Fix race condition in Fraction.hashCode()

Author: garydgregory

src/changes/changes.xml

@@ -74,6 +74,7 @@ The <action> type attribute can be add,update,fix,remove.
     <action                   type="fix" dev="ggregory" due-to="David Du, Gary Gregory">Remove redundant length check in StringUtils.isBlank(CharSequence) #1516.</action>
     <action issue="LANG-1800" type="fix" dev="ggregory" due-to="IcoreE">Incorrect grammar and unclear wording in RandomStringUtils#random method #1520.</action>
     <action issue="LANG-1802" type="fix" dev="ggregory" due-to="Gary Gregory, IcoreE">Fix collision in CharRange.hashCode().</action>
+    <action                   type="fix" dev="ggregory" due-to="Gary Gregory">Fix race condition in Fraction.hashCode().</action>
     <!-- ADD -->
     <!-- UPDATE -->
     <action                   type="update" dev="ggregory" due-to="Gary Gregory, Dependabot">Bump org.apache.commons:commons-parent from 92 to 93 #1498.</action>

src/main/java/org/apache/commons/lang3/math/Fraction.java

@@ -467,7 +467,7 @@ private static int subAndCheck(final int x, final int y) {
     /**
      * Cached output hashCode (class is immutable).
      */
-    private transient int hashCode;
+    private final int hashCode;
 
     /**
      * Cached output toString (class is immutable).
@@ -489,6 +489,7 @@ private static int subAndCheck(final int x, final int y) {
     private Fraction(final int numerator, final int denominator) {
         this.numerator = numerator;
         this.denominator = denominator;
+        this.hashCode = Objects.hash(denominator, numerator);
     }
 
     /**
@@ -720,10 +721,6 @@ public int getProperWhole() {
      */
     @Override
     public int hashCode() {
-        if (hashCode == 0) {
-            // hash code update should be atomic.
-            hashCode = Objects.hash(denominator, numerator);
-        }
         return hashCode;
     }