敏捷开发的必要技巧：慎用继承（1）

示例

　　这是一个会议管理系统。用来管理各种各样的会议参与者信息。数据库里面有个表Participants，里面的每条记录表示一个参会者。因为经常会发生用户误删掉某个参会者的信息。所以现在，用户删除时，并不会真的删除那参会者的信息，而只是将该记录的删除标记设为true。24小时以后，系统会自动将这条记录删除。但是在这24小时以内，如果用户改变主意了，系统还可以将这条记录还原，将删除标记设置为false。

　　请认真的读下面的代码：

    

public class DBTable {                                                                         
       protected Connection conn;                                                                  
       protected tableName;                                                                        
       public DBTable(String tableName) {                                                          
           this.tableName = tableName;                                                             
           this.conn = ...;                                                                        
       }                                                                                           
       public void clear() {                                                                       
           PreparedStatement st = conn.prepareStatement("DELETE FROM "+tableName);                 
           try {                                                                                   
               st.executeUpdate();                                                                 
           }finally{                                                                               
               st.close();                                                                         
           }                                                                                       
       }                                                                                           
       public int getCount() {                                                                     
           PreparedStatement st = conn.prepareStatement("SELECT COUNT(*) FROM"+tableName); 
           try {                                                                                   
               ResultSet rs = st.executeQuery();                                                   
               rs.next();                                                                          
               return rs.getInt(1);                                                                
           }finally{                                                                               
               st.close();                                                                         
           }                                                                                       
       }                                                                                           
    }                               
    
    public class ParticipantsInDB extends DBTable {                                                
       public ParticipantsInDB() {                                                                 
           super("participants");                                                                  
       }                                                                                           
       public void addParticipant(Participant part) {                                              
           ...                                                                                     
       }                                                                                           

       public void deleteParticipant(String participantId) {
           setDeleteFlag(participantId, true);
       }                      
       public void restoreParticipant(String participantId) {
           setDeleteFlag(participantId, false);
       }                      
       private void setDeleteFlag(String participantId, boolean b) {
           ...                
       }                      
       public void reallyDelete() {
           PreparedStatement st = conn.prepareStatement(
                             "DELETE FROM "+
                             tableName+
                             " WHERE deleteFlag=true");
           try {              
               st.executeUpdate();
           }finally{          
               st.close();    
           }                  
       }                      
       public int countParticipants() {
           PreparedStatement st = conn.prepareStatement(
                             "SELECT COUNT(*) FROM "+
                             tableName+
                             " WHERE deleteFlag=false");
           try {              
               ResultSet rs = st.executeQuery();
               rs.next();     
               return rs.getInt(1);
           }finally{          
               st.close();    
           }                  
       }                      
    }

                        

　　注意到，countParticipants这个方法只计算那些deleteFlags为false的记录。也就是，被删除的那些参会者不被计算在内。

　　上面的代码看起来还不错，但却有一个很严重的问题。什么问题？先看看下面的代码：

    ParticipantsInDB partsInDB = ...;
    Participant kent = new Participant(...);
    Participant paul = new Participant(...);
    partsInDB.clear();        
    partsInDB.addParticipant(kent);
    partsInDB.addParticipant(paul);
    partsInDB.deleteParticipant(kent.getId());
    System.out.println("There are "+partsInDB.getCount()+ "participants");

　　最后一行代码，会打印出"There are 1 participants"这样信息，对不？错！它打印的是"There are 2 participants"！因为最后一行调用的是DBTable里面的这个方法getCount,而不是ParticipantsInDB的countParticipants。getCount一点都不知道删除标记这回事，它只是简单的计算记录数量，并不知道要计算那些真正有效的参会者（就是删除标记为false的）。


继承了一些不合适（或者没用的）的功能

　　ParticipantsInDB继承了来自DBTable的方法，比如clear和getCount。对于ParticipantsInDB来讲，clear这个方法的确是有用的：清空所有的参会者。但getCount就造成了一点点小意外了：通过ParticipantsInDB调用getCount这个方法时，是取得participants这个表里面所有的记录，不管删除标记是true还是false的。而实际上，没人想知道这个数据。即使有人想知道，这个方法也不应该叫做getCount，因为这名字很容易就会跟“计算所有的（有效）参会者数量”联系在一起。

　　因此，ParticipantsInDB是不是真的应该继承这个方法getCount呢？或者我们应该怎么做比较恰当呢？

它们之间是否真的有继承关系？

　　当我们继承了一些我们不想要的东西，我们应该再三的想想：它们之间是不是真的有继承关系？ParticipantsInDB必须是一个DBTable吗？ParticipantsInDB希不希望别人知道它是一个DBTable？

　　实际上，ParticipantsInDB描述的是系统中所有的参会者的集合，该系统可以是个单数据库的，也可以是多数据库的，也就是说，这个类可以代表一个数据库里的一个Participants表，也可以代表两个数据库各自的两个Participants表的总和。

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