Java，TDD 不等于时为真？

Question

for some reason while testing this method during a lesson in class we found an issue we couldn't understand. When writing System.out.println(); for some reason it passes?. Can someone explain why this is happening?

public class Zones {

public ZoneId getZoneId(String input) {

    if (input.equalsIgnoreCase("Stockholm")) {
        return ZoneId.of("Europe/Stockholm");
    }
    else if (input.equalsIgnoreCase("Shanghai")) {
        return ZoneId.of("Asia/Shanghai");
    } else if (input.equalsIgnoreCase("Toronto")) {
        return ZoneId.of("America/Toronto");
    }
    else if (input.equalsIgnoreCase("Hamburg")) {
        return ZoneId.of("Europe/Berlin");
    }
    else return null;
}

public LocalDateTime getZoneTime(ZoneId zoneId) {
    LocalDateTime lt = LocalDateTime.now(zoneId);
    return lt;
}

}

private Zones z = new Zones();

@Test
public void getZoneTimeTest () {
    System.out.println(z.getZoneTime(zIDToronto).getNano() );
    System.out.println(LocalDateTime.now(zIDToronto).getNano() );
    assertTrue(z.getZoneTime(zIDToronto).getNano() == LocalDateTime.now(zIDToronto).getNano());
}

Answer 1

The test involves race condition and passes (sometimes) because of the timing and adding statements changes the timing and therefore test outcome.

The condition that is checked in the assertion basically is the equality of the nanoseconds parts of two date-times taken one after another in succession.

Given that by default System.currentTimeMillis() is used internally by LocalDateTime.now and it has at most millisecond precision the check will succeed if the second sequence of invocations to get the number of nanoseconds is quick enough (that is the actual sequence of calls leading to invocation of System.currentTimeMillis finished within the same millisecond as the first).

When you invoke the functions used to obtain the value of nanosecond before the actual assertion the corresponding classes are loaded, the code of corresponding methods get into the CPU caches etc. This makes the second pair of calls to get the number of nanoseconds run much quicker.