@Override
	public String toString() {
		String symbol = "";
		if (startIndex >= 0 && startIndex < getInputStream().size()) {
			symbol = getInputStream().getText(Interval.of(startIndex,startIndex));
			symbol = Utils.escapeWhitespace(symbol, false);
		}

		return String.format(Locale.getDefault(), "%s('%s')", LexerNoViableAltException.class.getSimpleName(), symbol);
	}
Which configurations did we try at input.index() that couldn't match input.LA(1)?
runtime/Java/src/org/antlr/v4/runtime/LexerNoViableAltException.java