Código de Java - Proporción de números

public static void printStatistics(int... a) {

    for (NumberValue nv : NumberValue.values()) {

        System.out.println(nv.name() + " = " + sum(nv.getPredicate(), a));

    }

}

private static enum NumberValue {

    GREATER_THAN_0, LESS_THAN_0, EQUAL_TO_0;

    private IntPredicate getPredicate() {

        return this == GREATER_THAN_0 ? i -> i > 0 : this == LESS_THAN_0 ? i -> i < 0 : i -> i == 0;

    }

}

public static double sum(IntPredicate predicate, int... a) {

    return Arrays.stream(a).filter(predicate).mapToDouble(i -> 1.0 / a.length).sum();

}

public static void main(String[] args) {

    printStatistics(-5, 5, 4, 3, 2, 1, 0, -1);

}