Loopy Puzzlers - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 84

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Puzzle 32: Curse of Looper
Provide declarations for i and j that turn this loop into an infinite loop:
while (i <= j && j <= i && i != j) {
}
Solution 32: Curse of Looper
Oh, no, not another seemingly impossible looper! If i <= j and j <= i , surely i must equal j ? This
property certainly holds for the real numbers. In fact, it is so important that it has a name: The
relation on the real numbers is said to be antisymmetric . Java's <= operator used to be antisymmetric
before release 5.0, but no longer.
Until release 5.0, Java's numerical comparison operators ( <, <= , > , and >= ) required both of their
operands to be of a primitive numeric type ( byte , char , short , int , long , float , or double ) [JLS2
15.20.1]. In release 5.0, the specification was changed to say that the type of each operand must be
convertible to a primitive numeric type [JLS 15.20.1, 5.1.8]. Therein lies the rub.
In release 5.0, autoboxing and auto-unboxing were added to the language. If you are unfamiliar with
them see: http://java.sun.com/j2se/5.0/docs/guide/language/autoboxing.html [Boxing] . The <=
operator is still antisymmetric on the set of primitive numeric values, but now it applies to operands
of boxed numeric types as well. (The boxed numeric types are Byte , Character , Short , Integer ,
Long , Float , and Double .) The <= operator is not antisymmetric on operands of these types, because
Java's equality operators ( == and != ) perform reference identity comparison rather than value
comparison when applied to object references.
To make this concrete, the following declarations give the expression ( i <= j && j <= i && i !=
j ) the value true , turning the loop into an infinite loop:
Integer i = new Integer(0);
Integer j = new Integer(0);
The first two subexpressions ( i <= j and j <= i ) perform unboxing conversions [JLS 5.1.8] on i
and j and compare the resulting int values numerically. Both i and j represent 0, so both of these