Added support for runtime type determination while serializing array elements.

Created a utility class Reflection to hold methods to find Runtime type and creating new Instances.

gson/src/main/java/com/google/gson/internal/bind/ArrayTypeAdapter.java

@@ -44,15 +44,17 @@ public final class ArrayTypeAdapter<E> extends TypeAdapter<Object> {
       TypeAdapter<?> componentTypeAdapter = context.getAdapter(TypeToken.get(componentType));
       @SuppressWarnings("unchecked") // create() doesn't define a type parameter
       TypeAdapter<T> result = new ArrayTypeAdapter(
-          componentTypeAdapter, $Gson$Types.getRawType(componentType));
+          context, componentTypeAdapter, $Gson$Types.getRawType(componentType));
       return result;
     }
   };
 
+  private final MiniGson context;
   private final Class<E> componentType;
   private final TypeAdapter<E> componentTypeAdapter;
 
-  public ArrayTypeAdapter(TypeAdapter<E> componentTypeAdapter, Class<E> componentType) {
+  public ArrayTypeAdapter(MiniGson context, TypeAdapter<E> componentTypeAdapter, Class<E> componentType) {
+    this.context = context;
     this.componentTypeAdapter = componentTypeAdapter;
     this.componentType = componentType;
   }
@@ -86,7 +88,10 @@ public final class ArrayTypeAdapter<E> extends TypeAdapter<Object> {
     writer.beginArray();
     for (int i = 0, length = Array.getLength(array); i < length; i++) {
       final E value = (E) Array.get(array, i);
-      componentTypeAdapter.write(writer, value);
+      Type runtimeType = Reflection.getRuntimeTypeIfMoreSpecific(componentType, array, value);
+      TypeAdapter t = runtimeType != componentType ?
+          context.getAdapter(TypeToken.get(runtimeType)) : componentTypeAdapter;
+      t.write(writer, value);
     }
     writer.endArray();
   }

gson/src/main/java/com/google/gson/internal/bind/CollectionTypeAdapter.java

@@ -89,7 +89,7 @@ public final class CollectionTypeAdapter<E> extends TypeAdapter<Collection<E>> {
       return null;
     }
 
-    Collection<E> collection = MiniGson.newInstance(constructor);
+    Collection<E> collection = Reflection.newInstance(constructor);
     reader.beginArray();
     while (reader.hasNext()) {
       E instance = elementTypeAdapter.read(reader);

gson/src/main/java/com/google/gson/internal/bind/MiniGson.java

@@ -16,18 +16,17 @@
 
 package com.google.gson.internal.bind;
 
-import com.google.gson.reflect.TypeToken;
-import com.google.gson.stream.JsonReader;
-import com.google.gson.stream.JsonWriter;
 import java.io.IOException;
-import java.lang.reflect.Constructor;
-import java.lang.reflect.InvocationTargetException;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+
 /**
  * A basic binding between JSON and Java objects.
  */
@@ -67,23 +66,6 @@ public final class MiniGson {
     this.factories = Collections.unmodifiableList(factories);
   }
 
-  // TODO: this should use Joel's unsafe constructor stuff
-  static <T> T newInstance(Constructor<T> constructor) {
-    try {
-      Object[] args = null;
-      return constructor.newInstance(args);
-    } catch (InstantiationException e) {
-      // TODO: JsonParseException ?
-      throw new RuntimeException(e);
-    } catch (InvocationTargetException e) {
-      // TODO: don't wrap if cause is unchecked!
-      // TODO: JsonParseException ?
-      throw new RuntimeException(e.getTargetException());
-    } catch (IllegalAccessException e) {
-      throw new AssertionError(e);
-    }
-  }
-
   /**
    * Returns the type adapter for {@code} type.
    *

gson/src/main/java/com/google/gson/internal/bind/Reflection.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright (C) 2011 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.gson.internal.bind;
+
+import java.lang.reflect.Constructor;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Type;
+import java.lang.reflect.TypeVariable;
+
+final class Reflection {
+  /**
+   * Finds a compatible runtime type if it is more specific
+   * In case of a field of an object, parent is the object instance, and child is the field value.
+   * In case of an Array, parent is the array instance, and the child is the array element.
+   */
+  public static Type getRuntimeTypeIfMoreSpecific(Type type, Object parent, Object child) {
+    if (parent == null || child == null) {
+      return type;
+    }
+    if (type == Object.class || type instanceof TypeVariable || type instanceof Class<?>) {
+      type = (Class<?>) child.getClass();
+    }
+    return type;
+  }
+
+  // TODO: this should use Joel's unsafe constructor stuff
+  public static <T> T newInstance(Constructor<T> constructor) {
+    try {
+      Object[] args = null;
+      return constructor.newInstance(args);
+    } catch (InstantiationException e) {
+      // TODO: JsonParseException ?
+      throw new RuntimeException(e);
+    } catch (InvocationTargetException e) {
+      // TODO: don't wrap if cause is unchecked!
+      // TODO: JsonParseException ?
+      throw new RuntimeException(e.getTargetException());
+    } catch (IllegalAccessException e) {
+      throw new AssertionError(e);
+    }
+  }
+
+}

gson/src/main/java/com/google/gson/internal/bind/ReflectiveTypeAdapter.java

@@ -60,7 +60,7 @@ public final class ReflectiveTypeAdapter<T> extends TypeAdapter<T>  {
 
     T instance;
     if (constructor != null) {
-      instance = (T) MiniGson.newInstance(constructor);
+      instance = (T) Reflection.newInstance(constructor);
     } else {
       try {
         instance = (T) unsafeAllocator.newInstance(rawType);
@@ -122,7 +122,7 @@ public final class ReflectiveTypeAdapter<T> extends TypeAdapter<T>  {
           throws IOException, IllegalAccessException {
         Object fieldValue = field.get(value);
         Type declaredTypeOfField = fieldType.getType();
-        Type resolvedTypeOfField = getMoreSpecificType(declaredTypeOfField, value, fieldValue);
+        Type resolvedTypeOfField = Reflection.getRuntimeTypeIfMoreSpecific(declaredTypeOfField, value, fieldValue);
         TypeAdapter t = resolvedTypeOfField != declaredTypeOfField ?
             context.getAdapter(TypeToken.get(resolvedTypeOfField)) : this.typeAdapter;
         t.write(writer, fieldValue);
@@ -135,16 +135,6 @@ public final class ReflectiveTypeAdapter<T> extends TypeAdapter<T>  {
     };
   }
 
-  private static Type getMoreSpecificType(Type type, Object obj, Object fieldValue) {
-    if (obj == null || fieldValue == null) {
-      return type;
-    }
-    if (type == Object.class || type instanceof TypeVariable || type instanceof Class<?>) {
-      type = (Class<?>) fieldValue.getClass();
-    }
-    return type;
-  }
-
   public static class FactoryImpl implements Factory {
     public boolean serializeField(Class<?> declaringClazz, Field f, Type declaredType) {
       return true;

gson/src/main/java/com/google/gson/internal/bind/StringToValueMapTypeAdapter.java

@@ -79,7 +79,7 @@ public final class StringToValueMapTypeAdapter<V> extends TypeAdapter<Map<String
       return null;
     }
 
-    Map<String, V> map = MiniGson.newInstance(constructor);
+    Map<String, V> map = Reflection.newInstance(constructor);
     reader.beginObject();
     while (reader.hasNext()) {
       String key = reader.nextName();