MET30-J. Follow the general contract while overriding the equals method

An object is characterized by its identity (location in memory) and state (actual data). While the '==' operator compares only the identities of two objects (to check if both the references are actually the same object), the equals method defined in java.lang.Object can compare the state as well, when customized by overriding it.

The equals method only applies to Objects, not primitives. Also, immutable objects do not need to override equals. There is no need to override equals if checking logical equality is not useful. Enum types are an example. If a subclass inherits an implementation of equals from a superclass and does not need additional functionality, one can forgo overriding equals().

The general usage contract for equals() has been put forth verbatim from the Java specification:

• It is reflexive: For any reference value x, x.equals(x) must return true.
• It is symmetric: For any reference values x and y, x.equals(y) must return true if and only if y.equals(x) returns true.
• It is transitive: For any reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) must return true.
• It is consistent: For any reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the object is modified.
• For any non-null reference value x, x.equals(null) must return false.

Do not violate any of these five conditions while overriding the equals method. Mistakes due to the first postulate are quite infrequent; it is thus omitted from this discussion. The second and third conditions are highlighted. The rule for consistency implies that mutable objects may not satisfy the equals contract. It is good practice to avoid defining equals() implementations so as to use unreliable sources such as IP addresses [[Bloch 08]] and caches.

Noncompliant Code Example

This noncompliant code example violates the second condition in the contract (symmetry). This requirement means that if one object is equal to another then the other must also be equal to this one. Consider a CaseInsensitiveString class that defines a String and overrides the equals method. The CaseInsensitiveString knows about ordinary strings but the String class has no idea about case-insensitive strings. As a result, s.equalsIgnoreCase(((CaseInsensitiveString)o).s) returns true but s.equalsIgnoreCase((String)o) always returns false.

public final class CaseInsensitiveString {
  private String s;

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

//This method violates asymmetry
  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
  }
}

Compliant Solution

Do not try to inter-operate with String from the equals method. The new equals method is highlighted in this compliant solution.

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 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
  }
}

Noncompliant Code Example

This noncompliant example violates transitivity though it follows the symmetry condition. This is because the first two statements print true while the third prints false. A practical implementation issue is intermingling of java.sql.Timestamp and java.util.Date classes. There is a disclaimer about the erratic behavior in the documentation for the Timestamp class.

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
  }
}

Compliant Solution

It is currently not possible to extend an instantiable class (as opposed to an abstract class) and add a value or field in the subclass while preserving the equals contract. This implies that composition must be preferred over inheritance. This technique does qualify as a reasonable workaround [[Bloch 08]]. It can be implemented by giving the XCard class a private card field and providing a a public viewCard method.

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;
  private Card card;
  
  public XCard(int number, String type) {
    super(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");
    System.out.println(p1.equals(p2)); //returns false
    System.out.println(p2.equals(p3)); //returns false
    System.out.println(p1.equals(p3)); //returns false
  }
}

"There are some classes in the Java platform libraries that do extend an instantiable class and add a value component. For example, java.sql.Timestamp extends java.util.Date and adds a nanoseconds field. The equals implementation for Timestamp does violate symmetry and can cause erratic behavior if Timestamp and Date objects are used in the same collection or are otherwise intermixed." [[Bloch 08]]

Risk Assessment

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

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[API 06]] method equals()
[[Bloch 08]] Item 8: Obey the general contract when overriding equals
[[Darwin 04]] 9.2 Overriding the equals method

MET03-J. For methods that return an array or collection prefer returning an empty array or collection over a null value      09. Methods (MET)      MET31-J. Ensure that hashCode() is overridden when equals() is overridden