code: /*
   * 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 tools.com.google.gson;
  
  import tools.com.google.gson.internal.bind.JsonTreeWriter;
  import tools.com.google.gson.internal.bind.JsonTreeReader;
  import tools.com.google.gson.stream.JsonReader;
  import tools.com.google.gson.stream.JsonToken;
  import tools.com.google.gson.stream.JsonWriter;
  import java.io.IOException;
  import java.io.Reader;
  import java.io.StringReader;
  import java.io.StringWriter;
  import java.io.Writer;
  
  /**
   * Converts Java objects to and from JSON.
   *
   * <h3>Defining a type's JSON form</h3>
   * By default Gson converts application classes to JSON using its built-in type
   * adapters. If Gson's default JSON conversion isn't appropriate for a type,
   * extend this class to customize the conversion. Here's an example of a type
   * adapter for an (X,Y) coordinate point: <pre>   {@code
   *
   *   public class PointAdapter extends TypeAdapter<Point> {
   *     public Point read(JsonReader reader) throws IOException {
   *       if (reader.peek() == JsonToken.NULL) {
   *         reader.nextNull();
   *         return null;
   *       }
   *       String xy = reader.nextString();
   *       String[] parts = xy.split(",");
   *       int x = Integer.parseInt(parts[0]);
   *       int y = Integer.parseInt(parts[1]);
   *       return new Point(x, y);
   *     }
   *     public void write(JsonWriter writer, Point value) throws IOException {
   *       if (value == null) {
   *         writer.nullValue();
   *         return;
   *       }
   *       String xy = value.getX() + "," + value.getY();
   *       writer.value(xy);
   *     }
   *   }}</pre>
   * With this type adapter installed, Gson will convert {@code Points} to JSON as
   * strings like {@code "5,8"} rather than objects like {@code {"x":5,"y":8}}. In
   * this case the type adapter binds a rich Java class to a compact JSON value.
   *
   * <p>The {@link #read(JsonReader) read()} method must read exactly one value
   * and {@link #write(JsonWriter,Object) write()} must write exactly one value.
   * For primitive types this is means readers should make exactly one call to
   * {@code nextBoolean()}, {@code nextDouble()}, {@code nextInt()}, {@code
   * nextLong()}, {@code nextString()} or {@code nextNull()}. Writers should make
   * exactly one call to one of <code>value()</code> or <code>nullValue()</code>.
   * For arrays, type adapters should start with a call to {@code beginArray()},
   * convert all elements, and finish with a call to {@code endArray()}. For
   * objects, they should start with {@code beginObject()}, convert the object,
   * and finish with {@code endObject()}. Failing to convert a value or converting
   * too many values may cause the application to crash.
   *
   * <p>Type adapters should be prepared to read null from the stream and write it
   * to the stream. Alternatively, they should use {@link #nullSafe()} method while
   * registering the type adapter with Gson. If your {@code Gson} instance
   * has been configured to {@link GsonBuilder#serializeNulls()}, these nulls will be
   * written to the final document. Otherwise the value (and the corresponding name
   * when writing to a JSON object) will be omitted automatically. In either case
   * your type adapter must handle null.
   *
   * <p>To use a custom type adapter with Gson, you must <i>register</i> it with a
   * {@link GsonBuilder}: <pre>   {@code
   *
   *   GsonBuilder builder = new GsonBuilder();
   *   builder.registerTypeAdapter(Point.class, new PointAdapter());
   *   // if PointAdapter didn't check for nulls in its read/write methods, you should instead use
   *   // builder.registerTypeAdapter(Point.class, new PointAdapter().nullSafe());
   *   ...
   *   Gson gson = builder.create();
   * }</pre>
   *
   * @since 2.1
   */
  // non-Javadoc:
  //
  // <h3>JSON Conversion</h3>
  // <p>A type adapter registered with Gson is automatically invoked while serializing
  // or deserializing JSON. However, you can also use type adapters directly to serialize
  // and deserialize JSON. Here is an example for deserialization: <pre>   {@code
  //
  //   String json = "{'origin':'0,0','points':['1,2','3,4']}";
  //   TypeAdapter<Graph> graphAdapter = gson.getAdapter(Graph.class);
  //   Graph graph = graphAdapter.fromJson(json);
  // }</pre>
  // And an example for serialization: <pre>   {@code
  //
  //   Graph graph = new Graph(...);
  //   TypeAdapter<Graph> graphAdapter = gson.getAdapter(Graph.class);
  //   String json = graphAdapter.toJson(graph);
  // }</pre>
  //
  // <p>Type adapters are <strong>type-specific</strong>. For example, a {@code
  // TypeAdapter<Date>} can convert {@code Date} instances to JSON and JSON to
  // instances of {@code Date}, but cannot convert any other types.
  //
  public abstract class TypeAdapter<T> {
  
    /**
     * Writes one JSON value (an array, object, string, number, boolean or null)
     * for {@code value}.
     *
     * @param value the Java object to write. May be null.
     */
    public abstract void write(JsonWriter out, T value) throws IOException;
  
