Puzzle 86: Poison-Paren Litter

Can you come up with a legal Java expression that can be made illegal by parenthesizing a subexpression, where the added parentheses serve only to document the order of evaluation that would take place in their absence?

Solution 86: Poison-Paren Litter

It seems that inserting a pair of parentheses serving only to document the existing order of evaluation should have no effect on the legality of a program. Indeed, this is true in nearly all cases. In two cases, however, inserting a seemingly innocuous pair of parentheses can make a legal Java program illegal. This strange state of affairs stems from the asymmetry of the two's-complement binary numbers, discussed in Puzzle 33 and Puzzle 64.

You may recall that the most negative int value has a magnitude that is one greater than the most positive: Integer.MIN_VALUE is -2³¹, or -2,147,483,648, whereas Integer.MAX_VALUE is 2³¹ - 1, or 2,147,483,647. Java does not support negative decimal literals; negative int and long constants are constructed by prefixing positive decimal literals with the unary minus operator (-). A special language rule governs this construction: The largest decimal literal of type int is 2147483648. Decimal literals from 0 to 2147483647 may appear anywhere an int literal may appear, but the literal 2147483648 may appear only as the operand of the unary negation operator [JLS 3.10.1].

Once you know this rule, the puzzle is easy. The characters -2147483648 form a legal Java expression consisting of the unary minus operator followed by the int literal 2147483648. Adding a pair of parentheses to document the (trivial) order of evaluation gives -(2147483648), which violates the rule. Believe it or not, this program really does generate a compile-time error, and the error goes away if you remove the parentheses:

public class PoisonParen {

    int i = -(2147483648);

}

The situation for long literals is analogous. This program too generates a compile-time error that goes away if you remove the parentheses:

public class PoisonParen {

    long j = -(9223372036854775808L);

}

As for a lesson, this puzzle has none. It's a corner case, pure and simple. But you must admit, it's amusing.