CardFlip.java

import java.util.Scanner;

public class CardFlip {
	static boolean DEBUG = false;

	private enum CardValue {
		A, B, C, D;
	}

	private static class Card {
		final CardValue cardValue;
		boolean flipped = false;

		public Card(CardValue cardValue) {
			this.cardValue = cardValue;
		}

		void setFlipped(boolean flipped) {
			this.flipped = flipped;
		}

		boolean isFlipped() {
			return flipped;
		}

	}

	private final Card[] cards;
	private int firstFlip = -1;

	public boolean isGameOver() {
		for (int i = 0; i < cards.length; i++) {
			if (!cards[i].isFlipped()) {
				return false;
			}
		}
		return true;
	}

	// If the result of the flip is desired, add a FlipResult enum with values 
	//   INVALID (< 0 || > n)
	//   FIRST_FLIP (1st card successfully flipped)
	//   ALREADY_FLIPPED (card is already flipped over)
	//   MATCH (the two selected cards matched)
	//   MISMATCH (the two selected cards did not match)
	//   SAME_CARD (tried to match with same card)
	// Change this method to return a FlipResult
	public void flip(int card) {
		if (card < 0 || card > cards.length -1) {
			return;
		}
		if (firstFlip == -1) {
			firstFlip = card;
			cards[firstFlip].setFlipped(true);
		} else {
			if (firstFlip != card && cards[firstFlip].cardValue == cards[card].cardValue) {
				cards[card].setFlipped(true);
				firstFlip = -1;
			} else {
				cards[firstFlip].setFlipped(false);
				firstFlip = -1;
			}
		}
	}

	public String toString() {
		StringBuilder sb = new StringBuilder();
		for (Card card : cards) {
			sb.append(card.isFlipped() ? card.cardValue.toString() : DEBUG ? card.cardValue.toString().toLowerCase() : '?');
		}
		return sb.toString();
	}

	public CardFlip(int sets) {
		cards = new Card[sets * 4];
		for (int i = 0; i < sets * 4; i++) {
			cards[i] = new Card(CardValue.values()[i % 4]);
		}
	}

	public static void main(String[] args) {
		if (args.length > 0 && args[0].toLowerCase().equals("--debug")) {
			CardFlip.DEBUG = true;
		}
		CardFlip theGame = new CardFlip(4);
		Scanner scanner = new Scanner(System.in);
		while (!theGame.isGameOver()) {
			System.out.println(theGame.toString());
			System.out.print("\nCard to flip> ");
			theGame.flip(scanner.nextInt());
		}
	}
}