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TypeAdapterFactory (Gson 2.3.1 API)
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<H2>
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com.google.gson</FONT>
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Interface TypeAdapterFactory</H2>
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<HR>
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<DL>
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<DT><PRE>public interface <B>TypeAdapterFactory</B></DL>
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</PRE>
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<P>
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Creates type adapters for set of related types. Type adapter factories are
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most useful when several types share similar structure in their JSON form.
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<h3>Example: Converting enums to lowercase</h3>
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In this example, we implement a factory that creates type adapters for all
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enums. The type adapters will write enums in lowercase, despite the fact
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that they're defined in <code>CONSTANT_CASE</code> in the corresponding Java
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model: <pre> <code>public class LowercaseEnumTypeAdapterFactory implements TypeAdapterFactory {
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public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
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Class<T> rawType = (Class<T>) type.getRawType();
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if (!rawType.isEnum()) {
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return null;
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}
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final Map<String, T> lowercaseToConstant = new HashMap<String, T>();
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for (T constant : rawType.getEnumConstants()) {
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lowercaseToConstant.put(toLowercase(constant), constant);
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}
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return new TypeAdapter<T>() {
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public void write(JsonWriter out, T value) throws IOException {
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if (value == null) {
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out.nullValue();
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} else {
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out.value(toLowercase(value));
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}
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}
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public T read(JsonReader reader) throws IOException {
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if (reader.peek() == JsonToken.NULL) {
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reader.nextNull();
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return null;
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} else {
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return lowercaseToConstant.get(reader.nextString());
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}
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}
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};
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}
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private String toLowercase(Object o) {
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return o.toString().toLowerCase(Locale.US);
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}
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}
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</code></pre>
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<p>Type adapter factories select which types they provide type adapters
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for. If a factory cannot support a given type, it must return null when
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that type is passed to <A HREF="../../../com/google/gson/TypeAdapterFactory.html#create(com.google.gson.Gson, com.google.gson.reflect.TypeToken)"><CODE>create(com.google.gson.Gson, com.google.gson.reflect.TypeToken<T>)</CODE></A>. Factories should expect <code>create()</code> to be called on them for many types and should return null for
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most of those types. In the above example the factory returns null for
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calls to <code>create()</code> where <code>type</code> is not an enum.
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<p>A factory is typically called once per type, but the returned type
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adapter may be used many times. It is most efficient to do expensive work
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like reflection in <code>create()</code> so that the type adapter's <code>read()</code> and <code>write()</code> methods can be very fast. In this example the
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mapping from lowercase name to enum value is computed eagerly.
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<p>As with type adapters, factories must be <i>registered</i> with a <A HREF="../../../com/google/gson/GsonBuilder.html" title="class in com.google.gson"><CODE>GsonBuilder</CODE></A> for them to take effect: <pre> <code>GsonBuilder builder = new GsonBuilder();
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builder.registerTypeAdapterFactory(new LowercaseEnumTypeAdapterFactory());
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...
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Gson gson = builder.create();
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</code></pre>
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If multiple factories support the same type, the factory registered earlier
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takes precedence.
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<h3>Example: composing other type adapters</h3>
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In this example we implement a factory for Guava's <code>Multiset</code>
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collection type. The factory can be used to create type adapters for
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multisets of any element type: the type adapter for <code>Multiset<String></code> is different from the type adapter for <code>Multiset<URL></code>.
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<p>The type adapter <i>delegates</i> to another type adapter for the
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multiset elements. It figures out the element type by reflecting on the
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multiset's type token. A <code>Gson</code> is passed in to <code>create</code> for
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just this purpose: <pre> <code>public class MultisetTypeAdapterFactory implements TypeAdapterFactory {
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public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
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Type type = typeToken.getType();
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if (typeToken.getRawType() != Multiset.class
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|| !(type instanceof ParameterizedType)) {
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return null;
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}
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Type elementType = ((ParameterizedType) type).getActualTypeArguments()[0];
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TypeAdapter<?> elementAdapter = gson.getAdapter(TypeToken.get(elementType));
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return (TypeAdapter<T>) newMultisetAdapter(elementAdapter);
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}
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private <E> TypeAdapter<Multiset<E>> newMultisetAdapter(
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final TypeAdapter<E> elementAdapter) {
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return new TypeAdapter<Multiset<E>>() {
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public void write(JsonWriter out, Multiset<E> value) throws IOException {
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if (value == null) {
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out.nullValue();
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return;
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}
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out.beginArray();
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for (Multiset.Entry<E> entry : value.entrySet()) {
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out.value(entry.getCount());
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elementAdapter.write(out, entry.getElement());
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}
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out.endArray();
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}
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public Multiset<E> read(JsonReader in) throws IOException {
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if (in.peek() == JsonToken.NULL) {
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in.nextNull();
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return null;
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}
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Multiset<E> result = LinkedHashMultiset.create();
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in.beginArray();
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while (in.hasNext()) {
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int count = in.nextInt();
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E element = elementAdapter.read(in);
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result.add(element, count);
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}
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in.endArray();
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return result;
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}
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};
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}
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}
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</code></pre>
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Delegating from one type adapter to another is extremely powerful; it's
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the foundation of how Gson converts Java objects and collections. Whenever
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possible your factory should retrieve its delegate type adapter in the
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<code>create()</code> method; this ensures potentially-expensive type adapter
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creation happens only once.
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<P>
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<P>
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<DL>
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<DT><B>Since:</B></DT>
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<DD>2.1</DD>
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</DL>
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<HR>
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<P>
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<!-- ========== METHOD SUMMARY =========== -->
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<TR ALIGN="right" VALIGN="">
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<TD NOWRAP><FONT SIZE="-1">
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<CODE><T> <A HREF="../../../com/google/gson/TypeAdapter.html" title="class in com.google.gson">TypeAdapter</A><T></CODE></FONT></TD>
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</TR>
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</TABLE>
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</CODE></FONT></TD>
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<TD><CODE><B><A HREF="../../../com/google/gson/TypeAdapterFactory.html#create(com.google.gson.Gson, com.google.gson.reflect.TypeToken)">create</A></B>(<A HREF="../../../com/google/gson/Gson.html" title="class in com.google.gson">Gson</A> gson,
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<A HREF="../../../com/google/gson/reflect/TypeToken.html" title="class in com.google.gson.reflect">TypeToken</A><T> type)</CODE>
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<BR>
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Returns a type adapter for <code>type</code>, or null if this factory doesn't
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support <code>type</code>.</TD>
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<A NAME="create(com.google.gson.Gson, com.google.gson.reflect.TypeToken)"><!-- --></A><H3>
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create</H3>
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<PRE>
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<T> <A HREF="../../../com/google/gson/TypeAdapter.html" title="class in com.google.gson">TypeAdapter</A><T> <B>create</B>(<A HREF="../../../com/google/gson/Gson.html" title="class in com.google.gson">Gson</A> gson,
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<A HREF="../../../com/google/gson/reflect/TypeToken.html" title="class in com.google.gson.reflect">TypeToken</A><T> type)</PRE>
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<DL>
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<DD>Returns a type adapter for <code>type</code>, or null if this factory doesn't
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support <code>type</code>.
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