Getting Fooled By Randomness

I ended up spending last one hour in debugging a program which runs perfectly while debugging but does not work as expected; when I run it normally. Bizarre...

Well, problem was the way in which I was generating random numbers. Look at the following snippet.


Random randomGen = new Random(System.currentTimeMillis());

Looks very simple and straightforward. But this line was part of a function. Which was structured as

fun()
{
    for 1 to 1000
        doRandom()
}
...

doRandom()
{
    Random randomGen = new Random(System.currentTimeMillis());
    number = randomGen.nextInt();
    ...
}

Has anyone reading it, figured out the problem with above code and why it will work fine while debugging?

If you have. Kudos.

The problem here is seed I am using i.e. System.currentTimeMillis().

1. This seed will remain same for next 1/1000th of a second
2. That's lots of time for a computer
3. While debugging, every break point I place will result in delay and hence will lead to generation of different seed

Obvious solution is to make randomGen a class level variable. Modified code is like below

private Random _randomGen = new Random(System.currentTimeMillis());

...

doRandom()
{
    number = _randomGen.nextInt();
    ...
}