敏捷开发中需要慎用继承

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);
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");