Simple "SafeListTypeAdapter" > Always return array for non-array objects that can be array together with non-arrays..

SafeListTypeAdapter.java

import com.google.gson.Gson;
import com.google.gson.TypeAdapter;
import com.google.gson.TypeAdapterFactory;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;
import com.google.gson.stream.JsonWriter;
import com.google.gson.stream.MalformedJsonException;

import java.io.IOException;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.List;

public class SafeListTypeAdapter<E> implements TypeAdapterFactory {

    private SafeListTypeAdapter() {
    }

    @SuppressWarnings("unchecked")
    @Override
    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
        if (!List.class.isAssignableFrom(typeToken.getRawType())) {
            return null;
        }
        return (TypeAdapter<T>) new AlwaysListTypeAdapter<>((TypeAdapter<E>) gson.getAdapter(TypeToken.get(resolveTypeArgument(typeToken.getType())))).nullSafe();
    }

    private static Type resolveTypeArgument(Type type) {
        if (!(type instanceof ParameterizedType)) {
            return Object.class;
        }
        return ((ParameterizedType) type).getActualTypeArguments()[0];
    }

    private static final class AlwaysListTypeAdapter<E> extends TypeAdapter<List<E>> {

        private final TypeAdapter<E> elementTypeAdapter;

        private AlwaysListTypeAdapter(TypeAdapter<E> elementTypeAdapter) {
            this.elementTypeAdapter = elementTypeAdapter;
        }

        @Override
        public void write(JsonWriter out, List<E> list) {
            throw new UnsupportedOperationException();
        }

        @Override
        public List<E> read(JsonReader in) throws IOException {
            List<E> candidates = new ArrayList<>();
            JsonToken token = in.peek();
            switch (token) {
                case BEGIN_ARRAY:
                    in.beginArray();
                    while (in.hasNext()) {
                        candidates.add(elementTypeAdapter.read(in));
                    }
                    in.endArray();
                    break;
                case BEGIN_OBJECT:
                case STRING:
                case NUMBER:
                case BOOLEAN:
                    candidates.add(elementTypeAdapter.read(in));
                    break;
                case NULL:
                    throw new AssertionError("Must never happen: check if the type adapter configured with .nullSafe()");
                case NAME:
                case END_ARRAY:
                case END_OBJECT:
                case END_DOCUMENT:
                    throw new MalformedJsonException("Unexpected token: " + token);
                default:
                    throw new AssertionError("Must never happen: " + token);
            }
            return candidates;
        }

    }

}

Usage:

    @JsonAdapter(SafeListTypeAdapter.class)
    @SerializedName("application-message")
    private List<Message> messages;

Helps on translating such JSON (where it can variates between an object and an array):

Array-based:

[
  {
    "title": "Test2",
    "type": "error",
    "application": "open_tracking",
    "application-message": [
      {
        "message-title": "yep yep - en",
        "message-body": "yep yep yep - en",
        "language": "en"
      },
      {
        "message-title": "yep yep - no",
        "message-body": "yep yep yep - no",
        "language": "nb"
      }
    ]
  }
]

Object-based:

[
  {
    "title": "test3",
    "type": "error",
    "application": "controltower",
    "application-message": {
      "message-title": "11",
      "message-body": "12",
      "language": "nb"
    }
  }
]

..both ends up in an array :-)