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Composition or inheritance may be used to create a new class that both encapsulates an existing class and adds one or more fields. When a subclass extends another in this way, the concept of equality for the subclass may or may not involve its new fields. That is, when comparing two subclass objects for equality, sometimes their respective fields must also be equal, and other times they need not be equal.  Depending on the concept of equality for the subclass, the subclass might override {{equals()}}. Furthermore, this method must follow the general contract for {{equals()}} as specified by the Java Language Specification \[[JLS 2005|AA. Bibliography#JLS 05]\].

public final class CaseInsensitiveString {
  private String s;

  public CaseInsensitiveString(String s) {
    if (s == null) {
      throw new NullPointerException();
    } 
    this.s = s;
  }

  // This method violates symmetry
  public boolean equals(Object o) {
    if (o instanceof CaseInsensitiveString) {
      return s.equalsIgnoreCase(((CaseInsensitiveString)o).s);
    }

    if (o instanceof String) {
      return s.equalsIgnoreCase((String)o);
    }
    return false;
  }

  public static void main(String[] args) {
    CaseInsensitiveString cis = new CaseInsensitiveString("Java");
    String s = "java";
    System.out.println(cis.equals(s)); // Returns true
    System.out.println(s.equals(cis)); // Returns false
  }
}

public final class CaseInsensitiveString {
  private String s;

  public CaseInsensitiveString(String s) {
    if (s == null) {
      throw new NullPointerException();
    }
    this.s = s;
  }

  public boolean equals(Object o) {
    return o instanceof CaseInsensitiveString &&
    ((CaseInsensitiveString)o).s.equalsIgnoreCase(s);
  }

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

  public static void main(String[] args) {
    CaseInsensitiveString cis = new CaseInsensitiveString("Java");
    String s = "java";
    System.out.println(cis.equals(s)); // Returns false now
    System.out.println(s.equals(cis)); // Returns false now
  }
}

public class Card {
  private final int number;

  public Card(int number) {
    this.number = number;
  }

  public boolean equals(Object o) {
    if (!(o instanceof Card)) {
      return false;
    } 
    
    Card c = (Card)o;
    return c.number == number;
  }
}

class XCard extends Card {
  private String type;
  public XCard(int number, String type) {
    super(number);
    this.type = type;
  }

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

    // Normal Card, do not compare type 
    if (!(o instanceof XCard)) {
      return o.equals(this);
    } 

    // It is an XCard, compare type as well
    XCard xc = (XCard)o;
    return super.equals(o) && xc.type == type;
  }	  
  
  public static void main(String[] args) {
    XCard p1 = new XCard(1, "type1"); 
    Card p2 = new Card(1);
    XCard p3 = new XCard(1, "type2");
    System.out.println(p1.equals(p2)); // Returns true
    System.out.println(p2.equals(p3)); // Returns true
    System.out.println(p1.equals(p3)); // Returns false, violating transitivity
  }
}

Unfortunately, it is impossible to extend an instantiable class (as opposed to an {{abstract}} class) by adding a value or field in the subclass while preserving the {{equals()}} contract. Use composition rather than inheritance to achieve the desired effect \[[Bloch 2008|AA. Bibliography#Bloch 08]\]. 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 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
  }
}

*MET12-EX0:* This guideline may be violated provided that the incompatible types are never compared.  There are classes in the Java platform libraries (and elsewhere) that extend an instantiable class by adding a value component.  For example, {{java.sql.Timestamp}} extends {{java.util.Date}} and adds a nanoseconds field. The {{equals}} implementation for {{Timestamp}} violates symmetry and can cause erratic behavior if {{Timestamp}} and {{Date}} objects are used in the same collection or are otherwise intermixed. \[[Bloch 2008|AA. Bibliography#Bloch 08]\]

\[[API 2006|AA. Bibliography#API 06]\] [method equals()|http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Object.html#equals(java.lang.Object)]
\[[Bloch 2008|AA. Bibliography#Bloch 08]\] Item 8: Obey the general contract when overriding equals
\[[Darwin 2004|AA. Bibliography#Darwin 04]\] 9.2 Overriding the equals method