private com.google.protobuf.SingleFieldBuilderV3<
              com.google.devtools.cloudtrace.v2.Span.TimeEvent.MessageEvent,
              com.google.devtools.cloudtrace.v2.Span.TimeEvent.MessageEvent.Builder,
              com.google.devtools.cloudtrace.v2.Span.TimeEvent.MessageEventOrBuilder>
          getMessageEventFieldBuilder() {
        if (messageEventBuilder_ == null) {
          if (!(valueCase_ == 3)) {
            value_ =
                com.google.devtools.cloudtrace.v2.Span.TimeEvent.MessageEvent.getDefaultInstance();
          }
          messageEventBuilder_ =
              new com.google.protobuf.SingleFieldBuilderV3<
                  com.google.devtools.cloudtrace.v2.Span.TimeEvent.MessageEvent,
                  com.google.devtools.cloudtrace.v2.Span.TimeEvent.MessageEvent.Builder,
                  com.google.devtools.cloudtrace.v2.Span.TimeEvent.MessageEventOrBuilder>(
                  (com.google.devtools.cloudtrace.v2.Span.TimeEvent.MessageEvent) value_,
                  getParentForChildren(),
                  isClean());
          value_ = null;
        }
        valueCase_ = 3;
        onChanged();
        return messageEventBuilder_;
      }
<pre>
An event describing a message sent/received between Spans.
</pre>

<code>.google.devtools.cloudtrace.v2.Span.TimeEvent.MessageEvent message_event = 3;</code>
java-trace/proto-google-cloud-trace-v2/src/main/java/com/google/devtools/cloudtrace/v2/Span.java