javax.annotation.Nonnull;
 import javax.xml.datatype.DatatypeFactory;
 import javax.xml.datatype.Duration;
 
@@ -68,8 +70,21 @@ private GsonFactory() {
      * @param logger the logger
      * @return the new instance
      */
-    public static Gson getGsonInstance(final ILogger logger) {
+    @Nonnull
+    public static Gson getGsonInstance(@Nonnull final ILogger logger) {
+        return getGsonInstance(logger, false);
+    }
 
+    /**
+     * Creates an instance of GSON
+     *
+     * @param logger         the logger
+     * @param serializeNulls the setting of whether or not to serialize the null values in the JSON object
+     * @return the new instance
+     */
+    @Nonnull
+    public static Gson getGsonInstance(@Nonnull final ILogger logger, final boolean serializeNulls) {
+        Objects.requireNonNull(logger, "parameter logger cannot be null");
         final JsonSerializer<OffsetDateTime> calendarJsonSerializer = new JsonSerializer<OffsetDateTime>() {
             @Override
             public JsonElement serialize(final OffsetDateTime src,
@@ -328,7 +343,11 @@ public Float deserialize(final JsonElement json,
             }
         };
 
-        return new GsonBuilder()
+        GsonBuilder builder = new GsonBuilder();
+        if (serializeNulls) {
+            builder.serializeNulls();
+        }
+        return builder
                 .excludeFieldsWithoutExposeAnnotation()
                 .registerTypeAdapter(Boolean.class, booleanJsonDeserializer)
                 .registerTypeAdapter(String.class, stringJsonDeserializer)

src/test/java/com/microsoft/graph/serializer/DefaultSerializerTest.java

@@ -3,12 +3,14 @@
 import com.google.gson.Gson;
 import com.google.gson.JsonElement;
 import com.microsoft.graph.logger.ILogger;
+import com.microsoft.graph.models.MessageStub;
 import com.microsoft.graph.models.ReactionStub;
 import com.microsoft.graph.models.SubReactionStub1;
 import org.junit.jupiter.api.Test;
 import org.mockito.Mockito;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.mockito.Mockito.mock;
 
@@ -33,4 +35,33 @@ public void testDeserializationOfObjectWithODataTypeProperty() {
         Mockito.verify(reaction.additionalDataManager()).setAdditionalData(gson.fromJson(testJsonResponse, JsonElement.class).getAsJsonObject());
     }
 
+    @Test
+    public void testDefaultSerializerDoesNotIncludeNullValuesByDefault() {
+        // Given
+        final String testJsonResponse =
+            "{\"@odata.type\": \"#microsoft.graph.messageStub\", \"body\": null}";
+
+        // When
+        final MessageStub message = defaultSerializer.deserializeObject(testJsonResponse, MessageStub.class);
+
+        // Then
+        assertNotNull(message);
+        assertEquals("{}", defaultSerializer.serializeObject(message));
+    }
+
+    @Test
+    public void testDefaultNullSerializerDoesIncludeNullValues() {
+        // Given
+        final String testJsonResponse =
+            "{\"@odata.type\": \"#microsoft.graph.messageStub\",\"body\":null}";
+
+        // When
+        final DefaultSerializer nullSerializer = new DefaultSerializer(logger, true);
+        final MessageStub message = nullSerializer.deserializeObject(testJsonResponse, MessageStub.class);
+
+        // Then
+        assertNotNull(message);
+        assertEquals("{\"body\":null,\"reaction\":null}", nullSerializer.serializeObject(message));
+    }
+
 }