返回我的字符串列表的随机元素，在 Android 项目中不重复

Question

I have an ArrayList of 5 Strings in my Android Eclipse project as folowing:

//ArrayList<String> Declaration
    ArrayList<String> al= new ArrayList<String>();
    //add method for String ArrayList
    al.add("1st_string");
    al.add("2nd_string");
    al.add("3rd_string");
    al.add("4th_string");
    al.add("5th_string");

And I want to randomly select strings without duplicates according to the following plan :

I will have a variable named string_selection and a Textview to save me results named displayed_string

1) for string_selection == 0 ---> display the 1st element in the TextView displayed_string (and next time choose from the remaining 4 strings )

2) for string_selection == 1 ---> display the 2nd element in the TextView displayed_string (and next time choose from the remaining 3 strings )

and finally

3)for string_selection == 2 ---> display the 3rd element in the TextView displayed_string( and that's it.Just set string_selection == 0 )

How can I do it please?

Answer 1

Remove items when randomly selected (store your strings in a separated array if you need to keep the original one):

items = 1, 2, 3, 4, 5

Random select: 2

Use items[2] and remove. items = 1, 2, 4, 5

Random select: 1

Use items[1] and remove. items = 1, 4, 5

So on...

Answer 2

private List<SameType> randomList (List<Type> targetList){

    List<TypeAsTarget> newUniqueList = new ArrayList<>(targetList.size());

    newUniqueList.add(0, targetList.get(new Random().nextInt(targetList.size())));
    newUniqueList.add(1, targetList.get(new Random().nextInt(targetList.size())));
    newUniqueList.add(2, targetList.get(new Random().nextInt(targetList.size())));
    newUniqueList.add(3, targetList.get(new Random().nextInt(targetList.size())));
    newUniqueList.add(4, targetList.get(new Random().nextInt(targetList.size())));

    while (newUniqueList.get(1) == newUniqueList.get(0))
        newUniqueList.set(1, targetList.get(new Random().nextInt(targetList.size())));

    while (newUniqueList.get(2) == newUniqueList.get(0) || newUniqueList.get(2) == newUniqueList.get(1))
        newUniqueList.set(2, targetList.get(new Random().nextInt(targetList.size())));

    while (newUniqueList.get(3) == newUniqueList.get(0) || newUniqueList.get(3) == newUniqueList.get(1) || newUniqueList.get(3) == newUniqueList.get(2))
        newUniqueList.set(3, targetList.get(new Random().nextInt(targetList.size())));

    while (newUniqueList.get(4) == newUniqueList.get(0) || newUniqueList.get(4) == newUniqueList.get(1) || newUniqueList.get(4) == newUniqueList.get(2) || newUniqueList.get(4) == newUniqueList.get(3))
        newUniqueList.set(4, targetList.get(new Random().nextInt(targetList.size())));

    return newUniqueList;
}

Answer 3

You can either randomly remove strings from your array if you don't need them or use some set to keep track of already used random indices.

1) Remove

String getRandom(List<String> strings) {
    return strings.remove(new Random().nextInt(strings.size()));
}

2) Track used indices:

String getRandom(List<String> strings, Set<Integer> used) {
    int next;
    do {
        next = new Random().nextInt(strings.size());
    } while (used.contains(next));
    used.add(next);
    return strings.get(next);
}