@java.lang.Override
  public Builder toBuilder() {
    return this == DEFAULT_INSTANCE ? new Builder() : new Builder().mergeFrom(this);
  }
<pre>
Optional. An expression for filtering the results of the request.
Filter is not supported yet.
</pre>

<code>string filter = 4 [(.google.api.field_behavior) = OPTIONAL];</code>

@return The bytes for filter.
java-translate/proto-google-cloud-translate-v3/src/main/java/com/google/cloud/translate/v3/ListAdaptiveMtDatasetsRequest.java