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;
  import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.base.Joiner;
  
  /**
   * {@link PTransform PTransforms} for converting a {@link PCollection PCollection&lt;?&gt;}, {@link
   * PCollection PCollection&lt;KV&lt;?,?&gt;&gt;}, or {@link PCollection
   * PCollection&lt;Iterable&lt;?&gt;&gt;} to a {@link PCollection PCollection&lt;String&gt;}.
   *
   * <p><b>Note</b>: For any custom string conversion and formatting, we recommend applying your own
   * {@link ProcessFunction} using {@link MapElements#via(ProcessFunction)}
   */
  @SuppressWarnings({
    "nullness" // TODO(https://github.com/apache/beam/issues/20497)
  })
  public final class ToString {
    private ToString() {
      // do not instantiate
    }
  
    /**
     * Transforms each element of the input {@link PCollection} to a {@link String} using the {@link
     * Object#toString} method.
     */
    public static PTransform<PCollection<?>, PCollection<String>> elements() {
      return new Elements();
    }
  
    /**
     * Transforms each element of the input {@link PCollection} to a {@link String} by using the
     * {@link Object#toString} on the key followed by a "," followed by the {@link Object#toString} of
     * the value.
     */
    public static PTransform<PCollection<? extends KV<?, ?>>, PCollection<String>> kvs() {
      return kvs(",");
    }
  
    /**
     * Transforms each element of the input {@link PCollection} to a {@link String} by using the
     * {@link Object#toString} on the key followed by the specified delimiter followed by the {@link
     * Object#toString} of the value.
     *
     * @param delimiter The delimiter to put between the key and value
     */
    public static PTransform<PCollection<? extends KV<?, ?>>, PCollection<String>> kvs(
        String delimiter) {
      return new KVs(delimiter);
    }
  
    /**
     * Transforms each item in the iterable of the input {@link PCollection} to a {@link String} using
     * the {@link Object#toString} method followed by a "," until the last element in the iterable.
     * There is no trailing delimiter.
     */
    public static PTransform<PCollection<? extends Iterable<?>>, PCollection<String>> iterables() {
      return iterables(",");
    }
  
    /**
     * Transforms each item in the iterable of the input {@link PCollection} to a {@link String} using
     * the {@link Object#toString} method followed by the specified delimiter until the last element
     * in the iterable. There is no trailing delimiter.
     *
     * @param delimiter The delimiter to put between the items in the iterable.
     */
    public static PTransform<PCollection<? extends Iterable<?>>, PCollection<String>> iterables(
        String delimiter) {
      return new Iterables(delimiter);
    }
  
    /**
     * A {@link PTransform} that converts a {@code PCollection} to a {@code PCollection<String>} using
     * the {@link Object#toString} method.
     *
     * <p>Example of use:
     *
     * <pre>{@code
     * PCollection<Long> longs = ...;
     * PCollection<String> strings = longs.apply(ToString.elements());
     * }</pre>
     *
     * <p><b>Note</b>: For any custom string conversion and formatting, we recommend applying your own
     * {@link ProcessFunction} using {@link MapElements#via(ProcessFunction)}
     */
    private static final class Elements extends PTransform<PCollection<?>, PCollection<String>> {
      @Override
      public PCollection<String> expand(PCollection<?> input) {
        return input.apply(
            MapElements.via(
                new SimpleFunction<Object, String>() {
                  @Override
                  public String apply(Object input) {
                    return input.toString();
                  }
                }));
      }
    }
  
    /**
     * A {@link PTransform} that converts a {@code PCollection} of {@code KV} to a {@code
     * PCollection<String>} using the {@link Object#toString} method for the key and value and an
     * optional delimiter.
     *
     * <p>Example of use:
     *
     * <pre>{@code
     * PCollection<KV<String, Long>> nameToLong = ...;
     * PCollection<String> strings = nameToLong.apply(ToString.kv());
     * }</pre>
     *
     * <p><b>Note</b>: For any custom string conversion and formatting, we recommend applying your own
     * {@link ProcessFunction} using {@link MapElements#via(ProcessFunction)}
     */
    private static final class KVs
        extends PTransform<PCollection<? extends KV<?, ?>>, PCollection<String>> {
      private final String delimiter;
  
      public KVs(String delimiter) {
        this.delimiter = delimiter;
      }
  
      @Override
      public PCollection<String> expand(PCollection<? extends KV<?, ?>> input) {
        return input.apply(
            MapElements.via(
                new SimpleFunction<KV<?, ?>, String>() {
                  @Override
                  public String apply(KV<?, ?> input) {
                    return input.getKey().toString() + delimiter + input.getValue().toString();
                  }
                }));
      }
    }
  
    /**
     * A {@link PTransform} that converts a {@code PCollection} of {@link Iterable} to a {@code
     * PCollection<String>} using the {@link Object#toString} method and an optional delimiter.
     *
     * <p>Example of use:
     *
     * <pre>{@code
     * PCollection<Iterable<Long>> longs = ...;
     * PCollection<String> strings = nameToLong.apply(ToString.iterable());
     * }</pre>
     *
     * <p><b>Note</b>: For any custom string conversion and formatting, we recommend applying your own
     * {@link ProcessFunction} using {@link MapElements#via(ProcessFunction)}
     */
    private static final class Iterables
        extends PTransform<PCollection<? extends Iterable<?>>, PCollection<String>> {
      private final String delimiter;
  
      public Iterables(String delimiter) {
        this.delimiter = delimiter;
      }
  
      @Override
      public PCollection<String> expand(PCollection<? extends Iterable<?>> input) {
        return input.apply(
            MapElements.via(
                new SimpleFunction<Iterable<?>, String>() {
                  @Override
                  public String apply(Iterable<?> input) {
                    return Joiner.on(delimiter).join(input);
                  }
                }));
      }
    }
  }
  