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   * Licensed to the Apache Software Foundation (ASF) under one
   * or more contributor license agreements.  See the NOTICE file
   * distributed with this work for additional information
   * regarding copyright ownership.  The ASF licenses this file
   * to you under the Apache License, Version 2.0 (the
   * "License"); you may not use this file except in compliance
   * with the License.  You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.beam.runners.samza.util;
  
  import java.util.Collection;
  import java.util.List;
  import java.util.concurrent.CompletableFuture;
  import java.util.concurrent.CompletionStage;
  import java.util.concurrent.CountDownLatch;
  import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.ImmutableList;
  import org.junit.Assert;
  import org.junit.Test;
  
  /** Unit tests for {@linkplain FutureUtils}. */
  public final class FutureUtilsTest {
    private static final List<String> RESULTS = ImmutableList.of("hello", "world");
  
    @Test
    public void testFlattenFuturesForCollection() {
      CompletionStage<Collection<String>> resultFuture =
          FutureUtils.flattenFutures(
              ImmutableList.of(
                  CompletableFuture.completedFuture("hello"),
                  CompletableFuture.completedFuture("world")));
  
      CompletionStage<Void> validationFuture =
          resultFuture.thenAccept(
              actualResults -> {
                Assert.assertEquals(
                    "Expected flattened results to contain {hello, world}", RESULTS, actualResults);
              });
  
      validationFuture.toCompletableFuture().join();
    }
  
    @Test
    public void testFlattenFuturesForFailedFuture() {
      CompletionStage<Collection<String>> resultFuture =
          FutureUtils.flattenFutures(
              ImmutableList.of(
                  CompletableFuture.completedFuture("hello"),
                  createFailedFuture(new RuntimeException())));
  
      CompletionStage<Void> validationFuture =
          resultFuture.handle(
              (results, ex) -> {
                Assert.assertTrue(
                    "Expected exception to be of RuntimeException", ex instanceof RuntimeException);
                return null;
              });
  
      validationFuture.toCompletableFuture().join();
    }
  
    @Test
    public void testWaitForAllFutures() {
      CountDownLatch latch = new CountDownLatch(1);
      CompletionStage<Collection<String>> resultFuture =
          FutureUtils.flattenFutures(
              ImmutableList.of(
                  CompletableFuture.supplyAsync(
                      () -> {
                        try {
                          latch.await();
                        } catch (InterruptedException e) {
                          return "";
                        }
  
                        return "hello";
                      }),
                  CompletableFuture.supplyAsync(
                      () -> {
                        latch.countDown();
                        return "world";
                      })));
  
      CompletionStage<Void> validationFuture =
          resultFuture.thenAccept(
              actualResults -> {
                Assert.assertEquals(
                    "Expected flattened results to contain {hello, world}", RESULTS, actualResults);
              });
  
      validationFuture.toCompletableFuture().join();
    }
  
    private static CompletionStage<String> createFailedFuture(Throwable t) {
      CompletableFuture<String> future = new CompletableFuture<>();
      future.completeExceptionally(t);
      return future;
    }
  }
  