Library Puzzlers - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 172

When you look at the array, a pattern becomes evident: The difference between consecutive elements
is 111. Therefore, the difference between two elements is a function of how far apart they are in the
array. If two elements are identical, the difference is 0; if they're adjacent, it's 111; if they're separated
by one element, it's 222; and so on. It would appear that the number of differences is the same as the
number of distances, which is the size of the array, or 8. If you ran the program, however, you found
that it prints 14 . What's going on?
This analysis contains one small flaw. To investigate the flaw, we can print out the contents of the set
by removing the characters .size() from the println statement. Doing this produces the following
output:
[111, 222, 446, 557, 668, 113, 335, 444, 779, 224, 0, 333, 555, 666]
Not all these numbers are multiples of 111. There must be two adjacent elements in the vals array
whose difference is 113. If you look at the array declaration, it may not be clear why this is so:
int vals[] = { 789, 678, 567, 456,
345, 234, 123, 012 };
But if you print the contents of the array, here is what you will see:
[789, 678, 567, 456, 345, 234, 123, 10]
Why is the final element of the array 10 instead of 12 ? Because integer literals beginning with a 0
are interpreted as octal values [JLS 3.10.1]. This obscure construct is a holdover from the C
programming language. C dates from the 1970s, when octal was much more commonly used than it
is today.
Once you know that 012 == 10 , it is obvious why the program prints 14 : There are 6 unique nonzero
differences not involving the final array element, 7 unique nonzero differences involving the final
array element, and there is zero, for a total of 14 unique differences. It is even more obvious how to
fix the program: Replace the octal integer literal 012 with the decimal integer literal 12 . If you do
this, the program will print 8 as expected.
The lesson of this puzzle is simple: Never pad an integer literal with zeros; this turns it into an
octal literal. The intentional use of octal integer literals is so rare that you should probably comment