[java] UnusedPrivateMethod: Add test case for #3209

pmd-java/src/test/resources/net/sourceforge/pmd/lang/java/rule/bestpractices/xml/UnusedPrivateMethod.xml

@@ -1744,4 +1744,26 @@ public class OuterClass {
 }
         ]]></code>
     </test-code> 
+
+    <test-code>
+        <description>[java] UnusedPrivateMethod false positive with static method and cast expression #3209</description>
+        <expected-problems>0</expected-problems>
+        <code><![CDATA[
+public class UnusedAssignmentRule {
+    private static class ReachingDefsVisitor {
+        private static String getVarIfUnaryAssignment(String node) {
+            return null;
+        }
+    }
+    public static class AssignmentEntry {
+        final String var;
+        final Object rhs;
+        public boolean isUnaryReassign() {
+            return rhs instanceof String
+                && var.equals(ReachingDefsVisitor.getVarIfUnaryAssignment((String) rhs));
+        }
+    }
+}
+        ]]></code>
+    </test-code>
 </test-data>