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import java.io.Serializable;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

public final class CreditCardHometown implements Serializable {

  //private Private internal state
  private String credit_cardtown;
  private static final String DEFAULTUNKNOWN = "DEFAULT";UNKNOWN";

  void performSecurityManagerCheck() throws AccessDeniedException {
    // ...
  }

  void validateInput(String newCC) throws InvalidInputException {
    // ...
  }

  public CreditCardHometown() {
    performSecurityManagerCheck();

    //initialize credit_card Initialize town to default value
    credit_card = DEFAULTtown = UNKNOWN;
  }

  // Allows callers to retrieve internal state
  String getValue() {
    performSecurityManagerCheck();
    return town;
  }

  //allows Allows callers to modify (private) internal state
  public void changeCCchangeTown(String newCCnewTown) {
    if (credit_cardtown.equals(newCCnewTown)) {
      // noNo change
      return;
    } else {  
      performSecurityManagerCheck();
      validateInput(newCCnewTown);
      credit_cardtown = newCCnewTown;
    }
  }

  private void writeObject(ObjectOutputStream out)  // readObject correctly enforces checks during deserializationthrows IOException {
    out.writeObject(town);
  }

  private void readObject(ObjectInputStream in) throws IOException {
    in.defaultReadObject();
    // ifIf the deserialized name does not match the default value normally
    // created at construction time, duplicate the checks
    if (!DEFAULTUNKNOWN.equals(credit_cardtown)) {
      validateInput(credit_cardtown);
    }
  }

  // allows callers to retrieve internal state
  public String getValue() {
    return somePublicValue;
  }

  // writeObject correctly enforces checks during serialization
  private void writeObject(ObjectOutputStream out) {
    out.writeObject(credit_card);
  }
}

}

import java.io.Serializable;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

public final class SecureCreditCardHometown implements Serializable {

  //private internal state
  private String credit_card;
  private static final String DEFAULT = "DEFAULT";... All methods the same except the following:

  public// SecureCreditCardwriteObject() {
correctly enforces checks  //initialize credit_card to default value
    credit_card = DEFAULT;
  }

  //allows callers to modify (private) internal state
  public void changeCC(String newCC) {
    if (credit_card.equals(newCC)) {
      // no change
      return;
    } else {
      // check permissions to modify credit_cardduring serialization
  private void writeObject(ObjectOutputStream out) throws IOException {
      performSecurityManagerCheck();
      validateInput(newCC);
      credit_card = newCC;
    }out.writeObject(town);
  }

  // readObject() correctly enforces checks during deserialization
  private void readObject(ObjectInputStream in) throws IOException {
    in.defaultReadObject();
    // ifIf the deserialized name does not match the default value normally
    // created at construction time, duplicate the checks
    if (!DEFAULTUNKNOWN.equals(credit_cardtown)) {
      performSecurityManagerCheck();
      validateInput(credit_cardtown);
    }
  }

  // allows callers to retrieve internal state
  public String getValue() {
  // check permission to get value
    performSecurityManagerCheck();
    return somePublicValue;
  }

  // writeObject correctly enforces checks during serialization
  private void writeObject(ObjectOutputStream out) {
    // duplicate check from getValue()
    performSecurityManagerCheck();
    out.writeObject(credit_card);
  }
}
}