@java.lang.Override
  public void writeTo(com.google.protobuf.CodedOutputStream output) throws java.io.IOException {
    if (startIndex_ != 0) {
      output.writeInt32(1, startIndex_);
    }
    if (endIndex_ != 0) {
      output.writeInt32(2, endIndex_);
    }
    if (!com.google.protobuf.GeneratedMessageV3.isStringEmpty(uri_)) {
      com.google.protobuf.GeneratedMessageV3.writeString(output, 3, uri_);
    }
    if (!com.google.protobuf.GeneratedMessageV3.isStringEmpty(title_)) {
      com.google.protobuf.GeneratedMessageV3.writeString(output, 4, title_);
    }
    if (!com.google.protobuf.GeneratedMessageV3.isStringEmpty(license_)) {
      com.google.protobuf.GeneratedMessageV3.writeString(output, 5, license_);
    }
    if (((bitField0_ & 0x00000001) != 0)) {
      output.writeMessage(6, getPublicationDate());
    }
    getUnknownFields().writeTo(output);
  }
<pre>
Output only. Publication date of the attribution.
</pre>

<code>.google.type.Date publication_date = 6 [(.google.api.field_behavior) = OUTPUT_ONLY];
</code>
java-vertexai/proto-google-cloud-vertexai-v1/src/main/java/com/google/cloud/vertexai/api/Citation.java