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.
   */
  package org.apache.beam.sdk.transforms;
  
  import org.apache.beam.sdk.values.KV;
  import org.apache.beam.sdk.values.PCollection;
  
  /**
   * {@code KvSwap<K, V>} takes a {@code PCollection<KV<K, V>>} and returns a {@code PCollection<KV<V,
   * K>>}, where all the keys and values have been swapped.
   *
   * <p>Example of use:
   *
   * <pre>{@code
   * PCollection<String, Long> wordsToCounts = ...;
   * PCollection<Long, String> countsToWords =
   *     wordToCounts.apply(KvSwap.<String, Long>create());
   * }</pre>
   *
   * <p>Each output element has the same timestamp and is in the same windows as its corresponding
   * input element, and the output {@code PCollection} has the same {@link
   * org.apache.beam.sdk.transforms.windowing.WindowFn} associated with it as the input.
   *
   * @param <K> the type of the keys in the input {@code PCollection} and the values in the output
   *     {@code PCollection}
   * @param <V> the type of the values in the input {@code PCollection} and the keys in the output
   *     {@code PCollection}
   */
  public class KvSwap<K, V> extends PTransform<PCollection<KV<K, V>>, PCollection<KV<V, K>>> {
    /**
     * Returns a {@code KvSwap<K, V>} {@code PTransform}.
     *
     * @param <K> the type of the keys in the input {@code PCollection} and the values in the output
     *     {@code PCollection}
     * @param <V> the type of the values in the input {@code PCollection} and the keys in the output
     *     {@code PCollection}
     */
    public static <K, V> KvSwap<K, V> create() {
      return new KvSwap<>();
    }
  
    private KvSwap() {}
  
    @Override
    public PCollection<KV<V, K>> expand(PCollection<KV<K, V>> in) {
      return in.apply(
          "KvSwap",
          MapElements.via(
              new SimpleFunction<KV<K, V>, KV<V, K>>() {
                @Override
                public KV<V, K> apply(KV<K, V> kv) {
                  return KV.of(kv.getValue(), kv.getKey());
                }
              }));
    }
  }
  