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.values;
  
  import java.util.Collections;
  import java.util.Map;
  import org.apache.beam.sdk.Pipeline;
  import org.apache.beam.sdk.io.TextIO.Read;
  import org.apache.beam.sdk.transforms.PTransform;
  
  /**
   * {@link PBegin} is the "input" to a root {@link PTransform}, such as {@link Read Read} or {@link
   * org.apache.beam.sdk.transforms.Create}.
   *
   * <p>Typically elided by simply calling {@link Pipeline#apply(String, PTransform)} or {@link
   * Pipeline#apply(PTransform)}, but one can be explicitly created by calling {@link Pipeline#begin}
   * on a Pipeline.
   */
  public class PBegin implements PInput {
    /** Returns a {@link PBegin} in the given {@link Pipeline}. */
    public static PBegin in(Pipeline pipeline) {
      return new PBegin(pipeline);
    }
  
    /**
     * Like {@link #apply(String, PTransform)} but defaulting to the name of the {@link PTransform}.
     */
    public <OutputT extends POutput> OutputT apply(PTransform<? super PBegin, OutputT> t) {
      return Pipeline.applyTransform(this, t);
    }
  
    /**
     * Applies the given {@link PTransform} to this {@link PBegin}, using {@code name} to identify
     * this specific application of the transform.
     *
     * <p>This name is used in various places, including the monitoring UI, logging, and to stably
     * identify this application node in the job graph.
     */
    public <OutputT extends POutput> OutputT apply(
        String name, PTransform<? super PBegin, OutputT> t) {
      return Pipeline.applyTransform(name, this, t);
    }
  
    @Override
    public Pipeline getPipeline() {
      return pipeline;
    }
  
    @Override
    public Map<TupleTag<?>, PValue> expand() {
      // A PBegin contains no PValues.
      return Collections.emptyMap();
    }
  
    /////////////////////////////////////////////////////////////////////////////
  
    /** Constructs a {@link PBegin} in the given {@link Pipeline}. */
    protected PBegin(Pipeline pipeline) {
      this.pipeline = pipeline;
    }
  
    private final Pipeline pipeline;
  }
  