TITLE: Java Trivia: Unary Operators in String Concatenation
AUTHOR: Eugene Wallingford
DATE: February 02, 2006 3:47 PM
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BODY:
Via another episode of College Kids Say the Darnedest
Things, here is a fun little Java puzzler, suitable for
your compiler students or, if you're brave, even for
your CS1 students.
What is the output of this snippet of code?
int price = 75;
int discount = -25;
System.out.println( "Price is " + price + discount + " dollars" );
System.out.println( "Price is " + price - discount + " dollars" );
System.out.println( "Price is " + price + - + discount + " dollars" );
System.out.println( "Price is " + price + - - discount + " dollars" );
System.out.println( "Price is " + price+-+discount + " dollars" );
System.out.println( "Price is " + price+--discount + " dollars" );
Okay, we all that the second println causes
a compile-time error, so comment that line out before
going on. After you've made your guesses, check out
the
answers.
Many students, even upper-division ones, are sometimes
surprised that all of the rest are legal Java. The
unary operators + and - are applied
to the value of discount before it is appended
to the string.
Even some who knew that the code would compile got caught
by the fact that the output of the last two printlns
is not identical to the output of the middle two. These
operators are self-delimiting, so the scanner does not
require that they be surrounded by white space. But in
the last line, the scanner is able to match a single
token in -- (the unary operator for pre-decrement)
rather than two unary - operators, and so it does.
This is an example of how most compilers match the longest
possible token whenever they have the choice.
So whitespace does matter -- sometimes!
This turned into a good exercise for my compiler students
today, as we just last time finished talking about lexical
analysis and were set to talk about syntax analysis today.
Coupled with the fact that they are in the midst of writing
a scanner for their compiler, we were able to discuss some
issues they need to keep in mind.
For me, this wasn't another example of
Why Didn't I Know This Already?, but in the process
of looking up "official" answers about Java I did learn
something new -- and, like that lesson, it involved
implicit type conversions of integral types. On Page 27,
the Java Language Reference, says:
The unary plus operator (+) ... does no explicit
computation .... However, the unary + operator
may perform a type conversion on its operand. ... If
the type of the operand is byte, short,
or char, the unary + operator produces
an int value; otherwise the operator produces a
value of the same type as its operand.
The unary - operator works similarly, with the
type conversion made before the value is negated. So,
again, an operation promotes to int in order
to do arithmetic. I assume that this done for the same
reason that the binary operators promote bytes,
shorts, and chars.
Like many CS professors and students, I enjoy this sort
of language trivia. I don't imagine that all our students
do. If you'd like to see more Java trivia, check out
Random Java Trivia
at the Fishbowl. (You gotta love the fact that you can
change the value of a string constant!)
I've also enjoyed thumbing through Joshua Bloch's and
Neal Grafter's
Java Puzzlers.
I am glad that someone knows all these details, but I'm
also glad not to have encountered most of them in my
own programming experience.
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