Exception in thread "main" java.util.IllegalFormatConversionException: d != java.lang.String
Solution 1:
The problem is here:
System.out.printf("%d has ", number + "digits.");
the %d format specifier requires an integer to be passed as second parameter to printf, but by concatenating number and "digits.", you actually passed a String.
Fixed version:
System.out.printf("has %d digits ", number);
note that you cannot print both the original number and the number of digits, because you overwrote one with the other in the number variable. Maybe use two different ones.