content: // 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.
  
  // beam-playground:
  //   name: flatten
  //   description: Flatten example.
  //   multifile: false
  //   context_line: 40
  //   categories:
  //     - Quickstart
  //   complexity: MEDIUM
  //   tags:
  //     - hellobeam
  
  import org.apache.beam.sdk.Pipeline;
  import org.apache.beam.sdk.options.PipelineOptions;
  import org.apache.beam.sdk.options.PipelineOptionsFactory;
  import org.apache.beam.sdk.transforms.*;
  import org.apache.beam.sdk.values.PCollection;
  import org.apache.beam.sdk.values.PCollectionList;
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  public class Task {
  
      private static final Logger LOG = LoggerFactory.getLogger(Task.class);
  
      public static void main(String[] args) {
          PipelineOptions options = PipelineOptionsFactory.fromArgs(args).create();
          Pipeline pipeline = Pipeline.create(options);
  
          // List of elements start with a
          PCollection<String> wordsStartingWithA =
                  pipeline.apply("Words starting with A",
                          Create.of("apple", "ant", "arrow")
                  );
  
          // List of elements start with b
          PCollection<String> wordsStartingWithB =
                  pipeline.apply("Words starting with B",
                          Create.of("ball", "book", "bow")
                  );
  
          // The applyTransform() converts [wordsStartingWithA] and [wordsStartingWithB] to [output]
          PCollection<String> output = applyTransform(wordsStartingWithA, wordsStartingWithB);
  
          output.apply("Log", ParDo.of(new LogOutput<String>()));
  
          pipeline.run();
      }
  
      // The applyTransform two PCollection data types are the same combines and returns one PCollection
      static PCollection<String> applyTransform(
              PCollection<String> words1, PCollection<String> words2) {
  
          return PCollectionList.of(words1).and(words2)
                  .apply(Flatten.pCollections());
      }
  
      static class LogOutput<T> extends DoFn<T, T> {
          private String prefix;
  
          LogOutput() {
              this.prefix = "Processing element";
          }
  
          LogOutput(String prefix) {
              this.prefix = prefix;
          }
  
          @ProcessElement
          public void processElement(ProcessContext c) throws Exception {
              LOG.info(prefix + ": {}", c.element());
          }
      }
  
  }