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final static char ALPH_LOW = 'a';   // erster Buchstabe des Alphabets
final static char ALPH_HIGH = 'z';  // letzter Buchstabe des Alphabets

public static double coincidenceIndex(Vector text) {
    int[] alphabet = new int[ALPH_HIGH - ALPH_LOW + 1];
    // bestimme Frequenzverteilung
    for (int i = 0; i < text.size(); i++) {
        char c = ((Character)text.get(i)).charValue();
        alphabet[c - ALPH_LOW]++;
    }
    // bestimme Konizidenzindex
    long sum = 0;
    for (int i = 0; i < alphabet.length; i++)
        sum += alphabet[i] * (alphabet[i] - 1);
    return (float)sum / text.size() / (text.size() - 1);
}