Fix LongSerializationPolicy null handling being inconsistent with Gson (#1990)

Gson does not actually use the specified LongSerializationPolicy but instead
uses type adapters which emulate the behavior. However, previously Gson's
implementation did not match LongSerializationPolicy regarding null handling.

Because it is rather unlikely that LongSerializationPolicy has been used on
its own, this commit adjusts its implementation to match Gson's behavior
(instead of the other way around).

Author: Marcono1234

Diff for: gson/src/main/java/com/google/gson/LongSerializationPolicy.java

@@ -17,7 +17,7 @@
 package com.google.gson;
 
 /**
- * Defines the expected format for a {@code long} or {@code Long} type when its serialized.
+ * Defines the expected format for a {@code long} or {@code Long} type when it is serialized.
  *
  * @since 1.3
  *
@@ -26,25 +26,35 @@
  */
 public enum LongSerializationPolicy {
   /**
-   * This is the "default" serialization policy that will output a {@code long} object as a JSON
+   * This is the "default" serialization policy that will output a {@code Long} object as a JSON
    * number. For example, assume an object has a long field named "f" then the serialized output
    * would be:
-   * {@code {"f":123}}.
+   * {@code {"f":123}}
+   *
+   * <p>A {@code null} value is serialized as {@link JsonNull}.
    */
   DEFAULT() {
     @Override public JsonElement serialize(Long value) {
+      if (value == null) {
+        return JsonNull.INSTANCE;
+      }
       return new JsonPrimitive(value);
     }
   },
 
   /**
    * Serializes a long value as a quoted string. For example, assume an object has a long field 
    * named "f" then the serialized output would be:
-   * {@code {"f":"123"}}.
+   * {@code {"f":"123"}}
+   *
+   * <p>A {@code null} value is serialized as {@link JsonNull}.
    */
   STRING() {
     @Override public JsonElement serialize(Long value) {
-      return new JsonPrimitive(String.valueOf(value));
+      if (value == null) {
+        return JsonNull.INSTANCE;
+      }
+      return new JsonPrimitive(value.toString());
     }
   };
 

Diff for: gson/src/test/java/com/google/gson/LongSerializationPolicyTest.java

@@ -29,21 +29,31 @@ public class LongSerializationPolicyTest extends TestCase {
   public void testDefaultLongSerialization() throws Exception {
     JsonElement element = LongSerializationPolicy.DEFAULT.serialize(1556L);
     assertTrue(element.isJsonPrimitive());
-    
+
     JsonPrimitive jsonPrimitive = element.getAsJsonPrimitive();
     assertFalse(jsonPrimitive.isString());
     assertTrue(jsonPrimitive.isNumber());
     assertEquals(1556L, element.getAsLong());
   }
-  
+
   public void testDefaultLongSerializationIntegration() {
     Gson gson = new GsonBuilder()
-        .setLongSerializationPolicy(LongSerializationPolicy.DEFAULT)
-        .create();
+      .setLongSerializationPolicy(LongSerializationPolicy.DEFAULT)
+      .create();
     assertEquals("[1]", gson.toJson(new long[] { 1L }, long[].class));
     assertEquals("[1]", gson.toJson(new Long[] { 1L }, Long[].class));
   }
 
+  public void testDefaultLongSerializationNull() {
+    LongSerializationPolicy policy = LongSerializationPolicy.DEFAULT;
+    assertTrue(policy.serialize(null).isJsonNull());
+
+    Gson gson = new GsonBuilder()
+      .setLongSerializationPolicy(policy)
+      .create();
+    assertEquals("null", gson.toJson(null, Long.class));
+  }
+
   public void testStringLongSerialization() throws Exception {
     JsonElement element = LongSerializationPolicy.STRING.serialize(1556L);
     assertTrue(element.isJsonPrimitive());
@@ -56,9 +66,19 @@ public void testStringLongSerialization() throws Exception {
 
   public void testStringLongSerializationIntegration() {
     Gson gson = new GsonBuilder()
-        .setLongSerializationPolicy(LongSerializationPolicy.STRING)
-        .create();
+      .setLongSerializationPolicy(LongSerializationPolicy.STRING)
+      .create();
     assertEquals("[\"1\"]", gson.toJson(new long[] { 1L }, long[].class));
     assertEquals("[\"1\"]", gson.toJson(new Long[] { 1L }, Long[].class));
   }
+
+  public void testStringLongSerializationNull() {
+    LongSerializationPolicy policy = LongSerializationPolicy.STRING;
+    assertTrue(policy.serialize(null).isJsonNull());
+
+    Gson gson = new GsonBuilder()
+      .setLongSerializationPolicy(policy)
+      .create();
+    assertEquals("null", gson.toJson(null, Long.class));
+  }
 }