Java randomly select list with maintain order-CodePudding

I want to make a random list from the list, but the conditions are a bit tricky, so I'm thinking about how to implement it.

list [A1,A2,A3,B1,B2,B3,C1,C2,C3]

example

result1 [A1,B1,A2,C1,A3,B2,C2,C3,B3] result2 [C1,B1,B2,A1,A2,B3,C2,A3,C3]

I want to put it with maintain order into a new list.

Is there an algorithmically good way?

CodePudding user response：

If you conceptually arrange your data as a HashMap<Integer, Queue<T>>, structured as

0: A1,A2,A3
1: B1,B2,B3
2: C1,C2,C3

Then once you've chosen your random number you can just do map.get(number).dequeue() to get your value

CodePudding user response：

You can do that by randomly arranging them using Collections.shuffle() and then correcting them using a map.

static List<String> shuffleTrickey(List<String> list) {
    List<String> copy = new ArrayList<>(list);
    Collections.shuffle(copy);
    Map<String, Deque<String>> map = list.stream()
        .collect(Collectors.groupingBy(s -> s.substring(0, 1),
            Collectors.toCollection(ArrayDeque::new)));
    return copy.stream()
        .map(s -> map.get(s.substring(0, 1)).pop())
        .toList();
}

public static void main(String[] args) {
    List<String> list = Arrays.asList("A1", "A2", "A3", "B1", "B2", "B3", "C1", "C2", "C3");
    List<String> result = shuffleTricky(list);
    System.out.println(result);
}

output:

[C1, A1, B1, C2, B2, B3, A2, C3, A3]

Elements with the same first letter will remain in order after shuffling.