App.java

import java.util.function.Predicate;

public class Calculations {

    public static void main(String[] args) {
        char[] chars = { 'a', 'b', 'c', 'd' };
        // looking for bba
        calculate(chars, 3, i -> i[0] == 1 && i[1] == 1 && i[2] == 0);
    }

    static void calculate(char[] a, int k, Predicate<int[]> decider) {
        int n = a.length;

        if (k < 1 || k > n)
            throw new IllegalArgumentException("Forbidden");

        int[] indexes = new int[n];
        int total = (int) Math.pow(n, k);

        while (total-- > 0) {

            for (int i = 0; i < n - (n - k); i++)
                System.out.print(a[indexes[i]]);
            System.out.println();

            if (decider.test(indexes))
                break;

            for (int i = 0; i < n; i++) {
                if (indexes[i] >= n - 1) {
                    indexes[i] = 0;
                } else {
                    indexes[i]++;
                    break;
                }
            }
        }
    }
}