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An object is characterized both by its identity (location in memory) and by its state (actual data). The == operator compares only the identities of two objects (to check whether the references refer to the same object); the equals method defined in java.lang.Object can be customized by overriding to compare the state as well. When a class defines an equals() method, it implies that the method compares state. When the class lacks a customized equals() method (either locally declared, or inherited from a parent class), it uses the default Object.equals() implementation that is inherited from Object which compares only the references and may produce counter-intuitive results. For example, the classes String and StringBuffer should override the Object.equals() method because they do not provide their own implementation.

The equals() method only applies to objects, not primitives. There is no need to override the equals method when checking logical equality is not useful. For example, enumerated types have a fixed set of distinct values that may be compared using == instead of the equals() method. Note that enumerated types provide an equals() implementation that uses == internally; this default cannot be overridden. More generally, subclasses that both inherit an implementation of equals() from a superclass and also lack a requirement for additional functionality need not override the equals() method.

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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
  }
}

Exceptions

*MET12-EX1:* This guideline may be violated provided that the incompatible types are not compared.  There are some classes in the Java platform libraries (and elsewhere) that extend an instantiable class andby addadding 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." \[[Bloch 2008|AA. Bibliography#Bloch 08]\]

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