Show all the possible combinations of n elements in an array???

Question

If I have an array of n elements, how to show all the possible combinations of n elements.
What is the java library for doing the combinations??
For example,
public class CombinationTest
{     public static void main(String[] args)     
     {     ArrayList letters = new ArrayList();
          leters.add("s1");
          leters.add("s2");
          leters.add("s3");
Find all the combinations of 3 elements: C(3,2). Here's the result:
1) s1, s2
2) s1, s3
3) s2, s3
}

Answer 1

There isn't a built-in API method for this. Still, combinations are pretty easy. Pseudocode (borrowing notation from ML and GJ, and assuming immutable sets): Set<Set<Object>> getCombinations(int itemsInCombo, Set<Object> itemsToCombine)
    // Handle base case and exceptional cases
    if (itemsInCombo = 0) return Set.emptySet();
    if (itemsInCombo < 0) throw new Exception("?!");
    // Recursive case
    Object o = itemsToCombine.first(); // use iterator - may throw exception
    // Get combos with first element
    Set<Set<Object>> rv = map (x => x.add(o)) getCombinations(itemsInCombo - 1, itemsToCombine.remove(o));
    // Add combos without first element
    rv.add(getCombinations(itemsInCombo, itemsToCombine.remove(o)));
    return rv;
}