code: /*
   * 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 tools.com.google.gson;
  
  import java.lang.reflect.Type;
  
  /**
   * This interface is implemented to create instances of a class that does not define a no-args
   * constructor. If you can modify the class, you should instead add a private, or public
   * no-args constructor. However, that is not possible for library classes, such as JDK classes, or
   * a third-party library that you do not have source-code of. In such cases, you should define an
   * instance creator for the class. Implementations of this interface should be registered with
   * {@link GsonBuilder#registerTypeAdapter(Type, Object)} method before Gson will be able to use
   * them.
   * <p>Let us look at an example where defining an InstanceCreator might be useful. The
   * {@code Id} class defined below does not have a default no-args constructor.</p>
   *
   * <pre>
   * public class Id&lt;T&gt; {
   *   private final Class&lt;T&gt; clazz;
   *   private final long value;
   *   public Id(Class&lt;T&gt; clazz, long value) {
   *     this.clazz = clazz;
   *     this.value = value;
   *   }
   * }
   * </pre>
   *
   * <p>If Gson encounters an object of type {@code Id} during deserialization, it will throw an
   * exception. The easiest way to solve this problem will be to add a (public or private) no-args
   * constructor as follows:</p>
   *
   * <pre>
   * private Id() {
   *   this(Object.class, 0L);
   * }
   * </pre>
   *
   * <p>However, let us assume that the developer does not have access to the source-code of the
   * {@code Id} class, or does not want to define a no-args constructor for it. The developer
   * can solve this problem by defining an {@code InstanceCreator} for {@code Id}:</p>
   *
   * <pre>
   * class IdInstanceCreator implements InstanceCreator&lt;Id&gt; {
   *   public Id createInstance(Type type) {
   *     return new Id(Object.class, 0L);
   *   }
   * }
   * </pre>
   *
   * <p>Note that it does not matter what the fields of the created instance contain since Gson will
   * overwrite them with the deserialized values specified in Json. You should also ensure that a
   * <i>new</i> object is returned, not a common object since its fields will be overwritten.
   * The developer will need to register {@code IdInstanceCreator} with Gson as follows:</p>
   *
   * <pre>
   * Gson gson = new GsonBuilder().registerTypeAdapter(Id.class, new IdInstanceCreator()).create();
   * </pre>
   *
   * @param <T> the type of object that will be created by this implementation.
   *
   * @author Inderjeet Singh
   * @author Joel Leitch
   */
  public interface InstanceCreator<T> {
  
    /**
     * Gson invokes this call-back method during deserialization to create an instance of the
     * specified type. The fields of the returned instance are overwritten with the data present
     * in the Json. Since the prior contents of the object are destroyed and overwritten, do not
     * return an instance that is useful elsewhere. In particular, do not return a common instance,
     * always use {@code new} to create a new instance.
     *
     * @param type the parameterized T represented as a {@link Type}.
     * @return a default object instance of type T.
     */
    public T createInstance(Type type);
  }
  