Moshi Determine if JSON is array or single object

You can use a JsonQualifier to generalize this.From your example, you might use it like

final class Foo {  @SingleToArray final List<User> users;}

Here's the code with a test to demonstrate more thouroughly.

@Retention(RUNTIME)@JsonQualifier public @interface SingleToArray {  final class Adapter extends JsonAdapter<List<Object>> {    final JsonAdapter<List<Object>> delegateAdapter;    final JsonAdapter<Object> elementAdapter;    public static final Factory FACTORY = new Factory() {      @Nullable @Override      public JsonAdapter<?> create(Type type, Set<? extends Annotation> annotations,          Moshi moshi) {        Set<? extends Annotation> delegateAnnotations =            Types.nextAnnotations(annotations, SingleToArray.class);        if (delegateAnnotations == null) {          return null;        }        if (Types.getRawType(type) != List.class) {          throw new IllegalArgumentException(              "Only lists may be annotated with @SingleToArray. Found: " + type);        }        Type elementType = Types.collectionElementType(type, List.class);        JsonAdapter<List<Object>> delegateAdapter = moshi.adapter(type, delegateAnnotations);        JsonAdapter<Object> elementAdapter = moshi.adapter(elementType);        return new Adapter(delegateAdapter, elementAdapter);      }    };    Adapter(JsonAdapter<List<Object>> delegateAdapter, JsonAdapter<Object> elementAdapter) {      this.delegateAdapter = delegateAdapter;      this.elementAdapter = elementAdapter;    }    @Nullable @Override public List<Object> fromJson(JsonReader reader) throws IOException {      if (reader.peek() != JsonReader.Token.BEGIN_ARRAY) {        return Collections.singletonList(elementAdapter.fromJson(reader));      }      return delegateAdapter.fromJson(reader);    }    @Override public void toJson(JsonWriter writer, @Nullable List<Object> value)        throws IOException {      if (value.size() == 1) {        elementAdapter.toJson(writer, value.get(0));      } else {        delegateAdapter.toJson(writer, value);      }    }  }}@Test public void singleToArray() throws Exception {  Moshi moshi = new Moshi.Builder().add(SingleToArray.Adapter.FACTORY).build();  JsonAdapter<List<String>> adapter =      moshi.adapter(Types.newParameterizedType(List.class, String.class), SingleToArray.class);  assertThat(adapter.fromJson("[\"Tom\",\"Huck\"]")).isEqualTo(Arrays.asList("Tom", "Huck"));  assertThat(adapter.toJson(Arrays.asList("Tom", "Huck"))).isEqualTo("[\"Tom\",\"Huck\"]");  assertThat(adapter.fromJson("\"Jim\"")).isEqualTo(Collections.singletonList("Jim"));  assertThat(adapter.toJson(Collections.singletonList("Jim"))).isEqualTo("\"Jim\"");  assertThat(adapter.fromJson("[]")).isEqualTo(Collections.emptyList());  assertThat(adapter.toJson(Collections.<String>emptyList())).isEqualTo("[]");}

Using @Eric's comment, I came up with the correct code below:

public static <T> List<T> loadFakeData(String url, Class<T> cls){    List<T> list = new ArrayList<>();    Moshi moshi = new Moshi.Builder().build();    try {        JsonReader reader = JsonReader.of(runHttpClient(url));        JsonReader.Token token = reader.peek();        if (token.equals(JsonReader.Token.BEGIN_ARRAY)) {            Type type = Types.newParameterizedType(List.class, cls);            JsonAdapter<List<T>> adapter = moshi.adapter(type);            list = adapter.fromJson(reader);        } else if (token.equals(JsonReader.Token.BEGIN_OBJECT)){            JsonAdapter<T> adapter = moshi.adapter(cls);            T t = adapter.fromJson(reader);            list.add(t);        }    } catch (IOException e) {        e.printStackTrace();    }    return list;}