Task: Abundant, deficient and perfect number classifications
C#: using System;
using System.Linq;

public class Program
{
    public static void Main()
    {
        int abundant, deficient, perfect;
        var sw = System.Diagnostics.Stopwatch.StartNew();
        ClassifyNumbers.UsingSieve(20000, out abundant, out deficient, out perfect); sw.Stop();
        Console.WriteLine($"Abundant: {abundant}, Deficient: {deficient}, Perfect: {perfect}  {sw.Elapsed.TotalMilliseconds} ms");
        sw.Restart();
        ClassifyNumbers.UsingOptiDivision(20000, out abundant, out deficient, out perfect);
        Console.WriteLine($"Abundant: {abundant}, Deficient: {deficient}, Perfect: {perfect}  {sw.Elapsed.TotalMilliseconds} ms");
        sw.Restart();
        ClassifyNumbers.UsingDivision(20000, out abundant, out deficient, out perfect);
        Console.WriteLine($"Abundant: {abundant}, Deficient: {deficient}, Perfect: {perfect}  {sw.Elapsed.TotalMilliseconds} ms");
    }
}

public static class ClassifyNumbers
{
    
    public static void UsingSieve(int bound, out int abundant, out int deficient, out int perfect) {
        abundant = perfect = 0;
        
        int[] sum = new int[bound + 1];
        for (int divisor = 1; divisor <= bound >> 1; divisor++)
            for (int i = divisor << 1; i <= bound; i += divisor)
                sum[i] += divisor;
        for (int i = 1; i <= bound; i++) {
            if (sum[i] > i) abundant++;
            else if (sum[i] == i) perfect++;
        }
        deficient = bound - abundant - perfect;
    }

    
    public static void UsingOptiDivision(int bound, out int abundant, out int deficient, out int perfect) {
        abundant = perfect = 0; int sum = 0;
        for (int i = 2, d, r = 1; i <= bound; i++) {
            if ((d = r * r - i) < 0) r++;
            for (int x = 2; x < r; x++) if (i % x == 0) sum += x + i / x;
            if (d == 0) sum += r;
            switch (sum.CompareTo(i)) { case 0: perfect++; break; case 1: abundant++; break; }
            sum = 1;
        }
        deficient = bound - abundant - perfect;
    }

    
    public static void UsingDivision(int bound, out int abundant, out int deficient, out int perfect) {
        abundant = perfect = 0;
        for (int i = 2; i <= bound; i++) {
            int sum = Enumerable.Range(1, (i + 1) / 2)
                .Where(div => i % div == 0).Sum();
            switch (sum.CompareTo(i)) {
                case 0: perfect++; break;
                case 1: abundant++; break;
            }
        }
        deficient = bound - abundant - perfect;
    }
}

Java: import java.util.stream.LongStream;

public class NumberClassifications {
 
    public static void main(String[] args) {
        int deficient = 0;
        int perfect = 0;
        int abundant = 0;
 
        for (long i = 1; i <= 20_000; i++) {
            long sum = properDivsSum(i);
            if (sum < i)
                deficient++;
            else if (sum == i)
                perfect++;
            else
                abundant++;
        }
        System.out.println("Deficient: " + deficient);
        System.out.println("Perfect: " + perfect);
        System.out.println("Abundant: " + abundant);
    }
 
    public static long properDivsSum(long n) {
        return LongStream.rangeClosed(1, (n + 1) / 2).filter(i -> n != i && n % i == 0).sum();
    }
}