A simple command-line Tic Tac Toe game.

TicTacToe.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Random;
import java.util.Scanner;

public class TicTacToe {
	static ArrayList<Integer> playerPositions = new ArrayList<Integer>();
	static ArrayList<Integer> cpuPositions = new ArrayList<Integer>();

	
	@SuppressWarnings("unlikely-arg-type")
	public static void main(String[] args) {
		char[][] gameBoard = {
				{' ', '|', ' ', '|', ' '},
				{'-', '+', '-', '+', '-'},
				{' ', '|', ' ', '|', ' '},
				{'-', '+', '-', '+', '-'},
				{' ', '|', ' ', '|', ' '}
		};
		printGameBoard(gameBoard);
		
		while(true) {
			@SuppressWarnings("resource")
			Scanner scan = new Scanner(System.in);
			System.out.print("Enter your placement (1-9): ");
			int playerPos = scan.nextInt();
			while(playerPositions.contains(playerPos) || cpuPositions.contains(playerPositions)) {
				System.out.print("Position taken. Please choose a valid one: ");
				playerPos = scan.nextInt();
			}
			
			placePiece(gameBoard, playerPos, "player");
			
			String result = checkWinner();
			if(result.length() > 0) {
				System.out.println(result);
				break;
			}
			
			/* This is the computer's "AI", which makes its moves.
			 * No, I'm not going to _actually_ add an AI system in this, ever.
			 * That's too much work for a silly Tic Tac Toe game...
			 * Java's _Random_ library will have to do for now. ;) */
			Random rand = new Random();
			int cpuPos = rand.nextInt(9) + 1;
			while(playerPositions.contains(cpuPos) || cpuPositions.contains(cpuPos)) {
				cpuPos = rand.nextInt(9) + 1;
			}
			placePiece(gameBoard, cpuPos, "cpu");
			
			printGameBoard(gameBoard);
			if(result.length() > 0) {
				System.out.println(result);
				break;
			}
		}
	}
	
	public static void printGameBoard(char[][] 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) {
		char symbol = ' ';
		
		if(user.equals("player")) {
			symbol = 'X';
			playerPositions.add(pos);
		} else if(user.equals("cpu")) { // People say "CPU" for the computer in these games. But this is Java, so I might as well say RAM. :P
			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;
		}
	}
	
	@SuppressWarnings("rawtypes")
	public static String checkWinner() {
		List topRow = Arrays.asList(1, 2, 3);
		List midRow = Arrays.asList(4, 5, 6); // Middle row. "mid" instead of "middle" because formatting look pretty ooooo.
		List botRow = Arrays.asList(7, 8, 9); // Bottom row. Same reason. oooOOOoooo
		List leftCol = Arrays.asList(1, 4, 7);
		List midCol = Arrays.asList(2, 5, 8);
		List rightCol = Arrays.asList(3, 6, 9); // Yeah I guess the columns messed up my nice formatting...
		List diag1 = Arrays.asList(1, 5, 9);
		List diag2 = Arrays.asList(7, 5, 3); // Hmm the diagonals are better...
		
		List<List> winningConditions = new ArrayList<List>();
		winningConditions.add(topRow);
		winningConditions.add(midRow);
		winningConditions.add(botRow);
		winningConditions.add(leftCol);
		winningConditions.add(midCol);
		winningConditions.add(rightCol);
		winningConditions.add(diag1);
		winningConditions.add(diag2);
		
		for(List l : winningConditions) {
			if(playerPositions.containsAll(l)) {
				return "You won!";
			} else if(cpuPositions.containsAll(l)) {
				return "You lost. :(";
			} else if(playerPositions.size() + cpuPositions.size() == 9) {
				return "It's a tie.";
			}
		}
		
		return ""; // This is a lie. You have been scammed.
		// (It also has a length of zero thus helps out some code above...)
	}
}