Added custom message to ensure only one

modules/collect/src/main/java/com/opengamma/strata/collect/Guavate.java

@@ -682,6 +682,7 @@ public static <R> Predicate<R> not(Predicate<R> predicate) {
    *
    * @param <T>  the type of element in the stream
    * @return the operator
+   * @throws IllegalArgumentException if more than one element is present
    */
   public static <T> BinaryOperator<T> ensureOnlyOne() {
     return (a, b) -> {
@@ -690,6 +691,31 @@ public static <T> BinaryOperator<T> ensureOnlyOne() {
     };
   }
 
+  /**
+   * Reducer used in a stream to ensure there is no more than one matching element.
+   * <p>
+   * This method returns an operator that can be used with {@link Stream#reduce(BinaryOperator)}
+   * that returns either zero or one elements from the stream. Unlike {@link Stream#findFirst()}
+   * or {@link Stream#findAny()}, this approach ensures an exception is thrown if there
+   * is more than one element in the stream.
+   * <p>
+   * This would be used as follows (with a static import):
+   * <pre>
+   *   stream.filter(...).reduce(ensureOnlyOne()).get();
+   * </pre>
+   *
+   * @param <T>  the type of element in the stream
+   * @param message the message template for the {@link IllegalArgumentException} with "{}" placeholders
+   * @param args the arguments for the message
+   * @return the operator
+   * @throws IllegalArgumentException if more than one element is present
+   */
+  public static <T> BinaryOperator<T> ensureOnlyOne(String message, Object... args) {
+    return (a, b) -> {
+      throw new IllegalArgumentException(Messages.format(message, args));
+    };
+  }
+
   //-------------------------------------------------------------------------
   /**
    * Function used in a stream to cast instances to a particular type without filtering.

modules/collect/src/test/java/com/opengamma/strata/collect/GuavateTest.java

@@ -424,6 +424,17 @@ public void test_ensureOnlyOne() {
     assertThatIllegalArgumentException().isThrownBy(() -> Stream.of("a", "b").reduce(Guavate.ensureOnlyOne()));
   }
 
+  @Test
+  public void test_ensureOnlyOne_withCustomMessage() {
+    String message = "Expected one letter but found multiple for date {}";
+    LocalDate arg = LocalDate.of(2024, 4, 24);
+    assertThat(Stream.empty().reduce(Guavate.ensureOnlyOne(message, arg))).isEqualTo(Optional.empty());
+    assertThat(Stream.of("a").reduce(Guavate.ensureOnlyOne(message, arg))).isEqualTo(Optional.of("a"));
+    assertThatIllegalArgumentException().isThrownBy(() -> Stream.of("a", "b")
+            .reduce(Guavate.ensureOnlyOne(message, arg)))
+        .withMessage(Messages.format(message, arg));
+  }
+
   //-------------------------------------------------------------------------
   @Test
   public void test_casting() {