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Google Cloud Dataflow Java SDK provides a simple, Java-based interface for processing virtually any size data using Google cloud resources. This artifact includes entire Dataflow Java SDK.

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/*
 * Copyright (C) 2015 Google Inc.
 *
 * Licensed 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 com.google.cloud.dataflow.sdk.transforms;

import com.google.cloud.dataflow.sdk.values.KV;
import com.google.cloud.dataflow.sdk.values.PCollection;

/**
 * {@code Keys} takes a {@code PCollection} of {@code KV}s and
 * returns a {@code PCollection} of the keys.
 *
 * Example of use:
 * 
 {@code
 * PCollection> wordCounts = ...;
 * PCollection words = wordCounts.apply(Keys.create());
 * } 
 *
 * 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 com.google.cloud.dataflow.sdk.transforms.windowing.WindowFn}
 * associated with it as the input.
 *
 * See also {@link Values}.
 *
 * @param  the type of the keys in the input {@code PCollection},
 * and the type of the elements in the output {@code PCollection}
 */
public class Keys extends PTransform>,
                                        PCollection> {
  /**
   * Returns a {@code Keys} {@code PTransform}.
   *
   * @param  the type of the keys in the input {@code PCollection},
   * and the type of the elements in the output {@code PCollection}
   */
  public static  Keys create() {
    return new Keys<>();
  }

  private Keys() { }

  @Override
  public PCollection apply(PCollection> in) {
    return
        in.apply(ParDo.named("Keys")
                 .of(new DoFn, K>() {
                     @Override
                     public void processElement(ProcessContext c) {
                       c.output(c.element().getKey());
                     }
                    }));
  }
}