单人纸牌(Elevens Lab)活动9：实现Elevens牌局

导言

在活动8中，我们重构（refactor，重新组织）了原本的class ElevensBoard，将其改写为新的class Board和一个更小的class ElevensBoard。这个改动的目的是为着在诸如Tens和Thirteens等新游戏中重用代码。现在你需要完成重构后的class ElevensBoard中相关method的实现。

练习

1. 完成目录中的class ElevensBoard，实现下列method。
   class Board的abstract method：
   1. isLegal——该method的行为由其名字和注释描述。其实现应该检查所选择扑克牌的数目并利用ElevensBoard的助手method。

      /**  
       * Determines if the selected cards form a valid group for removal.  
       * In Elevens, the legal groups are (1) a pair of non-face cards  
       * whose values add to 11, and (2) a group of three cards consisting of  
       * a jack, a queen, and a king in some order.  
       * @param selectedCards the list of the indexes of the selected cards.  
       * @return true if the selected cards form a valid group for removal;  
       *         false otherwise.  
       */  
      @Override  
      public boolean isLegal(List<Integer> selectedCards)

   2. anotherPlayIsPossible——该method也利用助手method，正确的实现不会太复杂。

      /** 
       * Determine if there are any legal plays left on the board. 
       * In Elevens, there is a legal play if the board contains 
       * (1) a pair of non-face cards whose values add to 11, or (2) a group 
       * of three cards consisting of a jack, a queen, and a king in some order. 
       * @return true if there is a legal play left on the board; 
       *         false otherwise. 
       */ 
      @Override  
      public boolean anotherPlayIsPossible()

   ElevensBoard助手method：

   1. containsPairSum11——该method判断所选的cards元素是否包括一堆点数值之和为11的牌。

      /** 
       * Check for an 11-pair in the selected cards. 
       * @param selectedCards selects a subset of this board.  It is this list 
       *                      of indexes into this board that are searched 
       *                      to find an 11-pair. 
       * @return true if the board entries indexed in selectedCards 
       *              contain an 11-pair; false otherwise. 
       */
      private boolean containsPairSum11(List<Integer> selectedCards)

   2. containsJQK——该method判断所选的cards元素是否为J、Q、K的一种组合。

      /**  
       * Check for a JQK in the selected cards.  
       * @param selectedCards selects a subset of this board.  It is this list  
       *                      of indexes into this board that are searched  
       *                      to find a JQK-triplet.  
       * @return true if the board entries indexed in selectedCards  
       *              include a jack, a queen, and a king; false otherwise.  
       */ 
      private boolean containsJQK(List<Integer> selectedCards)

完成这些method后，运行ElevensGUIRunner.java中的main method。确保Elevens游戏能够正确工作。注意cards目录应该与.class文件在同一级。

问题

1. Elevens和Thirteens的桌面牌数相差一。为什么size不是abstract method？
2. 为什么没有处理选择数组cards中被移除/替换牌的abstract method？
3. 创建is-a的另一种方式是实现interface。不创建abstract class Board的话，我们可以写如下这个interface Board，并让ElevensBoard实现它。这个新方案能让Elevens的GUI多态调用isLegal和anotherPlayIsPossible么？这种设计方案会和abstract class Board一样好么？为什么？

   public interface Board
   {
     boolean isLegal(List<Integer> selectedCards); 
     boolean anotherPlayIsPossible();
   }