TypeAdapterFactory (Gson 2.3 API)
Date: 2014-08-11

com.google.gson

## Interface TypeAdapterFactory

-     public interface TypeAdapterFactory

  Creates type adapters for set of related types. Type adapter factories are most useful when several types share similar structure in their JSON form.

  ### Example: Converting enums to lowercase

  In this example, we implement a factory that creates type adapters for all enums. The type adapters will write enums in lowercase, despite the fact that they're defined in `CONSTANT_CASE` in the corresponding Java model:

         public class LowercaseEnumTypeAdapterFactory implements TypeAdapterFactory {
           public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
             Class<T> rawType = (Class<T>) type.getRawType();
             if (!rawType.isEnum()) {
               return null;
             }

             final Map<String, T> lowercaseToConstant = new HashMap<String, T>();
             for (T constant : rawType.getEnumConstants()) {
               lowercaseToConstant.put(toLowercase(constant), constant);
             }

             return new TypeAdapter<T>() {
               public void write(JsonWriter out, T value) throws IOException {
                 if (value == null) {
                   out.nullValue();
                 } else {
                   out.value(toLowercase(value));
                 }
               }

               public T read(JsonReader reader) throws IOException {
                 if (reader.peek() == JsonToken.NULL) {
                   reader.nextNull();
                   return null;
                 } else {
                   return lowercaseToConstant.get(reader.nextString());
                 }
               }
             };
           }

           private String toLowercase(Object o) {
             return o.toString().toLowerCase(Locale.US);
           }
         }

  Type adapter factories select which types they provide type adapters for. If a factory cannot support a given type, it must return null when that type is passed to `create(com.google.gson.Gson, com.google.gson.reflect.TypeToken<T>)`. Factories should expect `create()` to be called on them for many types and should return null for most of those types. In the above example the factory returns null for calls to `create()` where `type` is not an enum.

  A factory is typically called once per type, but the returned type adapter may be used many times. It is most efficient to do expensive work like reflection in `create()` so that the type adapter's `read()` and `write()` methods can be very fast. In this example the mapping from lowercase name to enum value is computed eagerly.

  As with type adapters, factories must be registered with a `GsonBuilder` for them to take effect:

        GsonBuilder builder = new GsonBuilder();
        builder.registerTypeAdapterFactory(new LowercaseEnumTypeAdapterFactory());
        ...
        Gson gson = builder.create();

  If multiple factories support the same type, the factory registered earlier takes precedence.

  ### Example: composing other type adapters

  In this example we implement a factory for Guava's `Multiset` collection type. The factory can be used to create type adapters for multisets of any element type: the type adapter for `Multiset<String>` is different from the type adapter for `Multiset<URL>`.

  The type adapter delegates to another type adapter for the multiset elements. It figures out the element type by reflecting on the multiset's type token. A `Gson` is passed in to `create` for just this purpose:

         public class MultisetTypeAdapterFactory implements TypeAdapterFactory {
           public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
             Type type = typeToken.getType();
             if (typeToken.getRawType() != Multiset.class
                 || !(type instanceof ParameterizedType)) {
               return null;
             }

             Type elementType = ((ParameterizedType) type).getActualTypeArguments()[0];
             TypeAdapter<?> elementAdapter = gson.getAdapter(TypeToken.get(elementType));
             return (TypeAdapter<T>) newMultisetAdapter(elementAdapter);
           }

           private <E> TypeAdapter<Multiset<E>> newMultisetAdapter(
               final TypeAdapter<E> elementAdapter) {
             return new TypeAdapter<Multiset<E>>() {
               public void write(JsonWriter out, Multiset<E> value) throws IOException {
                 if (value == null) {
                   out.nullValue();
                   return;
                 }

                 out.beginArray();
                 for (Multiset.Entry<E> entry : value.entrySet()) {
                   out.value(entry.getCount());
                   elementAdapter.write(out, entry.getElement());
                 }
                 out.endArray();
               }

               public Multiset<E> read(JsonReader in) throws IOException {
                 if (in.peek() == JsonToken.NULL) {
                   in.nextNull();
                   return null;
                 }

                 Multiset<E> result = LinkedHashMultiset.create();
                 in.beginArray();
                 while (in.hasNext()) {
                   int count = in.nextInt();
                   E element = elementAdapter.read(in);
                   result.add(element, count);
                 }
                 in.endArray();
                 return result;
               }
             };
           }
         }

  Delegating from one type adapter to another is extremely powerful; it's the foundation of how Gson converts Java objects and collections. Whenever possible your factory should retrieve its delegate type adapter in the `create()` method; this ensures potentially-expensive type adapter creation happens only once.
  Since:
    2.1

- - ### Method Summary

    All Methods Instance Methods Abstract Methods
    Modifier and Type | Method and Description
    --- | ---
    `<T> TypeAdapter<T>` | `create(Gson gson, TypeToken<T> type)` Returns a type adapter for `type`, or null if this factory doesn't support `type`.

- - ### Method Detail

    - #### create

          <T> TypeAdapter<T> create(Gson gson,
                                    TypeToken<T> type)

      Returns a type adapter for `type`, or null if this factory doesn't support `type`.

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