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public final class CreditCard implements Serializable {
  // Private internal state
  private String credit_card;
  private static final String DEFAULT = "DEFAULT""DEFAULT";

  public CreditCard() {
    // 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 {  
      validateInput(newCC);
      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 (!DEFAULT.equals(credit_card)) {
      validateInput(credit_card);
    }
  }

  // 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 SecureCreditCard implements Serializable {
  // Private internal state
  private String credit_card;
  private static final String DEFAULT = "DEFAULT""DEFAULT";

  public SecureCreditCard() {
    // 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_card
      performSecurityManagerCheck();
      validateInput(newCC);
      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 (!DEFAULT.equals(credit_card)) {
      performSecurityManagerCheck();
      validateInput(credit_card);
    }
  }

  // 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)  throws IOException {
    // Duplicate check from getValue()
    performSecurityManagerCheck();
    out.writeObject(credit_card);
  }
}