Task: 21 game
C#: 

using System;

namespace _21Game
{
    public class Program
    {
        private const string computerPlayer = "Computer";
        private const string humanPlayer = "Player 1";

        public static string SwapPlayer(string currentPlayer)
        {
            if (currentPlayer == computerPlayer)
            {
                currentPlayer = humanPlayer;
            }
            else
            {
                currentPlayer = computerPlayer;
            }

            return currentPlayer;
        }

        public static void PlayGame()
        {
            bool playAnother = true;
            int total = 0;
            int final = 21;
            int roundChoice = 0;
            string currentPlayer = RandomPLayerSelect();
            int compWins = 0;
            int humanWins = 0;

            while (playAnother)
            {
                Console.WriteLine($"Now playing: {currentPlayer}");
                try
                {
                    if (currentPlayer == computerPlayer)
                    {
                       roundChoice =  CompMove(total);
                    }
                    else
                    {
                        roundChoice = int.Parse(Console.ReadLine());
                    }
                    

                    if (roundChoice != 1 && roundChoice != 2 && roundChoice != 3)
                    {
                        throw new Exception();
                    }

                    total += roundChoice;
                }
                catch (Exception)
                {
                    Console.WriteLine("Invalid choice! Choose from numbers: 1, 2, 3.");
                    continue;
                }

                Console.WriteLine(total);

                if (total == final)
                {
                    if (currentPlayer == computerPlayer)
                    {
                        compWins++;
                    }
                    if (currentPlayer == humanPlayer)
                    {
                        humanWins++;
                    }
                    Console.WriteLine($"Winner: {currentPlayer}");
                    Console.WriteLine($"Comp wins: {compWins}. Human wins: {humanWins}");
                    Console.WriteLine($"do you wan to play another round? y/n");
                    var choice = Console.ReadLine();
                    if (choice == "y")
                    {
                        total = 0;
                    }
                    else if (choice == "n")
                    {
                        break;
                    }
                    else
                    {
                        Console.WriteLine("Invalid choice! Choose from y or n");
                        continue;
                    }
                }

                else if (total > 21)
                {
                    Console.WriteLine("Not the right time to play this game :)");
                    break;
                }

                currentPlayer = SwapPlayer(currentPlayer);
            }
        }

        public static bool CheckIfCanWin(int total)
        {
            bool result = false;
            if (total == 18)
            {
                result = true;
            }
            return result;
        }

        public static int CompMove(int total)
        {
            int choice = 0;

            if (CheckIfCanWin(total))
            {
                choice = 21 - total;
            }
            else
            {
                choice = new Random().Next(1,4);
            }

            return choice;
        }

        public static string RandomPLayerSelect()
        {
            string[] players = new string[] { computerPlayer, humanPlayer };
            var random = new Random().Next(0,2);
            return players[random];
        }

        public static void Main(string[] args)
        {
            
            Console.WriteLine("Welcome to 21 game \n");
            Console.WriteLine(@"21 is a two player game. 
The game is played by choosing a number.
1, 2, or 3 to be added a total sum. \n
The game is won by the player reaches exactly 21. \n" );            
            Console.WriteLine("Choose your number: (1, 2 or 3)");

            PlayGame();
        }
    }
}

Java: import java.util.Random;
import java.util.Scanner;

public class TwentyOneGame {

    public static void main(String[] args) {
        new TwentyOneGame().run(true, 21, new int[] {1, 2, 3});
    }
    
    public void run(boolean computerPlay, int max, int[] valid) {
        String comma = "";
        for ( int i = 0 ; i < valid.length ; i++ ) {
            comma += valid[i];
            if ( i < valid.length - 2 && valid.length >= 3 ) {
                comma += ", ";
            }
            if ( i == valid.length - 2 ) {
                comma += " or ";
            }
        }
        System.out.printf("The %d game.%nEach player chooses to add %s to a running total.%n" + 
                "The player whose turn it is when the total reaches %d will win the game.%n" + 
                "Winner of the game starts the next game.  Enter q to quit.%n%n", max, comma, max);
        int cGames = 0;
        int hGames = 0;
        boolean anotherGame = true;
        try (Scanner scanner = new Scanner(System.in);) {
            while ( anotherGame ) {
                Random r = new Random();
                int round = 0;
                int total = 0;
                System.out.printf("Start game %d%n", hGames + cGames + 1);
                DONE:
                    while ( true ) {
                        round++;
                        System.out.printf("ROUND %d:%n%n", round);
                        for ( int play = 0 ; play < 2 ; play++ ) {
                            if ( computerPlay ) {
                                int guess = 0;
                                
                                for ( int test : valid ) {
                                    if ( total + test == max ) {
                                        guess = test;
                                        break;
                                    }
                                }
                                
                                if ( guess == 0 ) {
                                    for ( int test : valid ) {
                                        if ( total + test >= max ) {
                                            guess = test;
                                            break;
                                        }
                                    }
                                }
                                if ( guess == 0 ) {
                                    guess = valid[r.nextInt(valid.length)];
                                }
                                total += guess;
                                System.out.printf("The computer chooses %d%n", guess);
                                System.out.printf("Running total is now %d%n%n", total);
                                if ( total >= max ) {
                                    break DONE;
                                }
                            }
                            else {
                                while ( true ) {
                                    System.out.printf("Your choice among %s: ", comma);
                                    String line = scanner.nextLine();
                                    if ( line.matches("^[qQ].*") ) {
                                        System.out.printf("Computer wins %d game%s, human wins %d game%s.  One game incomplete.%nQuitting.%n", cGames, cGames == 1 ? "" : "s", hGames, hGames == 1 ? "" : "s");
                                        return;
                                    }
                                    try {
                                        int input = Integer.parseInt(line);
                                        boolean inputOk = false;
                                        for ( int test : valid ) {
                                            if ( input == test ) {
                                                inputOk = true;
                                                break;
                                            }
                                        }
                                        if ( inputOk ) {
                                            total += input;
                                            System.out.printf("Running total is now %d%n%n", total);
                                            if ( total >= max ) {
                                                break DONE;
                                            }
                                            break;
                                        }
                                        else {
                                            System.out.printf("Invalid input - must be a number among %s.  Try again.%n", comma);
                                        }
                                    }
                                    catch (NumberFormatException e) {
                                        System.out.printf("Invalid input - must be a number among %s.  Try again.%n", comma);
                                    }
                                }
                            }
                            computerPlay = !computerPlay;
                        }
                    }
                String win;
                if ( computerPlay ) {
                    win = "Computer wins!!";
                    cGames++;
                }
                else {
                    win = "You win and probably had help from another computer!!";
                    hGames++;
                }
                System.out.printf("%s%n", win);
                System.out.printf("Computer wins %d game%s, human wins %d game%s%n%n", cGames, cGames == 1 ? "" : "s", hGames, hGames == 1 ? "" : "s");
                while ( true ) {
                    System.out.printf("Another game (y/n)? ");
                    String line = scanner.nextLine();
                    if ( line.matches("^[yY]$") ) {
                        
                        System.out.printf("%n");
                        break;
                    }
                    else if ( line.matches("^[nN]$") ) {
                        anotherGame = false;
                        System.out.printf("Quitting.%n");
                        break;
                    }
                    else {
                        System.out.printf("Invalid input - must be a y or n.  Try again.%n");
                    }
                }
            }
        }
    }

}