    /**
     * Converts {@code value} to a JSON document and writes it to {@code out}.
     * Unlike Gson's similar {@link Gson#toJson(JsonElement, Appendable) toJson}
     * method, this write is strict. Create a {@link
     * JsonWriter#setLenient(boolean) lenient} {@code JsonWriter} and call
     * {@link #write(com.google.gson.stream.JsonWriter, Object)} for lenient
     * writing.
     *
     * @param value the Java object to convert. May be null.
     * @since 2.2
     */
    public final void toJson(Writer out, T value) throws IOException {
      JsonWriter writer = new JsonWriter(out);
      write(writer, value);
    }
  
    /**
     * This wrapper method is used to make a type adapter null tolerant. In general, a
     * type adapter is required to handle nulls in write and read methods. Here is how this
     * is typically done:<br>
     * <pre>   {@code
     *
     * Gson gson = new GsonBuilder().registerTypeAdapter(Foo.class,
     *   new TypeAdapter<Foo>() {
     *     public Foo read(JsonReader in) throws IOException {
     *       if (in.peek() == JsonToken.NULL) {
     *         in.nextNull();
     *         return null;
     *       }
     *       // read a Foo from in and return it
     *     }
     *     public void write(JsonWriter out, Foo src) throws IOException {
     *       if (src == null) {
     *         out.nullValue();
     *         return;
     *       }
     *       // write src as JSON to out
     *     }
     *   }).create();
     * }</pre>
     * You can avoid this boilerplate handling of nulls by wrapping your type adapter with
     * this method. Here is how we will rewrite the above example:
     * <pre>   {@code
     *
     * Gson gson = new GsonBuilder().registerTypeAdapter(Foo.class,
     *   new TypeAdapter<Foo>() {
     *     public Foo read(JsonReader in) throws IOException {
     *       // read a Foo from in and return it
     *     }
     *     public void write(JsonWriter out, Foo src) throws IOException {
     *       // write src as JSON to out
     *     }
     *   }.nullSafe()).create();
     * }</pre>
     * Note that we didn't need to check for nulls in our type adapter after we used nullSafe.
     */
    public final TypeAdapter<T> nullSafe() {
      return new TypeAdapter<T>() {
        @Override public void write(JsonWriter out, T value) throws IOException {
          if (value == null) {
            out.nullValue();
          } else {
            TypeAdapter.this.write(out, value);
          }
        }
        @Override public T read(JsonReader reader) throws IOException {
          if (reader.peek() == JsonToken.NULL) {
            reader.nextNull();
            return null;
          }
          return TypeAdapter.this.read(reader);
        }
      };
    }
  
    /**
     * Converts {@code value} to a JSON document. Unlike Gson's similar {@link
     * Gson#toJson(Object) toJson} method, this write is strict. Create a {@link
     * JsonWriter#setLenient(boolean) lenient} {@code JsonWriter} and call
     * {@link #write(com.google.gson.stream.JsonWriter, Object)} for lenient
     * writing.
     *
     * @param value the Java object to convert. May be null.
     * @since 2.2
     */
    public final String toJson(T value) {
      StringWriter stringWriter = new StringWriter();
      try {
        toJson(stringWriter, value);
      } catch (IOException e) {
        throw new AssertionError(e); // No I/O writing to a StringWriter.
      }
      return stringWriter.toString();
    }
  
    /**
     * Converts {@code value} to a JSON tree.
     *
     * @param value the Java object to convert. May be null.
     * @return the converted JSON tree. May be {@link JsonNull}.
     * @since 2.2
     */
    public final JsonElement toJsonTree(T value) {
      try {
        JsonTreeWriter jsonWriter = new JsonTreeWriter();
        write(jsonWriter, value);
        return jsonWriter.get();
      } catch (IOException e) {
        throw new JsonIOException(e);
      }
    }
  
    /**
     * Reads one JSON value (an array, object, string, number, boolean or null)
     * and converts it to a Java object. Returns the converted object.
     *
     * @return the converted Java object. May be null.
     */
    public abstract T read(JsonReader in) throws IOException;
  
    /**
     * Converts the JSON document in {@code in} to a Java object. Unlike Gson's
     * similar {@link Gson#fromJson(java.io.Reader, Class) fromJson} method, this
     * read is strict. Create a {@link JsonReader#setLenient(boolean) lenient}
     * {@code JsonReader} and call {@link #read(JsonReader)} for lenient reading.
     *
     * @return the converted Java object. May be null.
     * @since 2.2
     */
    public final T fromJson(Reader in) throws IOException {
      JsonReader reader = new JsonReader(in);
      return read(reader);
    }
  
    /**
     * Converts the JSON document in {@code json} to a Java object. Unlike Gson's
     * similar {@link Gson#fromJson(String, Class) fromJson} method, this read is
     * strict. Create a {@link JsonReader#setLenient(boolean) lenient} {@code
     * JsonReader} and call {@link #read(JsonReader)} for lenient reading.
     *
     * @return the converted Java object. May be null.
     * @since 2.2
     */
    public final T fromJson(String json) throws IOException {
      return fromJson(new StringReader(json));
    }
  
    /**
     * Converts {@code jsonTree} to a Java object.
     *
     * @param jsonTree the Java object to convert. May be {@link JsonNull}.
     * @since 2.2
     */
    public final T fromJsonTree(JsonElement jsonTree) {
      try {
        JsonReader jsonReader = new JsonTreeReader(jsonTree);
        return read(jsonReader);
      } catch (IOException e) {
        throw new JsonIOException(e);
      }
    }
  }
  