title: "No known class method for selector 'validSuits'" Error in xCode 5.1
body: <p>I'm trying to follow along with a Stanford CS193p course, but I'm getting an objective C error that I cannot resolve.</p>
  
  <p>I'm getting the error in the below <code>PlayingCardDeck.m</code> file on the line that contains "[PlayingCard validSuits]".</p>
  
  <pre><code>#import "PlayingCard.h"
  #import "PlayingCardDeck.h"
  
  @implementation PlayingCardDeck
  
  - (instancetype)init {
      self = [super init];
  
      if (self){
          for (NSString *suit2 in [PlayingCard validSuits]) {   //Error on this line
  
          }
      }
  
      return self;
  }
  
  @end
  </code></pre>
  
  <p>Here is the PlayingCardDeck.h file:</p>
  
  <pre><code>#import "Deck.h"
  
  @interface PlayingCardDeck : Deck
  
  @end
  </code></pre>
  
  <p>Here is the <code>PlayingCard.m</code> file:</p>
  
  <pre><code>#import "PlayingCard.h"
  
  @implementation PlayingCard
  - (NSString *)contents {
  
      NSArray *rankStrings = [PlayingCard rankStrings];
      return [rankStrings[self.rank] stringByAppendingString:self.suit];
  }
  
  + (NSArray *)validSuits {
      return @[@"♣︎", @"♠︎", @"♥︎", @"♦︎"];
  }
  
  @synthesize suit = _suit;
  - (void)setSuit:(NSString *)suit {
      if ([[PlayingCard validSuits] containsObject:suit]) {
          _suit = suit;
      }
  }
  - (NSString *)suit {
      return _suit ? _suit : @"?";
  }
  + (NSArray *)rankStrings {
      return @[@"?", @"A", @"2", @"3", @"4", @"5",
               @"6", @"7", @"8", @"9", @"10", @"J",
               @"Q", @"K"];
  }
  + (NSUInteger)maxRank {
      return [[self rankStrings] count] - 1;
  }
  - (void)setRank:(NSUInteger)rank {
      if (rank &lt;= [PlayingCard maxRank]) {
          _rank = rank;
      }
  }
  @end
  </code></pre>
  
  <p>And the <code>PlayingCard.h</code> file:</p>
  
  <pre><code>#import "Card.h"
  
  @interface PlayingCard : Card
  
  @property (strong, nonatomic) NSString *suit;
  @property (nonatomic) NSUInteger rank;
  
  + (NSArray *)validSuits;
  + (NSUInteger)maxRank;
  @end
  </code></pre>
  
  <p>I'm new to objective C and have no idea what is causing this problem.  Or why identical code can work for one person and not for me.  Any help is appreciated.</p>