Get the Recurrence of Days Randomally

I faced a problem where I had to generate random picks in a week so that it  can include permutations of all days in a week, in every possible manner.

Computationally it creates following possibilities, plus one more fora none condition

P = 7C1 + 7C2 + 7C3 + 7c4 + 7C5 + 7C6 + 7C7
   = 7 + 21 + 35 + 35 + 21 + 7 + 1
   = 127

So 128 conditions including 1 none of all condition.

Where nCk is computed as   n!/((n-k)!*k!) or (n (n-1) (n-2) .... (n-k+1))/ (k (k-1) (k-2)........1)

So going by traditional programming it will take 128 if else conditions or switch cases to generate a random sample.

This will grow exponentially if the value of 'n' increases in case the requirement changes to some thing.

The following method returns a string of a possible random sample out of these 128 probabilities. Here days are marked as follows.

Sunday as 0
Monday as 1
Tuesday as 2
Wednesday as 3
Thursday as 4
Friday as 5
Saturday as 6

The output would come as a random value 0 or 0,7 or 0,1,7 and many more.

String getRecurrence() {

List<String> dayList=new ArrayList<String>();

dayList.add(0, "0");

dayList.add(1, "1");

dayList.add(2, "2");

dayList.add(3, "3");

dayList.add(4, "4");

dayList.add(5, "5");

dayList.add(6, "6");

int numofdays=(int) Math.floor(Math.random() * 8);

int indexCnt=7;

if(numofdays % 8==0)

return null;

String recurrence=null;

while(numofdays!=0){

int index=(int) Math.floor(Math.random() * (indexCnt));

recurrence=(recurrence==null?"":recurrence+",") +dayList.get(index);

dayList.remove(index);

numofdays--;

indexCnt--;

}

if(recurrence.length()>1){

String s[]=recurrence.split(",");

Arrays.sort(s);

recurrence=null;

for(String str:s){

recurrence=(recurrence==null?"":recurrence+",") +str;

}

}

return recurrence;

}