Clear up lack of clarity around equality/identity

src/main/java/org/opentripplanner/framework/lang/PredicateUtils.java

@@ -16,6 +16,8 @@ public class PredicateUtils {
    * <p>
    * This is useful for removing duplicates from a stream where the key to be compared is not the
    * entity itself but a field of it.
+   * <p>
+   * Note: Duplicate check is based on equality not identity.
    */
   public static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
     Map<Object, Boolean> seen = new ConcurrentHashMap<>();

src/test/java/org/opentripplanner/framework/lang/PredicateUtilsTest.java

@@ -8,13 +8,15 @@
 
 class PredicateUtilsTest {
 
-  private static final String HELLO = "HELLO";
+  private static String makeHello() {
+    return new String("HELLO");
+  }
 
   @Test
   void distinctByKey() {
-    var first = new Wrapper(10, HELLO);
+    var first = new Wrapper(10, makeHello());
     var last = new Wrapper(20, "HI");
-    var stream = Stream.of(first, new Wrapper(20, HELLO), last);
+    var stream = Stream.of(first, new Wrapper(20, makeHello()), last);
 
     var deduplicated = stream.filter(PredicateUtils.distinctByKey(w -> w.string)).toList();