Update: This is now in the Moshi adapters artifact. https://github.com/square/moshi/blob/5153295988e09e6a1bfe76eecff8b22bf49e25de/adapters/src/main/java/com/squareup/moshi/adapters/EnumJsonAdapter.java An enum JsonAdapter for Moshi that allows for a fallback value when deserializing unknown strings. NOTE: Allows null for the default.

EnumWithDefaultValueJsonAdapter.java

import com.squareup.moshi.Json;
import com.squareup.moshi.JsonAdapter;
import com.squareup.moshi.JsonReader;
import com.squareup.moshi.JsonWriter;
import java.io.IOException;

public final class EnumWithDefaultValueJsonAdapter<T extends Enum<T>> extends JsonAdapter<T> {
  private final Class<T> enumType;
  private final String[] nameStrings;
  private final T[] constants;
  private final JsonReader.Options options;
  private final T defaultValue;

  public EnumWithDefaultValueJsonAdapter(Class<T> enumType, T defaultValue) {
    this.enumType = enumType;
    this.defaultValue = defaultValue;
    try {
      constants = enumType.getEnumConstants();
      nameStrings = new String[constants.length];
      for (int i = 0; i < constants.length; i++) {
        T constant = constants[i];
        Json annotation = enumType.getField(constant.name()).getAnnotation(Json.class);
        String name = annotation != null ? annotation.name() : constant.name();
        nameStrings[i] = name;
      }
      options = JsonReader.Options.of(nameStrings);
    } catch (NoSuchFieldException e) {
      throw new AssertionError(e);
    }
  }

  @Override public T fromJson(JsonReader reader) throws IOException {
    int index = reader.selectString(options);
    if (index != -1) return constants[index];
    reader.nextString();
    return defaultValue;
  }

  @Override public void toJson(JsonWriter writer, T value) throws IOException {
    writer.value(nameStrings[value.ordinal()]);
  }

  @Override public String toString() {
    return "JsonAdapter(" + enumType.getName() + ").defaultValue( " + defaultValue + ")";
  }
}