HLRC: Use Optional in validation logic (#33104)

The Validatable class comes from an old class in server code, that
assumed null was returned in the event of validation having no
errors. This commit changes that to use Optional, which is cleaner than
passing around null objects.

Author: hub-cap

client/rest-high-level/src/main/java/org/elasticsearch/client/RestHighLevelClient.java

@@ -174,6 +174,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import java.util.Optional;
 import java.util.ServiceLoader;
 import java.util.Set;
 import java.util.function.Function;
@@ -1334,9 +1335,9 @@ protected final <Req extends Validatable, Resp> Resp performRequest(Req request,
                                                              RequestOptions options,
                                                              CheckedFunction<Response, Resp, IOException> responseConverter,
                                                              Set<Integer> ignores) throws IOException {
-        ValidationException validationException = request.validate();
-        if (validationException != null && validationException.validationErrors().isEmpty() == false) {
-            throw validationException;
+        Optional<ValidationException> validationException = request.validate();
+        if (validationException != null && validationException.isPresent()) {
+            throw validationException.get();
         }
         return internalPerformRequest(request, requestConverter, options, responseConverter, ignores);
     }
@@ -1445,9 +1446,9 @@ protected final <Req extends Validatable, Resp> void performRequestAsync(Req req
                                                                           RequestOptions options,
                                                                           CheckedFunction<Response, Resp, IOException> responseConverter,
                                                                           ActionListener<Resp> listener, Set<Integer> ignores) {
-        ValidationException validationException = request.validate();
-        if (validationException != null && validationException.validationErrors().isEmpty() == false) {
-            listener.onFailure(validationException);
+        Optional<ValidationException> validationException = request.validate();
+        if (validationException != null && validationException.isPresent()) {
+            listener.onFailure(validationException.get());
             return;
         }
         internalPerformRequestAsync(request, requestConverter, options, responseConverter, listener, ignores);

client/rest-high-level/src/main/java/org/elasticsearch/client/Validatable.java

@@ -18,24 +18,20 @@
  */
 package org.elasticsearch.client;
 
+import java.util.Optional;
+
 /**
  * Defines a validation layer for Requests.
  */
 public interface Validatable {
-    ValidationException EMPTY_VALIDATION = new ValidationException() {
-        @Override
-        public void addValidationError(String error) {
-            throw new UnsupportedOperationException("Validation messages should not be added to the empty validation");
-        }
-    };
-
     /**
-     * Perform validation. This method does not have to be overridden in the event that no validation needs to be done.
+     * Perform validation. This method does not have to be overridden in the event that no validation needs to be done,
+     * or the validation was done during object construction time. A {@link ValidationException} that is not null is
+     * assumed to contain validation errors and will be thrown.
      *
-     * @return potentially null, in the event of older actions, an empty {@link ValidationException} in newer actions, or finally a
-     * {@link ValidationException} that contains a list of all failed validation.
+     * @return An {@link Optional} {@link ValidationException} that contains a list of validation errors.
      */
-    default ValidationException validate() {
-        return EMPTY_VALIDATION;
+    default Optional<ValidationException> validate() {
+        return Optional.empty();
     }
 }