如何从数组中获取随机对象

Question

I am witing this program which removes 2 random objects at a time from a bag. Inside the bag there are objects of two different colours and depending what colour the objects have different things will happen. The way I am taking random values right now is:

while (bag.size() > 1) {
            count++;
            int select1 = rnd.nextInt(bag.size());
            Balls indexPos1 = bag.get(select1);
            int select2 = rnd.nextInt(bag.size());
            Balls indexPos2 = bag.get(select2);
            System.out.println(select1);
            System.out.println(select2);

But at the end of the program if it happens to take an index of the same object then the colour variation inside the bag is wrong and wrong outcome prints. I'll paste in the rest of the loop.

int count = 0;
        Random rnd = new Random();
// Depending on what colour the balls are either black ball is placed or white.
        while (bag.size() > 1) {
            count++;
            int select1 = rnd.nextInt(bag.size());
            Balls indexPos1 = bag.get(select1);
            int select2 = rnd.nextInt(bag.size());
            Balls indexPos2 = bag.get(select2);
            System.out.println(select1);
            System.out.println(select2);
            if (bag.size() != 1) {
                System.out.println(bag);
                System.out.println("Printing size: " + bag.size());
                if (bag.get(select1).equals(blackBalls) && indexPos2.equals(blackBalls)) {
                    System.out.println("Both balls are black.");
                    System.out.println("Removing black ball");
                    bag.add(blackBalls);
                    bag.remove(blackBalls);
                    bag.remove(blackBalls);
                    continue;
                }if (indexPos1.equals(whiteBalls) && indexPos2.equals(whiteBalls)) {
                    System.out.println("both Balls are white");
                    bag.add(blackBalls);
                    bag.remove(whiteBalls);
                    bag.remove(whiteBalls);
                    System.out.println("Removing 2 white balls. adding black.");
                    continue;
                }if (indexPos1.equals(whiteBalls) && indexPos2.equals(blackBalls) || indexPos1.equals(blackBalls) && indexPos2.equals(whiteBalls)) {
                    bag.add(whiteBalls);
                    bag.remove(select1);
                    bag.remove(select2);
                    System.out.println("Both balls are different.");
                    System.out.println("Removing white ball");
                }
            }
        }

I have no clue how else to take two random objects from an array, so it doesn't choose the same object twice. Any help or feedback is appreciated.

Answer 1

You have a few options.

1. Shuffle the entire collection, then take the first 2 elements.

   This would of course 'destroy' any order it already has, and it also a bit of a heavy cost to pay, given that you only need 2 random elements, but Collections.shuffle isn't going to be a performance issue unless you have tens of thousands of elements in there.

2. Keep picking random numbers.

   Turn your second 'get a random index' call into a do/while loop, which keeps picking a number until you get a number that isn't equal to the first element. Note that this will loop forever if you somehow end up with a bag with only 1 element in it, so ensure you check for that first and act appropriately.

3. Pick a random number 1 less, then adjust.

   Instead of .nextInt(bag.size()) , do .nextInt(bag.size() - 1) , and if the index so obtained is equal to, or larger than your first index, increment by 1. This then effectively translates into picking a uniform-distributed random number from the collection [0, 1, 2, 3, 5, 6, 7], assuming a bag of 8 elements, and the first index chosen was 4.