????3> interface????ó???

????A. ?????????????????????Э????????????????????

????B. ???????????????????????????????????????????е????????

????C. ????????????????????????????????????????????????????

????D. ?????????????????????Щ?????????????????κ??????
????4> abstract class????ó???

??????仰??????????????????????????????????????????￡???????????????????У?

????A. ????????????????????????????????????????е??????????abstract class??????鷽???壬???????????????壬??????????????????????????????????

????B. ?Щ?????￡????????????????????????????Э?????????????б??????????????????????abstract???н????????????????????

????C. ?淶???????Э??????????????Щ????????????????????????????????????????????????Щ??????????????????????????????????????????

????22??????????н??? ?????????????(implements)???? ?????????????????(concrete class)??

???????????н???????????????(implements)??????????????????????????????????????????????????

????23????????Constructor?????override??

??????????Constructor???????У?????????дOverriding?????????????Overloading??

????24??.??????????????????????synchronized???????????????????????????????????

????????????????????synchronized???????????????????

????25???????д???Singleton??????

public class Singleton {  
 
private Singleton(){}  
 
//?????????????????????????????????  
 
//???????private ??????????  
 
private static Singleton instance = new Singleton();  
 
//????????????????????class????????????????????  
 
public static Singleton getInstance() {  
 
return instance;   
 
 }  
 
}
 ??????????????

public class Singleton {  
 
private static Singleton instance = null;  
 
public static synchronized Singleton getInstance() {  
 
//????????????????????????????ζ??????????????????Ρ? ??  
 
//??????????????????Ч???  
 
if (instance==null)  
 
instance??new Singleton();  
 
return instance; }  
 
}