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Unfortunately, in this case it is impossible to extend the Card class by  class by adding a value or field in the subclass while preserving the Java equals() contract contract. This problem is not specific to the Card class but applies to any class hierarchy that can consider equal instances of distinct subclasses of some superclass. For such cases, use composition rather than inheritance to achieve the desired effect effect [Bloch 2008]. This compliant solution adopts this approach by adding a private card field to the XCard class and providing a public viewCard() method.], [Liskov 1994], [Cline, C++ Super-FAQ]. It is fundamentally impossible to have a class that both allows arbitrary subclass extensions and permits an equals() method that is reflexive, symmetric, and transitive, as is required by Object.equals(). In the interests of consistency and security, we forgo arbitrary subclass extensions, and assume that {{Card.equals()}} may impose certain restrictions on its subclasses.

This compliant solution adopts this approach by adding a private card field to the XCard class and providing a public viewCard() method.

class XCard {
  private String type;
  private Card card; // Composition

  public XCard(int number, String type) {
    card = new Card(number);
    this.type = type;
  }

  public Card viewCard() {
    return card;
  }

  public boolean equals(Object o) {
    if (!(o instanceof XCard)) {
      return false;
    }

    XCard cp = (XCard)o;
    return cp.card.equals(card) && cp.type.equals(type);
  }

  public int hashCode() {/* ... */}

  public static void main(String[] args) {
    XCard p1 = new XCard(1, "type1");
    Card p2 = new Card(1);
    XCard p3 = new XCard(1, "type2");
    XCard p4 = new XCard(1, "type1");
    System.out.println(p1.equals(p2)); // Prints false
    System.out.println(p2.equals(p3)); // Prints false
    System.out.println(p1.equals(p3)); // Prints false
    System.out.println(p1.equals(p4)); // Prints true
  }
}

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Violating the general contract when overriding the equals() method can lead to unexpected results.

Automated Detection

Related Guidelines

Bibliography

Related Guidelines

Bibliography

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