Listing 20.2: AddressValidator as an Interface

public interface AddressValidator { 
  public boolean isValidAddress(String inp_address, 
     String inp_zip, String inp_state); 
}//end of class 

　　让我们定义一个USAddress的验证类，来验证给定的U.S地址。

　　Listing 20.3: USAddress Class

class USAddress implements AddressValidator { 
  public boolean isValidAddress(String inp_address, 
     String inp_zip, String inp_state) { 
   if (inp_address.trim().length() < 10) 
     return false; 
   if (inp_zip.trim().length() < 5) 
     return false; 
   if (inp_zip.trim().length() > 10) 
     return false; 
   if (inp_state.trim().length() != 2) 
     return false; 
   return true; 
  } 
}//end of class 

　　USAddress类实现AddressValidator接口，因此Customer对象使用USAddress实例作为验证客户地址过程的一部分是没有任何问题的。

　　Listing 20.4: Customer Class Using the USAddress Class

class Customer { 
          … 
          … 
 public boolean isValidAddress() { 
   //get an appropriate address validator 
   AddressValidator validator = getValidator(type); 
   //Polymorphic call to validate the address 
   return validator.isValidAddress(address, zip, state); 
 } 
 private AddressValidator getValidator(String custType) { 
   AddressValidator validator = null;