public Builder mergeFileName(com.google.devtools.cloudtrace.v2.TruncatableString value) {
        if (fileNameBuilder_ == null) {
          if (((bitField0_ & 0x00000004) != 0)
              && fileName_ != null
              && fileName_
                  != com.google.devtools.cloudtrace.v2.TruncatableString.getDefaultInstance()) {
            getFileNameBuilder().mergeFrom(value);
          } else {
            fileName_ = value;
          }
        } else {
          fileNameBuilder_.mergeFrom(value);
        }
        if (fileName_ != null) {
          bitField0_ |= 0x00000004;
          onChanged();
        }
        return this;
      }
<pre>
The name of the source file where the function call appears (up to 256
bytes).
</pre>

<code>.google.devtools.cloudtrace.v2.TruncatableString file_name = 3;</code>
java-trace/proto-google-cloud-trace-v2/src/main/java/com/google/devtools/cloudtrace/v2/StackTrace.java