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diff --git a/gson/src/test/java/com/google/gson/functional/RuntimeTypeAdapterFactoryFunctionalTest.java b/gson/src/test/java/com/google/gson/functional/RuntimeTypeAdapterFactoryFunctionalTest.java
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+/*
+ * Copyright (C) 2008 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.functional;
+
+import java.io.IOException;
+import java.util.LinkedHashMap;
+import java.util.Map;
+
+import junit.framework.TestCase;
+
+import com.google.gson.Gson;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonParseException;
+import com.google.gson.JsonPrimitive;
+import com.google.gson.TypeAdapter;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.internal.Streams;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+
+/**
+ * Functional tests for the RuntimeTypeAdapterFactory feature in extras.
+ */
+public final class RuntimeTypeAdapterFactoryFunctionalTest extends TestCase {
+
+  private final Gson gson = new Gson();
+
+  /**
+   * This test also ensures that {@link TypeAdapterFactory} registered through {@link JsonAdapter}
+   * work correctly for {@link Gson#getDelegateAdapter(TypeAdapterFactory, TypeToken)}.
+   */
+  public void testSubclassesAutomaticallySerialzed() throws Exception {
+    Shape shape = new Circle(25);
+    String json = gson.toJson(shape);
+    shape = gson.fromJson(json, Shape.class);
+    assertEquals(25, ((Circle)shape).radius);
+
+    shape = new Square(15);
+    json = gson.toJson(shape);
+    shape = gson.fromJson(json, Shape.class);
+    assertEquals(15, ((Square)shape).side);
+    assertEquals(ShapeType.SQUARE, shape.type);
+  }
+
+  @JsonAdapter(Shape.JsonAdapterFactory.class)
+  static class Shape {
+    final ShapeType type;
+    Shape(ShapeType type) { this.type = type; }
+    private static final class JsonAdapterFactory extends RuntimeTypeAdapterFactory<Shape> {
+      public JsonAdapterFactory() {
+        super(Shape.class, "type");
+        registerSubtype(Circle.class, ShapeType.CIRCLE.toString());
+        registerSubtype(Square.class, ShapeType.SQUARE.toString());
+      }
+    }
+  }
+
+  public enum ShapeType {
+    SQUARE, CIRCLE
+  }
+
+  private static final class Circle extends Shape {
+    final int radius;
+    Circle(int radius) { super(ShapeType.CIRCLE); this.radius = radius; }
+  }
+
+  private static final class Square extends Shape {
+    final int side;
+    Square(int side) { super(ShapeType.SQUARE); this.side = side; }
+  }
+
+  // Copied from the extras package
+  static class RuntimeTypeAdapterFactory<T> implements TypeAdapterFactory {
+    private final Class<?> baseType;
+    private final String typeFieldName;
+    private final Map<String, Class<?>> labelToSubtype = new LinkedHashMap<String, Class<?>>();
+    private final Map<Class<?>, String> subtypeToLabel = new LinkedHashMap<Class<?>, String>();
+
+    protected RuntimeTypeAdapterFactory(Class<?> baseType, String typeFieldName) {
+      if (typeFieldName == null || baseType == null) {
+        throw new NullPointerException();
+      }
+      this.baseType = baseType;
+      this.typeFieldName = typeFieldName;
+    }
+
+    /**
+     * Creates a new runtime type adapter using for {@code baseType} using {@code
+     * typeFieldName} as the type field name. Type field names are case sensitive.
+     */
+    public static <T> RuntimeTypeAdapterFactory<T> of(Class<T> baseType, String typeFieldName) {
+      return new RuntimeTypeAdapterFactory<T>(baseType, typeFieldName);
+    }
+
+    /**
+     * Creates a new runtime type adapter for {@code baseType} using {@code "type"} as
+     * the type field name.
+     */
+    public static <T> RuntimeTypeAdapterFactory<T> of(Class<T> baseType) {
+      return new RuntimeTypeAdapterFactory<T>(baseType, "type");
+    }
+
+    /**
+     * Registers {@code type} identified by {@code label}. Labels are case
+     * sensitive.
