All possible combinations of an array

Question

I have an string array

{\"ted\", \"williams\", \"golden\", \"voice\", \"radio\"}

and I want all possible combinations of these keywords in

Answer 1

My optimized solution is based on the solution provided by Matthew McPeak. This version avoids unnecessary array copies.

public static  T[][] combinations(T[] a) {

    int len = a.length;
    if (len > 31)
        throw new IllegalArgumentException();

    int numCombinations = (1 << len) - 1;

    @SuppressWarnings("unchecked")
    T[][] combinations = (T[][]) java.lang.reflect.Array.newInstance(a.getClass(), numCombinations);

    // Start i at 1, so that we do not include the empty set in the results
    for (int i = 1; i <= numCombinations; i++) {

        @SuppressWarnings("unchecked")
        T[] combination = (T[]) java.lang.reflect.Array.newInstance(a.getClass().getComponentType(),
                Integer.bitCount(i));

        for (int j = 0, ofs = 0; j < len; j++)
            if ((i & (1 << j)) > 0)
                combination[ofs++] = a[j];

        combinations[i - 1] = combination;
    }

    return combinations;
}