Grabbing random object from ArrayList is not random

Question

I am creating a method where if you pass in a parameter of type Random, then it will return a random object. Here is basically what I am trying to do:

public T choose(Random r) {
    int randomInt = r.nextInt(randomList.size()); // randomList is just a instance variable
    return randomList.get(randomInt);   
}

The random list has this the following strings: [2, 2, 2, 1, 1, 1, 1, c, c, c, a, a, a, a]

Then I made a driver with the following code

 for (int i = 0; i < 10; i++) {
        System.out.print(rndList.choose(rnd)); // rnd is initialized as a static Random variable
    }

However my outputs are not coming out random. I used the debugger and found out that my choose method generates an integer that is relatively low, so it will always print out either 2's or 1's but never c's or a's. I can't figure out why this is happening and help would be greatly appreciated.

EDIT: Problem was solved. I left out alot of detail, but when I called the size() method, that was something I overwrote which had an error which would return a smaller number than I would have liked. Thanks to dtech for noticing my silly mistake. Thank you for everyone who tried to help me!

Answer 1

At first glance nothing seems wrong with the code, so it might just be a random result. But your "Print it and check" method is very unreliable. Just use something like this:

final int N = 10000; // test 10.000 times
HashTable<Object, Integer> count = new HashTable(N);
for(int i=0;i < N;i++){
    Object o = rndList.choose(rnd);
    count.put(o, (count.get(o)==null?0:count.get(o))+1);
}
for(Map.Entry<Object, Integer> map : count.entrySet()){
    System.out.println(String.format("%s: %d", map.getKey().toString(), map.getValue()));
}

This will print on average something like: 2: 1429 1: 2857 c: 2143 a: 2857

Only if the numbers differ creatly you should be concerned.

Also make sure that you use the new Random() constructor, not new Random(somenumber). If you use the latter you will get the same number sequence every time.

Answer 2

send you random initialization code, are you getting exactly the same results each time? are you using a seed to create the Random object?

Answer 3

Here is the code I used. You need to provide more code to see why yours doesn't work.

public class Main<T> {
    private List<T> randomList = new ArrayList<T>();

    public  T choose(Random r) {
        int randomInt = r.nextInt(randomList.size()); // randomList is just a instance variable
        return randomList.get(randomInt);
    }


    public static void main(String... args) throws IOException, InterruptedException, ExecutionException {
        Main<String> rndList = new Main<String>();
        rndList.randomList.addAll(Arrays.asList("2, 2, 2, 1, 1, 1, 1, c, c, c, a, a, a, a".split(", ")));

        Random rnd = new Random();
        for (int i = 0; i < 10; i++) {
               System.out.print(rndList.choose(rnd)); // rnd is initialized as a static Random variable
           }

    }
}

prints

1ca1caa1a2