+     *
+     * @throws IllegalArgumentException if either {@code type} or {@code label}
+     *     have already been registered on this type adapter.
+     */
+    public RuntimeTypeAdapterFactory<T> registerSubtype(Class<? extends T> type, String label) {
+      if (type == null || label == null) {
+        throw new NullPointerException();
+      }
+      if (subtypeToLabel.containsKey(type) || labelToSubtype.containsKey(label)) {
+        throw new IllegalArgumentException("types and labels must be unique");
+      }
+      labelToSubtype.put(label, type);
+      subtypeToLabel.put(type, label);
+      return this;
+    }
+
+    /**
+     * Registers {@code type} identified by its {@link Class#getSimpleName simple
+     * name}. Labels are case sensitive.
+     *
+     * @throws IllegalArgumentException if either {@code type} or its simple name
+     *     have already been registered on this type adapter.
+     */
+    public RuntimeTypeAdapterFactory<T> registerSubtype(Class<? extends T> type) {
+      return registerSubtype(type, type.getSimpleName());
+    }
+
+    public <R> TypeAdapter<R> create(Gson gson, TypeToken<R> type) {
+      if (type.getRawType() != baseType) {
+        return null;
+      }
+
+      final Map<String, TypeAdapter<?>> labelToDelegate
+          = new LinkedHashMap<String, TypeAdapter<?>>();
+      final Map<Class<?>, TypeAdapter<?>> subtypeToDelegate
+          = new LinkedHashMap<Class<?>, TypeAdapter<?>>();
+      for (Map.Entry<String, Class<?>> entry : labelToSubtype.entrySet()) {
+        TypeAdapter<?> delegate = gson.getDelegateAdapter(this, TypeToken.get(entry.getValue()));
+        labelToDelegate.put(entry.getKey(), delegate);
+        subtypeToDelegate.put(entry.getValue(), delegate);
+      }
+
+      return new TypeAdapter<R>() {
+        @Override public R read(JsonReader in) throws IOException {
+          JsonElement jsonElement = Streams.parse(in);
+          JsonElement labelJsonElement = jsonElement.getAsJsonObject().get(typeFieldName);
+          if (labelJsonElement == null) {
+            throw new JsonParseException("cannot deserialize " + baseType
+                + " because it does not define a field named " + typeFieldName);
+          }
+          String label = labelJsonElement.getAsString();
+          @SuppressWarnings("unchecked") // registration requires that subtype extends T
+          TypeAdapter<R> delegate = (TypeAdapter<R>) labelToDelegate.get(label);
+          if (delegate == null) {
+            throw new JsonParseException("cannot deserialize " + baseType + " subtype named "
+                + label + "; did you forget to register a subtype?");
+          }
+          return delegate.fromJsonTree(jsonElement);
+        }
+
+        @Override public void write(JsonWriter out, R value) throws IOException {
+          Class<?> srcType = value.getClass();
+          String label = subtypeToLabel.get(srcType);
+          @SuppressWarnings("unchecked") // registration requires that subtype extends T
+          TypeAdapter<R> delegate = (TypeAdapter<R>) subtypeToDelegate.get(srcType);
+          if (delegate == null) {
+            throw new JsonParseException("cannot serialize " + srcType.getName()
+                + "; did you forget to register a subtype?");
+          }
+          JsonObject jsonObject = delegate.toJsonTree(value).getAsJsonObject();
+          if (!jsonObject.has(typeFieldName)) {
+            JsonObject clone = new JsonObject();
+            clone.add(typeFieldName, new JsonPrimitive(label));
+            for (Map.Entry<String, JsonElement> e : jsonObject.entrySet()) {
+              clone.add(e.getKey(), e.getValue());
+            }
+            jsonObject = clone;
+          }
+          Streams.write(jsonObject, out);
+        }
+      };
+    }
+  }
+}