Java DecimalFormat returns a "?"

It's not a question mark, it's a U+FFFD REPLACEMENT CHARACTER, which is displayed as ? since it can't be mapped to the output encoding:

NaN is formatted as a string, which typically has a single character \uFFFD. This string is determined by the DecimalFormatSymbols object. This is the only value for which the prefixes and suffixes are not used.

Similarly, ? in representation of infinity is a U+221E INFINITY character (∞).

Infinity is formatted as a string, which typically has a single character \u221E, with the positive or negative prefixes and suffixes applied. The infinity string is determined by the DecimalFormatSymbols object.

See also:

• DecimalFormat javadoc

It'll return "?" if X is Float.NaN or Float.POSITIVE_INFINITY. It appears that Float.NEGATIVE_INFINITY returns "-?".