#2436 - Updates to User guide & comments to indicate exception cases …

…and fix for 7 test cases of Parameterized Type

UserGuide.md

@@ -405,7 +405,9 @@ gson.registerTypeAdapter(MyType.class, new MyDeserializer());
 gson.registerTypeAdapter(MyType.class, new MyInstanceCreator());
 ```
 
-`registerTypeAdapter` call checks if the type adapter implements more than one of these interfaces and register it for all of them.
+`registerTypeAdapter` call checks 
+1. if the type adapter implements more than one of these interfaces and register it for all of them.
+2. if the type adapter is for Object class or (JsonElements or any of its subclasses), it throws IllegalArgumentException.
 
 #### Writing a Serializer
 

gson/src/main/java/com/google/gson/GsonBuilder.java

@@ -41,6 +41,8 @@
 import com.google.gson.reflect.TypeToken;
 import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonWriter;
+
+import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
 import java.text.DateFormat;
 import java.util.ArrayDeque;
@@ -337,7 +339,7 @@ public GsonBuilder enableComplexMapKeySerialization() {
    *
    * <p>In general using inner classes with Gson should be avoided; they should be converted to {@code static}
    * nested classes if possible.
-   *
+   *Ø
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.3
    */
@@ -664,6 +666,7 @@ public GsonBuilder setDateFormat(int dateStyle, int timeStyle) {
    * @param typeAdapter This object must implement at least one of the {@link TypeAdapter},
    * {@link InstanceCreator}, {@link JsonSerializer}, and a {@link JsonDeserializer} interfaces.
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
+   * @throws IllegalArgumentException if the Type adapter being registered is for Object class or (JsonElements or any of its subclasses)
    */
   @CanIgnoreReturnValue
   public GsonBuilder registerTypeAdapter(Type type, Object typeAdapter) {
@@ -689,8 +692,9 @@ public GsonBuilder registerTypeAdapter(Type type, Object typeAdapter) {
   }
 
   private boolean isTypeObjectOrJsonElements(Type type) {
-    return (type == Object.class
-            || JsonElement.class.isAssignableFrom((Class<?>) type));
+    return (!(type instanceof ParameterizedType) &&
+            (type == Object.class
+            || JsonElement.class.isAssignableFrom((Class<?>) type)));
   }
 
   /**
@@ -724,6 +728,7 @@ public GsonBuilder registerTypeAdapterFactory(TypeAdapterFactory factory) {
    * @param typeAdapter This object must implement at least one of {@link TypeAdapter},
    *        {@link JsonSerializer} or {@link JsonDeserializer} interfaces.
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
+   * @throws IllegalArgumentException if the Type adapter being registered is for a JsonElements or any of its subclasses
    * @since 1.7
    */
   @CanIgnoreReturnValue

gson/src/test/java/com/google/gson/GsonBuilderTest.java

@@ -184,8 +184,7 @@ public void testRegisterTypeAdapterForCoreType() {
         String.class,
     };
     for (Type type : types) {
-      GsonBuilder gsonBuilder = new GsonBuilder();
-      assertThat(gsonBuilder.registerTypeAdapter(type, NULL_TYPE_ADAPTER)).isEqualTo(gsonBuilder);
+      new GsonBuilder().registerTypeAdapter(type, NULL_TYPE_ADAPTER);
     }
   }
 
@@ -282,6 +281,6 @@ public void testRegisterTypeHierarchyAdapterJsonElements() {
       assertThrows(IllegalArgumentException.class,
               () ->  gsonBuilder.registerTypeHierarchyAdapter((Class<?>) type, NULL_TYPE_ADAPTER));
     }
-    assertThat(gsonBuilder.registerTypeHierarchyAdapter(Object.class, NULL_TYPE_ADAPTER)).isEqualTo(gsonBuilder);
+    gsonBuilder.registerTypeHierarchyAdapter(Object.class, NULL_TYPE_ADAPTER);
   }
 }