Puzzle (Tweedledum)

Provide declarations for the variables x and i such that this is a legal statement:

x += i;

but this is not:

x = x + i;

This example was taken from PUZZLE 9: TWEEDLEDUM in Java™ Puzzlers: Traps, Pitfalls, and Corner Cases.

“Many programmers think that the first statement in this puzzle (x += i) is simply a shorthand for the second (x = x + i). This isn’t quite true. Both of these statements are assignment expressions (JLS 15.26). The second statement uses the simple assignment operator (=), whereas the first uses a compound assignment operator. The compound assignment operators are +=, -=, *=, /=, %=, <<=, >>=, >>>=, &=, ^=, and |=. The Java language specification says that the compound assignment E1 op= E2 is equivalent to the simple assignment E1 = (T) ((E1) op (E2)), where T is the type of E1, except that E1 is evaluated only once (JLS 15.26.2).”

“In other words, compound assignment expressions automatically cast the result of the computation they perform to the type of the variable on their left-hand side. If the type of the result is identical to the type of the variable, the cast has no effect. If, however, the type of the result is wider than that of the variable, the compound assignment operator performs a silent narrowing primitive conversion (JLS 5.1.3). Attempting to perform the equivalent simple assignment would generate a compilation error, with good reason.”