@Override
    public String getSourceName() {
		if (name == null || name.isEmpty()) {
			return UNKNOWN_SOURCE_NAME;
		}

		return name;
	}
Seek to absolute character index, which might not be in the current
 sliding window.  Move {@code p} to {@code index-bufferStartIndex}.
runtime/Java/src/org/antlr/v4/runtime/UnbufferedCharStream.java