public Builder setFileName(com.google.devtools.cloudtrace.v2.TruncatableString value) {
        if (fileNameBuilder_ == null) {
          if (value == null) {
            throw new NullPointerException();
          }
          fileName_ = value;
        } else {
          fileNameBuilder_.setMessage(value);
        }
        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