How to format a number to a certain number of significant figures, as opposed to decimal places?

That’s a job for the BigDecimal class

public static String formatToSignificant(double value,
int significant)
{
MathContext mathContext = new MathContext(significant,
RoundingMode.DOWN);
BigDecimal bigDecimal = new BigDecimal(value,
mathContext);
return bigDecimal.toPlainString();
}