敏捷开发的必要技巧：慎用继承

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();    
           }                  
       }                      
    }

    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");