Expressive Puzzlers - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 21

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Puzzle 8: Dos Equis
This puzzle tests your knowledge of the conditional operator , better known as the "question mark
colon operator." What does the following program print?
public class DosEquis {
public static void main(String[] args) {
char x = 'X';
int i = 0;
System.out.print(true ? x : 0);
System.out.print(false ? i : x);
}
}
Solution 8: Dos Equis
The program consists of two variable declarations and two print statements. The first print
statement evaluates the conditional expression (true ? x : 0) and prints the result. The result is
the value of the char variable x , which is 'X' . The second print statement evaluates the
conditional expression (false ? i : x) and prints the result. Again the result is the value of x ,
which is still 'X' , so the program ought to print XX . If you ran the program, however, you found that
it prints X88 . This behavior seems strange. The first print statement prints X and the second prints
88 . What accounts for their different behavior?
The answer lies in a dark corner of the specification for the conditional operator [JLS 15.25]. Note
that the types of the second and third operands are different from each other in both of the
conditional expressions: x is of type char , whereas 0 and i are both of type int . As mentioned in
the solution to Puzzle 5 , mixed-type computation can be confusing. Nowhere is this more
apparent than in conditional expressions . You might think that the result types of the two
conditional expressions in this program would be identical, as their operand types are identical,
though reversed, but it isn't so.
The rules for determining the result type of a conditional expression are too long and complex to
reproduce in their entirety, but here are three key points.