How can I "Round-Robin" 2 ArrayLists?

Question

I have an assignment for school where I need to make a programme that automatically makes couples for the lessons at school. In this case there are 6 lessons and each lesson you partner with a different person. So in week 1 person a partnered with person b and the following weeks he can't partner with person a.

I have written some code that splits the class in 2, but I have no idea on how I could change the couples every week.

Here is the code I already have (sorry it isn't in english):

public void maakKoppels() {
    if (leerlingenLijst.size() % 2 == 1) {
        // if you have an odd number of students it adds the "Bye" student
        leerlingenLijst.add(new Leerling("Bye"));
    }

    for (int i = 1; i <= 6; i++) {
        //its needed for 6 lessons so it does it 6 times
        maakKoppels(i, leerlingenLijst);
    }
}

public void maakKoppels(int weekNum, ArrayList<Leerling> leerlingenLijst) {
    int midden = leerlingenLijst.size() / 2; //split the arraylist in 2

    ArrayList lijst1 = new ArrayList();
    for (int j = 0; j < midden; j++) {
        lijst1.add(leerlingenLijst.get(j));
    }

    ArrayList lijst2 = new ArrayList();
    for (int j = leerlingenLijst.size() - 1; j >= midden; j--) {
        lijst2.add(leerlingenLijst.get(j));
    }
    // here it fills the lessons with the 2 lists. weekNum is the lesson
    // number and the name on lijst1 at index 0 couples with the name on
    // lijst2 at index zero
    practica.add(new Practicum(weekNum, lijst1, lijst2));
}

Answer 1

One way to solve is as below...

1. Split the whole class into two arraylists. L1 & L2
2. create a variable offset = 0
3. First week assign L1[0] & L2[0+offset] as one couple, L1[1] & L2[1+offset] as another couple and so on.
4. Second week set offset = 1
5. assign L1[0] & L2[0+offset] as one couple but this time, you will actually be assigning L1[0] and L2[1] as a couple because of increased offset value. 6.To ensure that you don't go beyond length of L2, use (0+offset)%L2.length()

This will ensure you have different couple everyweek.

Answer 2

Simplest way... start from 1st element in 1st arrayList and shuffle the second arrayList and select the first element.. 2nd in arylist1-->2nd in arraylist2.. and so on.. Now add this data as key value pair in a map.. next time you create couples, shuffle the second arraylist again.. now for each couple made, first check if the person from the second arraylist exists for the same key in the map . If no, add the value to the List (value of map ) else again shuffle the list and try.. You will need a key, List in the map...

Map<String,List> hm = new HashMap<String, List>();
List<String> l = new ArrayList<String>();
Collections.shuffle(arrayList2);

Now its your turn to implement the logic.. And sorry, I can't give you code...

Answer 3

Splitting the class in two is a good start. Now you can rotate list1 whilst keeping list2 as is and pair {list1.get(1),list2.get(1)}, {list1.get(2),list2.get(2)}, etc. Rotate list1 by one (week) and do it all over again.

If you have more then 12 students that will work for your six weeks. With this approach students that are in the same list can never be a pair.