MET12-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 refer to the same object), the equals method defined in java.lang.Object can compare the state as well, when customized by overriding. If a class defines an equals() method, the provider has accounted for state comparison. If not, the default Object.equals() implementation that is inherited from Object compares only the references and may produce conterintuitive 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. Also, immutable objects do not need to override it. There is no need to override equals if checking logical equality is not useful. For example, Enumerated types have a fixed set of distinct values that may be compared using ==}}instead of {{equals(). Note that Enumerated types do have an equals() implementation that uses == internally and this default cannot be overridden. More generally, if a subclass inherits an implementation of equals from a superclass and does not need additional functionality, overriding equals() can be forgone.

The general usage contract for equals() as specified by the Java Language Specification [[JLS 2005]] says

• 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 resulting from a violation of the first condition are infrequent; it is consequently 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 that use unreliable data sources such as IP addresses and caches. The final condition about the comparison with null is typically violated when the equals() code throws an exception instead of returning false. When this constitutes a security vulnerability (in the form of denial of service), it can be trivially fixed by returning false.

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 within 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 code example violates transitivity though it satisfies the symmetry condition. In the first print statement, the comparison between p1 and p2 returns true, in the second, the comparison between p2 and p3 also returns true but in the third, the comparison between p1 and p3 returns false. This contradicts the transitivity rule.

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 2008]]. It can be implemented by giving the XCard class a private card field 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
  }
}

"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 2008]]

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.

Bibliography

[[API 2006]] method equals()
[[Bloch 2008]] Item 8: Obey the general contract when overriding equals
[[Darwin 2004]] 9.2 Overriding the equals method

MET11-J. Understand the difference between overriding and hiding      16. Methods (MET)      MET13-J. Ensure that hashCode() is overridden when equals() is overridden