/*
* 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());
}
}));
}
}