== Why is this an issue?

According to the Java Language Specification, there is a contract between ``++equals(Object)++`` and ``++hashCode()++``:


____
If two objects are equal according to the ``++equals(Object)++`` method, then calling the ``++hashCode++`` method on each of the two objects must produce the same integer result. 

It is not required that if two objects are unequal according to the ``++equals(java.lang.Object)++`` method, then calling the ``++hashCode++`` method on each of the two objects must produce distinct integer results.

However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables.

____

In order to comply with this contract, those methods should be either both inherited, or both overridden.

== How to fix it

=== Code examples

==== Noncompliant code example

[source,java,diff-id=1,diff-type=noncompliant]
----
class MyClass {    // Noncompliant - should also override "hashCode()"

  @Override
  public boolean equals(Object obj) {
    /* ... */
  }

}
----

==== Compliant solution

[source,java,diff-id=1,diff-type=compliant]
----
class MyClass {    // Compliant

  @Override
  public boolean equals(Object obj) {
    /* ... */
  }

  @Override
  public int hashCode() {
    /* ... */
  }

}
----


== Resources

* CWE - https://cwe.mitre.org/data/definitions/581[CWE-581 - Object Model Violation: Just One of Equals and Hashcode Defined]
* https://wiki.sei.cmu.edu/confluence/x/7DVGBQ[CERT, MET09-J.] - Classes that define an equals() method must also define a hashCode() method

ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

This [enum|class] overrides "hashCode()|equals()" and should therefore also override "hashCode()|equals()".


'''
== Comments And Links
(visible only on this page)

include::../comments-and-links.adoc[]

endif::env-github,rspecator-view[]
