Random behavior -- please help!

Question

I posted yesterday and only received one response, so I'm posting again in the hopes that someone will be able to help. This time I'm including some of the code that's giving me problems.
This is what I wrote yesterday:
I'm writing a Java program that schedules employees into a monthly work schedule. I'm trying to write it such that the list of employees (which is read in from file) is in a different, random order for each day of the month. This way when the program accesses the list to start scheduling people into shifts, the list of employees will be in a different order each day.
I thought I had it, but when I execute the program, the employees show up in the exact same order for every day EXCEPT the very first day of the month. Why would the random generator use a unique list for the first day and then the same list for all subsequent days?
In my scheduling method, I iterate through each of the days in the month, and at the beginning of each new day, I call the method that supposedly rearranges the order of the list of employees. Here's that method:
    public ArrayList makeNewEmpList()
        ArrayList temp = new ArrayList();
        while (temp.size() < emps.size())
            int i = rand.nextInt(emps.size());
            Employee doe = (Employee) emps.get(i);
            if (!temp.contains(doe))
                temp.add(doe);
        return temp;
    }"emps" is my list of employees, and "rand" is my random generator. When I execute the scheduling method, the very first day of the month has a unique order, but every day for the rest of the month has the exact same list of employees in the exact same order.
Does anyone have any idea why this is happening? Any suggestions would be greatly appreciated!
Thanks again,
Erica

Answer 1

So do I need to specify that the random generator
should only choose numbers within the range of
emps.size() -- as I did in the original code above --
or will it automatically choose within that range
because I'm also giving it the list to shuffle?Check out the documentation, you'll see that it just requires a Random object. You don't have any control over how one of those works anyway. Or you can just call Collections.shuffle(emps) and it will create its own Random, which is most likely just as good.
Also, you're saying that I can just put this one line
of code in place of the line in which I call the
above method in my main scheduling method? I'm not
sure how to use the Collections class.I think so. Your description of your requirements seemed to be a description of shuffling the list of employees randomly. Again, read the documentation to ensure that it is going to do what you really want done.
What you're
suggesting will alter the order of the "emps" list
itself, right? Because I was trying to create a new
ArrayList for each time through, but I guess it
doesn't matter if I just keep altering the orignal
list. Yes, it will. You're right, you might have to reorganize some other code to account for that.
Thanks so much for helping! I really appreciate it.