public EnumTypeProtoConverter<E, O> build() {
      return new EnumTypeProtoConverter<>(
          Collections.unmodifiableMap(fromProtoEnumMap),
          Collections.unmodifiableMap(toProtoEnumMap));
    }
Adds bidirectional conversion mapping between {@code protoEnum} and {@code objectEnum}.
java_src/src/main/java/com/google/crypto/tink/internal/EnumTypeProtoConverter.java