Many calls to static method in single thread

Question

please look at this class, static method calls and output.

public class OneThreadManyStaticCalls {

public static final Calculator calculator = new Calculator();
    public static void main(String[] args) {
        dummy(0, 1, 1);
        dummy(0, 2, 2);
        dummy(0, 3, 5);
        dummy(0, 4, 44);
        dummy(0, 5, 5);
    }

    public static void dummy(int a, int b, int expected) {

        System.out.print(System.currentTimeMillis() + "\t");
        if (calculator.add(a, b) == expected) {
            System.out.println("OK");
        } else {
            System.err.println("NOK");
        }
    }
}

I got diffrent (order from System.out.print) ouputs running this program. Example:

   NOK
   NOK
   1342527389506    OK
   1342527389506    OK
   1342527389506    1342527389506   1342527389506   OK

Could any of you explain me (with details) why? Thanks in advance. sznury

Answer 1

System.err and System.out are two different streams which are interleaved in your console window - they're not necessarily synchronized. Try using System.*.flush() (nevermind, this doesn't seem to work) to force the output to be handled, or print all your output to the same stream.

public static void dummy(int a, int b, int expected) {
    System.out.print(System.currentTimeMillis() + "\t");
    if ((a + b) == expected) { // I don't have your Calculator :<
        System.out.println("OK");
    } else {
        System.out.println("NOK");
    }
}

Gives this result

1342528255764   OK
1342528255764   OK
1342528255764   NOK
1342528255764   NOK
1342528255764   OK

Answer 2

A simpler example is

for (int i = 0; i <= 20; i++)
    (i % 2 == 0 ? System.out : System.err).println(i);

There is NO guarantee of order between two streams even if they both go to the console.

prints on one run (changes with each run)

1
0
3
2
5
4
7
6
9
8
11
10
13
12
15
14
17
16
19
18
20

Note: In my IDE the System.err lines appear in red