3
abcdde
baccd
eeabg

2

public class GemCounter {

    private static final int LETTERCOUNT = 26;

    // Start with 1 bit set for each element/letter.
    private int gemsSoFar = (1 << LETTERCOUNT) - 1;

    public GemCounter() {
        // default constructor. Does nothing.
    }

    public void processRock(final String rock) {
        int gotElement = 0; // no bits are set.
        for (int i = 0; i < rock.length(); i++) {
            int element = rock.charAt(i) - 'a';
            if (element >= 0 && element < LETTERCOUNT) {
                // bitwise OR the bit that represents the element
                gotElement |= 1 << element;
            }
        }
        // only bits that were found in this rock
        // and also that have been seen before
        // will remain set.
        gemsSoFar = gemsSoFar & gotElement;
    }

    public int getGemCount() {
        // count the number of bits that are still set.
        return Integer.bitCount(gemsSoFar);
    }

    public static void main(String[] args) {
        String[] rocks = {"abcdde", "baccd", "eeabg"};

        GemCounter gemcount = new GemCounter();
        for (String rock : rocks) {
            gemcount.processRock(rock);
            System.out.printf("Processed %s, got %d gems still%n", rock, gemcount.getGemCount());
        }

        System.out.printf("%d rocks have %d gems%n", rocks.length, gemcount.getGemCount());

    }

}