public java.lang.String getResponseMimeType() {
      java.lang.Object ref = responseMimeType_;
      if (!(ref instanceof java.lang.String)) {
        com.google.protobuf.ByteString bs = (com.google.protobuf.ByteString) ref;
        java.lang.String s = bs.toStringUtf8();
        responseMimeType_ = s;
        return s;
      } else {
        return (java.lang.String) ref;
      }
    }
<pre>
Optional. Output response mimetype of the generated candidate text.
Supported mimetype:
- `text/plain`: (default) Text output.
- `application/json`: JSON response in the candidates.
The model needs to be prompted to output the appropriate response type,
otherwise the behavior is undefined.
This is a preview feature.
</pre>

<code>string response_mime_type = 13 [(.google.api.field_behavior) = OPTIONAL];</code>

@return The responseMimeType.
java-vertexai/proto-google-cloud-vertexai-v1/src/main/java/com/google/cloud/vertexai/api/GenerationConfig.java