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## 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)`. 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
`
`<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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