What's the difference between formatting an integer as %d and %s ?

Question

While both syntactically valid, are there any important underlying differences one should be aware of between:

String result = String.format("Here is a number - %s", someIntValue);

vs:

String result = String.format("Here is a number - %d", someIntValue);

where in both cases someIntValue is a int?

Answer 1

For formatter syntax see the documentation.

For %s:

If arg implements Formattable, then arg.formatTo is invoked. Otherwise, the result is obtained by invoking arg.toString().

Integer does not implement Formattable, so toString is called.

For %d:

The result is formatted as a decimal integer

In most cases the results are the same. However, %d is also subject to Locale. In Hindi, for example, 100000 will be formatted to १००००० (Devanagari numerals)

You can run this short code snippet to see locales with a "non-standard" output:

for (Locale locale : Locale.getAvailableLocales())
{
    String format = String.format(locale, "%d", 100_000);

    if (!format.equals("100000")) System.out.println(locale + " " + format);
}