SEI CERT Oracle Coding Standard for Java

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Comment: Edited by NavBot (vkp) v1.0

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The general usage contract for {{equals()}} as specified by the Java Language Specification \[[JLS 2005|AA. Java References#JLSBibliography#JLS 05]\] 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.

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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 \[[Bloch 2008|AA. Java References#BlochBibliography#Bloch 08]\]. It can be implemented by giving the {{XCard}} class a private {{card}} field and providing a {{public}} {{viewCard()}} method.

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"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." \[[Bloch 2008|AA. Java References#BlochBibliography#Bloch 08]\]

Risk Assessment

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

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

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