Task: Apply a digital filter (direct form II transposed)
C#: using System;

namespace ApplyDigitalFilter {
    class Program {
        private static double[] Filter(double[] a, double[] b, double[] signal) {
            double[] result = new double[signal.Length];
            for (int i = 0; i < signal.Length; ++i) {
                double tmp = 0.0;
                for (int j = 0; j < b.Length; ++j) {
                    if (i - j < 0) continue;
                    tmp += b[j] * signal[i - j];
                }
                for (int j = 1; j < a.Length; ++j) {
                    if (i - j < 0) continue;
                    tmp -= a[j] * result[i - j];
                }
                tmp /= a[0];
                result[i] = tmp;
            }
            return result;
        }

        static void Main(string[] args) {
            double[] a = new double[] { 1.00000000, -2.77555756e-16, 3.33333333e-01, -1.85037171e-17 };
            double[] b = new double[] { 0.16666667, 0.5, 0.5, 0.16666667 };

            double[] signal = new double[] {
                -0.917843918645, 0.141984778794, 1.20536903482, 0.190286794412,
                -0.662370894973, -1.00700480494, -0.404707073677, 0.800482325044,
                0.743500089861, 1.01090520172, 0.741527555207, 0.277841675195,
                0.400833448236, -0.2085993586, -0.172842103641, -0.134316096293,
                0.0259303398477, 0.490105989562, 0.549391221511, 0.9047198589
            };

            double[] result = Filter(a, b, signal);
            for (int i = 0; i < result.Length; ++i) {
                Console.Write("{0,11:F8}", result[i]);
                Console.Write((i + 1) % 5 != 0 ? ", " : "\n");
            }
        }
    }
}

Java: public class DigitalFilter {
    private static double[] filter(double[] a, double[] b, double[] signal) {
        double[] result = new double[signal.length];
        for (int i = 0; i < signal.length; ++i) {
            double tmp = 0.0;
            for (int j = 0; j < b.length; ++j) {
                if (i - j < 0) continue;
                tmp += b[j] * signal[i - j];
            }
            for (int j = 1; j < a.length; ++j) {
                if (i - j < 0) continue;
                tmp -= a[j] * result[i - j];
            }
            tmp /= a[0];
            result[i] = tmp;
        }
        return result;
    }

    public static void main(String[] args) {
        double[] a = new double[]{1.00000000, -2.77555756e-16, 3.33333333e-01, -1.85037171e-17};
        double[] b = new double[]{0.16666667, 0.5, 0.5, 0.16666667};

        double[] signal = new double[]{
            -0.917843918645, 0.141984778794, 1.20536903482, 0.190286794412,
            -0.662370894973, -1.00700480494, -0.404707073677, 0.800482325044,
            0.743500089861, 1.01090520172, 0.741527555207, 0.277841675195,
            0.400833448236, -0.2085993586, -0.172842103641, -0.134316096293,
            0.0259303398477, 0.490105989562, 0.549391221511, 0.9047198589
        };

        double[] result = filter(a, b, signal);
        for (int i = 0; i < result.length; ++i) {
            System.out.printf("% .8f", result[i]);
            System.out.print((i + 1) % 5 != 0 ? ", " : "\n");
        }
    }
}