Loopy Puzzlers - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 67

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Puzzle 25: Inclement Increment
This program increments a variable repeatedly and then prints its value. What is it?
public class Increment {
public static void main(String[] args) {
int j = 0;
for (int i = 0; i < 100; i++)
j = j++;
System.out.println(j);
}
}
Solution 25: Inclement Increment
At first glance, the program might appear to print 100 . After all, it does increment j 100 times.
Perhaps surprisingly, it does not print 100 but 0 . All that incrementing gets us nowhere. Why?
As the puzzle's title suggests, the problem lies in the statement that does the increment:
j = j++;
Presumably, the author of the statement meant for it to add 1 to the value of j , which is what the
expression j++ does. Unfortunately, the author inadvertently assigned the value of this expression
back to j . When placed after a variable, the ++ operator functions as the postfix increment operator
[JLS 15.14.2]: The value of the expression j++ is the original value of j before it was incremented.
Therefore, the preceding assignment first saves the value of j , then sets j to its value plus 1, and,
finally, resets j back to its original value. In other words, the assignment is equivalent to this
sequence of statements: