MainGame.java

// Sk Elaf Ahmed 
// Tic Tac Toe game in Java

import java.util.*;

public class MainGame {

    static ArrayList<Integer> playerPositions = new ArrayList<Integer>();
    static ArrayList<Integer> cpuPositions = new ArrayList<Integer>();
    public static void main(String[] args) {
        // Here we use 5 rows to create the game board
        char[][] gameBoard = {{' ', '|', ' ', '|', ' '},
                            {'=', '+', '=', '+', '='}, 
                            {' ', '|', ' ', '|', ' '},
                            {'=', '+', '=', '+', '='},
                            {' ', '|', ' ', '|', ' '}};

        printGameBoard(gameBoard);
        
        while(true){
            Scanner sc = new Scanner(System.in);
            System.out.println("Enter your placement(1-9):");
            int playerPos = sc.nextInt();
            while(playerPositions.contains(playerPos) || cpuPositions.contains(playerPos)){
                System.out.println("Position taken already!");
                playerPos = sc.nextInt();
            }

            System.out.println("Your choice is: "+playerPos);
            placePiece(gameBoard, playerPos, "player");
            // Here we will check result after placing my piece
            String result = checkWinner();
            if(result.length()>0){
                System.out.println(result);
                break;
            }
    
            Random rand = new Random();
            int cpuPos = rand.nextInt(9) + 1;
            // This code if we get player pos and cpu pos is same
            while(playerPositions.contains(cpuPos) || cpuPositions.contains(cpuPos)){
                //System.out.println("Position taken already!");
                cpuPos = rand.nextInt(9) + 1;
            }
    
            System.out.println("Computer choice is: "+cpuPos);
    
            placePiece(gameBoard, cpuPos, "cpu");
    
            printGameBoard(gameBoard);
            // Here we will check result after cpu placing piece
            result = checkWinner();
            if(result.length()>0){
                System.out.println(result);
                break;
            }
        }
    }

    public static void printGameBoard(char[][] gameBoard){
        // Printing the gameBoard
        for(char[] row: gameBoard){
            for(char c: row){
                System.out.print(c);
            }
            System.out.println();
        }
    }

    public static void placePiece(char[][] gameBoard,int pos, String user){
    // This method for place character that player want        
        char symbol = ' ';

        if(user.equals("player")){
            symbol = 'X';
            playerPositions.add(pos);
        } else if(user.equals("cpu")){
            symbol = 'O';
            cpuPositions.add(pos);
        }

        switch(pos){
            case 1: 
            gameBoard[0][0] = symbol;
            break;
            case 2: 
            gameBoard[0][2] = symbol;
            break;
            case 3: 
            gameBoard[0][4] = symbol;
            break;
            case 4: 
            gameBoard[2][0] = symbol;
            break;
            case 5: 
            gameBoard[2][2] = symbol;
            break;
            case 6: 
            gameBoard[2][4] = symbol;
            break;
            case 7: 
            gameBoard[4][0] = symbol;
            break;
            case 8: 
            gameBoard[4][2] = symbol;
            break;
            case 9: 
            gameBoard[4][4] = symbol;
            break;
            default:
                break;
        }
    }

    public static String checkWinner(){
        // This method will check wheather the user or the computer wins
        List<Integer> topRow = Arrays.asList(1,2,3);
        List<Integer> midRow = Arrays.asList(4,5,6);
        List<Integer> botRow = Arrays.asList(7,8,9);
        List<Integer> leftCol = Arrays.asList(1,4,7);
        List<Integer> midCol = Arrays.asList(2,5,8);
        List<Integer> rightCol = Arrays.asList(3,6,9);
        List<Integer> cross1 = Arrays.asList(1,5,9);
        List<Integer> cross2 = Arrays.asList(3,5,7);

        List<List> winning = new ArrayList<>();
        winning.add(topRow);
        winning.add(midRow);
        winning.add(botRow);
        winning.add(leftCol);
        winning.add(midCol);
        winning.add(rightCol);
        winning.add(cross1);
        winning.add(cross2);

        for(List l: winning){
            if(playerPositions.containsAll(l)){
                return "Congratulation you won!";
            }
            else if(cpuPositions.containsAll(l)){
                return "CPU Wins :(";
            }
        }
        /* This if statement is out of the loop because if we put the winning piece at the last
        turn it will show correct result otherwise it will show wrong result (BUG fixed) */
        if(playerPositions.size() + cpuPositions.size() == 9){
            return "DRAW!";
        }
        return "";
    }
}