Puzzle (Long Division)

Consider the following example.

package demo;

public class App {
  public static void main( String[] args ) {
    long microsPerDay = 24 * 60 * 60 * 1000 * 1000;
    long millisPerDay = 24 * 60 * 60 * 1000;
    System.out.println( microsPerDay / millisPerDay );
  }
}

The program should print 1000, but unfortunately, it prints 5. What exactly is going on here?

5

This example was taken from PUZZLE 3: LONG DIVISION in Java™ Puzzlers: Traps, Pitfalls, and Corner Cases.

“The problem is that the computation of the constant microsPerDay does overflow. Although the result of the computation fits in a long with room to spare, it doesn’t fit in an int. The computation is performed entirely in int arithmetic, and only after the computation completes is the result promoted to a long. By then, it’s too late: The computation has already overflowed, returning a value that is too low by a factor of 200. The promotion from int to long is a widening primitive conversion JLS 5.1.2, which preserves the (incorrect) numerical value. This value is then divided by millisPerDay, which was computed correctly because it does fit in an int. The result of this division is 5.”