Here we do the old-school iteration, we canot call stream().count() because it use the forEachRemaining method which will was not mocked.

johnmay · egonw · commit f1248f81b03a · 2022-01-01T11:32:07.000+01:00
diff --git a/base/isomorphism/src/main/java/org/openscience/cdk/isomorphism/Mappings.java b/base/isomorphism/src/main/java/org/openscience/cdk/isomorphism/Mappings.java
@@ -534,7 +534,12 @@ public int[] first() {
      * @return number of matches
      */
     public int count() {
-        return Iterables.size(iterable);
+        // Note: doesn't work when mocked due to forEachRemaining
+        // return (int)stream().count();
+        int count = 0;
+        for (int[] m : iterable)
+            count++;
+        return count;
     }
 
     /**
