🎮 System Design + Java Program for Tic-Tac-Toe (TicTac)

In this mini system design, we’ll build a console-based Tic-Tac-Toe game in Java.

We’ll keep it:

• Simple

• Readable

• Data-structure focused

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🧠 High-Level Design

We’ll design 3 main parts:

1. Board – stores the grid and game state

2. Player – represents X and O

3. Game – controls turns, input, and winning logic

Data Structures Used

• char[][] board = new char[3][3];
  → 3x3 grid for the game

• char currentPlayer = 'X';
  → Tracks who is playing now

• Simple methods to:

  • printBoard()

  • makeMove(row, col)

  • checkWin()

  • isDraw()

No complex frameworks. Just core Java + basic data structures.

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📦 Class Design Overview

1️⃣ Board Class

Responsibilities:

• Initialize empty board

• Print the board

• Validate and update moves

• Check winner / draw

2️⃣ TicTacToeGame Class

Responsibilities:

• Run the game loop

• Switch players

• Take input from console

• Use Board to perform actions

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✅ Full Java Program – Simple Tic-Tac-Toe

import java.util.Scanner;

/**
 * Simple Tic-Tac-Toe game using basic OOP and array data structure.
 * 
 * Design:
 * - Board: handles the grid and game state
 * - TicTacToeGame: controls the game flow
 */
class Board {
    private char[][] grid; // 3x3 board

    public Board() {
        grid = new char[3][3];
        // Initialize board with spaces
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 3; j++) {
                grid[i][j] = ' ';
            }
        }
    }

    // Print board in a simple format
    public void printBoard() {
        System.out.println("-------------");
        for (int i = 0; i < 3; i++) {
            System.out.print("| ");
            for (int j = 0; j < 3; j++) {
                System.out.print(grid[i][j] + " | ");
            }
            System.out.println();
            System.out.println("-------------");
        }
    }

    // Try to place a move on the board
    public boolean placeMove(int row, int col, char playerSymbol) {
        // Check if row and col are in range and cell is empty
        if (row < 0 || row >= 3 || col < 0 || col >= 3) {
            System.out.println("Invalid position! Choose row and col between 0 and 2.");
            return false;
        }
        if (grid[row][col] != ' ') {
            System.out.println("Cell already taken! Choose another position.");
            return false;
        }
        grid[row][col] = playerSymbol;
        return true;
    }

    public boolean hasWinner(char playerSymbol) {
        // Check rows
        for (int i = 0; i < 3; i++) {
            if (grid[i][0] == playerSymbol &&
                grid[i][1] == playerSymbol &&
                grid[i][2] == playerSymbol) {
                return true;
            }
        }

        // Check columns
        for (int j = 0; j < 3; j++) {
            if (grid[0][j] == playerSymbol &&
                grid[1][j] == playerSymbol &&
                grid[2][j] == playerSymbol) {
                return true;
            }
        }

        // Check diagonals
        if (grid[0][0] == playerSymbol &&
            grid[1][1] == playerSymbol &&
            grid[2][2] == playerSymbol) {
            return true;
        }

        if (grid[0][2] == playerSymbol &&
            grid[1][1] == playerSymbol &&
            grid[2][0] == playerSymbol) {
            return true;
        }

        return false;
    }

    public boolean isFull() {
        // If any cell is an empty space, board is not full
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 3; j++) {
                if (grid[i][j] == ' ') {
                    return false;
                }
            }
        }
        return true;
    }
}

public class TicTacToeGame {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        Board board = new Board();
        char currentPlayer = 'X';
        boolean gameFinished = false;

        System.out.println("Welcome to Tic-Tac-Toe!");
        System.out.println("Player X and Player O take turns.");
        System.out.println("Enter row and column (0, 1, or 2).");
        System.out.println();

        while (!gameFinished) {
            board.printBoard();
            System.out.println("Player " + currentPlayer + ", it's your turn.");

            System.out.print("Enter row (0-2): ");
            int row = scanner.nextInt();

            System.out.print("Enter col (0-2): ");
            int col = scanner.nextInt();

            boolean moveSuccess = board.placeMove(row, col, currentPlayer);

            if (!moveSuccess) {
                // Invalid move, ask again
                continue;
            }

            // Check win
            if (board.hasWinner(currentPlayer)) {
                board.printBoard();
                System.out.println("Player " + currentPlayer + " wins! 🎉");
                gameFinished = true;
            } 
            // Check draw
            else if (board.isFull()) {
                board.printBoard();
                System.out.println("It's a draw! 🤝");
                gameFinished = true;
            } 
            else {
                // Switch player
                currentPlayer = (currentPlayer == 'X') ? 'O' : 'X';
            }
        }

        scanner.close();
        System.out.println("Game over. Thanks for playing!");
    }
}

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🧩 How This Relates to System Design (in a Simple Way)

Even though this is a small console program, it still uses good design ideas:

• Separation of Concerns

  • Board → game state & logic

  • TicTacToeGame → input + game loop

• Encapsulation

  • Board hides its internal grid and exposes only methods

• Data Structures

  • 2D array char[][] is the core structure