private void ensureFramesIsMutable() {
      if (!((bitField0_ & 0x00000010) != 0)) {
        frames_ =
            new java.util.ArrayList<
                com.google.cloud.videointelligence.v1p3beta1.ObjectTrackingFrame>(frames_);
        bitField0_ |= 0x00000010;
      }
    }
<pre>
Object category's labeling confidence of this track.
</pre>

<code>float confidence = 4;</code>

@return This builder for chaining.
java-video-intelligence/proto-google-cloud-video-intelligence-v1p3beta1/src/main/java/com/google/cloud/videointelligence/v1p3beta1/ObjectTrackingAnnotation.java