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public final class CreditCardHometown implements Serializable {
  // Private internal state
  private String credit_cardtown;
  private static final String DEFAULTUNKNOWN = "DEFAULT";

  public CreditCardUNKNOWN";

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

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

  public Hometown() {
    performSecurityManagerCheck();

    // Initialize credit_cardtown to default value
    credit_card = DEFAULTtown = UNKNOWN;
  }

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

  // Allows callers to modify (private) internal state
  public void changeCCchangeTown(String newCCnewTown) {
    if (credit_cardtown.equals(newCCnewTown)) {
      // No change
      return;
    } else {  
      performSecurityManagerCheck();
      validateInput(newCCnewTown);
      credit_cardtown = newCCnewTown;
    }
  }

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

  private void readObject(ObjectInputStream in)  throws IOException {
    in.defaultReadObject();
    // If 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
  String getValue() {
    return credit_card;
  }

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

public final class SecureCreditCardHometown implements Serializable {
  // Private... internalAll state
methods the privatesame String credit_card;
  private static final String DEFAULT = "DEFAULT";except the following:

  public// SecureCreditCardwriteObject() {
correctly enforces checks  performSecurityManagerCheck();

    // 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(newCCout.writeObject(town);
      credit_card = newCC;
    }
  }

  // readObject() correctly enforces checks during deserialization
  private void readObject(ObjectInputStream in)  throws IOException {
    in.defaultReadObject();
    // If 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_card);
    }
  }

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

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