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敏捷开发的必要技巧：慎用继承

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。

　　如果还不清楚的话，我们就这样举例吧，比如，现在我们已经有了2000个参会者，在两个数据库中存放，其中数据库A的participants表里面存放了1000个参会者，数据库B的participants这个表存放了1000个参会者。DBTable顶多只能描述一个数据库里面的一张表，也就是1000个参会者，而participants则可以完全的描述这2000年参会者的信息。前面可以当作数据库的数据表在系统中的代表，而后者表示的应该包含更多业务逻辑的一个域对象。（原谅这边我只能用域对象这样的词来断开这样的混淆。）

　　因此，我们可以判断，ParticipantsInDB跟DBTable之间不应该有什么继承的关系。ParticipantsInDB不能继承DBTable这个类了。于是，现在ParticipantsInDB也没有getCount这个方法了。可是ParticipantsInDB还需要DBTable类里面的其他方法啊，那怎么办？所以现在我们让ParticipantsInDB里面引用了一个DBTable：

    public class DBTable {                                                                         
       private Connection conn;                                                                    
       private String 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 String getTableName() {
          return tableName;    
       }                       
       public Connection getConn() {
          return conn;         
       }                       
   }                           
   
   public class ParticipantsInDB {
       private DBTable table;  
       public ParticipantsInDB() {
          table = new DBTable("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 = table.getConn().prepareStatement(
                              "DELETE FROM "+
                              table.getTableName()+
                              " WHERE deleteFlag=true");
          try {                
              st.executeUpdate();
          }finally{            
              st.close();      
          }                    
       }                       
       public void clear() {   
          table.clear();       
       }                       
       public int countParticipants() {
          PreparedStatement st = table.getConn().prepareStatement(
                              "SELECT COUNT(*) FROM "+
                              table.getTableName()+
                              " WHERE deleteFlag=false");
          try {                
              ResultSet rs = st.executeQuery();
              rs.next();       
              return rs.getInt(1);
          }finally{            
              st.close();      
          }                    
       }                       
   }

    ParticipantsInDB不再继承DBTable。代替的，它里面有一个属性引用了一个DBTable对象，然后调用这个DBTable的clear, getConn, getTableName 等等方法。

代理（delegation）
　　
　　其实我们这边可以看一下ParticipantsInDB的clear方法，这个方法除了直接调用DBTable的clear方法以外，什么也没做。或者说，ParticipantsInDB只是做为一个中间介让外界调用DBTable的方法，我们管这样传递调用的中间介叫“代理(delegation)”。

　　现在，之前有bug的那部分代码就编译不过了：

    ParticipantsInDB partsInDB = ...;                                                              
    Participant kent = new Participant(...);                                                       
    Participant paul = new Participant(...);                                                       
    partsInDB.clear();                                                                             
    partsInDB.addParticipant(kent);                                                                
    partsInDB.addParticipant(paul);                                                                
    partsInDB.deleteParticipant(kent.getId());                                                     
    //编译出错：因为在ParticipantsInDB里面已经没有getCount这个方法了！
    System.out.println("There are "+partsInDB.getCount()+ "participants");

　　总结一下：首先，我们发现，ParticipantsInDB 和 DBTableIn之间没有继承关系。然后我们就将“代理”来取代它们的继承。“代理”的优点就是，我们可以控制DBTable的哪些方法可以“公布（就是设为public）”（比如clear方法）。如果我们用了继承的话，我们就没得选择，DBTable里面的所有public方法都要对外公布！

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