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You will only be able to do this for ordered collections like lists, and not sets which are unordered.
List member function:
add(int index, E element)
Inserts the specified element at the specified position in this list (optional operation).
Then use:
import java.util.Random;
Random rand = new Random();
int n = rand.nextInt(x);
To get a random index for the list's add() function, ensuring you insert with an index that is in range (i.e. between 0 and length of list).