NumberFormat.parse("3.14") returning 314 when used with German Locale

When I try to read the string "3.14" as a float using German Locale I expect one of two things to happen:

(1) throw an error, because that is not a valid way to write 3.14 in German
(2) fallback to a more standard decimal notation and read the number as 3.14 because that is what any German would read in that number

But instead I am getting 314.

import java.text.NumberFormat;
import java.util.Locale;


public class MyClass {
    public static void main(String args[]) throws Exception {
        System.out.println(
            NumberFormat.getNumberInstance(Locale.GERMANY).parse("3.14")
        ); // prints 314
    }
}

The oracle-documentation for parse states:

Number parse(String source)
Parses text from the beginning of the given string to produce a number.

Which does not really explain what I am seeing here as it doesn't specify any non-happy path. What is the javas understanding of a German decimal number, and how can I fail-fast and safely convert Strings to numbers assuming a German decimal notation?

Solution

The basic assumption that NumberFormat would validate its input is wrong. A modern dev might expect a validation, especially because the method throws a ParseException as a checked exception, but with the magic of open-source I can look at the source and realize I am very wrong and this Java 1.1 code was written with different design principles than I am used to.

The critical code section in the concrete class that we are using here (for one implementation) is in openjdk > DecimalFormat.java > int subparseNumber, where the input string gets converted into a "DigitList". The digit-list for "3.14" with a German locale is indeed [3, 1, 4] because the thousands-separator is indeed ignored as @GiacomoCatenazzi pointed out in his comment ¹, so subsequent code has to interpret it as 314. Also, when an invalid character is encountered, the parsing just stops, so for example "0x134" -> 0 with no error.

There is more to learn from the source-code: NumberFormat is not threadsafe, you may not reuse the same instance across multiple threads. The modern assumption that a function like format.parse(input) -> obj would be trivially safe because input and format are only accessed readonly does not hold - parsing changes internal state of the NumberFormat-instance. You can only reuse the instance after parse completes.

So how do I make a failfast conversion of Strings to numbers in Java?

(1) If you know the target type and the number is in the standard decimal format, this works:

Float.valueOf("3,14"); // NumberFormatException
Float.valueOf("3.14"); // 3.14f

Note that NumberFormat.getNumberInstance().parse("3,14") will return 314 - not an error - so this no-validation-problem is in no way exclusive to the German Locale.

(2) If I have to use German-locale-number-strings for reading numbers, I must check if the input-string matches expectation and NumberFormat does not provide any way to do that, nor does there seem to be a satisfying fail-fast/non-gigo answer to this 12-year old question about the problem: Convert String with Dot or Comma to Float Number

The best idea I have is to validate the input myself and restrict it that way. Here is a solution that is stricter than necessary, banning thousands-separators completely, but for my usecase, this is fine:

if (inputString.contains(".")) {
           // throw
}
return Float.valueOf(inputString.replace(',', '.'));

¹ You can actually do format.setGroupingUsed(false), and then you can parse "3.14" as a 3 instead of a 314, so it is not entirely true they get fully ignored. But there is no code that uses the grouping-character to judge the correctness of the input String, even though there is format.setGroupingSize and getter which controls how many digits should be grouped together.