An pseudocode AI demonstrating the Minimax algorithm. Note that the algorithm has two helper functions: min and max.

AdversarialSearchAI.java

class AdversarialSearchAI {
	private PieceType piece;

	public AdversarialSearchAI(PieceType piece) {
		this.piece = piece;
	}

	/**
	 * Returns the best move given the state of the game
	 */
	public Move minimax(State state) {
		int highestScore = INTEGER.MIN_VALUE;
		Move bestMove = null;

		ArrayList<Move> moves = state.getMoves();
		int score = 0;
		for (Move move : moves) {
			State clone = state.clone();
			score = min(clone);
			if (score > highestScore) {
				highestScore = score;
				bestMove = move;
			}
		}

		return move;
	}

	/**
	 * Returns the lowest score received by the min player
	 */
	private int min(State state) {
		if (state.isEnd())
			return eval(state);

		int lowestScore = INTEGER.MAX_VALUE;

		ArrayList<Move> moves = state.getMoves();
		int score = 0;
		for (Move move : moves) {
			State clone = state.clone();
			score = max(clone);
			if (score < lowestScore)
				highestScore = score;
		}

		return move;
	}

	/**
	 * Returns the highest score received by the max player
	 */
	private int max(State state) {
		if (state.isEnd())
			return eval(state);

		int highestScore = INTEGER.MIN_VALUE;

		ArrayList<Move> moves = state.getMoves();
		int score = 0;
		for (Move move : moves) {
			State clone = state.clone();
			score = min(clone);
			if (score > highestScore)
				highestScore = score;
		}

		return move;
	}

	/**
	 * Returns 1 if the AI is the winner 
	 */
	private int eval(State state) {
		PieceType piece = state.winner();
		if (piece == this.piece)
			return 1;
		else
			return -1;
	}

	public static enum PieceType {
		X, O
	}

	/**
	 * Represents the state of the game board
	 */
	public static State {
		private static final WIDTH = 3;
		private static final HEIGHT = 3;

		public int board[WIDTH][HEIGHT];

		public ArrayList<Move> getMoves() {
			// TODO: Retrieve available moves
		}

		public boolean isEnd() {
			// TODO: Implement end-game check
		}

		public PieceType winner() {
			// TODO: Implement winner 
		}
	}

	public static Move {
		private int x;
		private int y;
		
		public Move(int x, int y) {
			this.x = x;
			this.y = y;
		}
	}

}