    001/*
    002 * Copyright (c) 2007-2017 Xplenty, Inc. All Rights Reserved.
    003 *
    004 * Project and contact information: http://www.cascading.org/
    005 *
    006 * This file is part of the Cascading project.
    007 *
    008 * Licensed under the Apache License, Version 2.0 (the "License");
    009 * you may not use this file except in compliance with the License.
    010 * You may obtain a copy of the License at
    011 *
    012 *     http://www.apache.org/licenses/LICENSE-2.0
    013 *
    014 * Unless required by applicable law or agreed to in writing, software
    015 * distributed under the License is distributed on an "AS IS" BASIS,
    016 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    017 * See the License for the specific language governing permissions and
    018 * limitations under the License.
    019 */
    020
    021package cascading.pipe.assembly;
    022
    023import java.beans.ConstructorProperties;
    024
    025import cascading.operation.Identity;
    026import cascading.pipe.Each;
    027import cascading.pipe.Pipe;
    028import cascading.pipe.SubAssembly;
    029import cascading.tuple.Fields;
    030
    031/**
    032 * Class Retain is a {@link SubAssembly} that will discard all incoming fields except those given on the constructor.
    033 * <p/>
    034 * Use this pipe to narrow a stream of tuples, removing unused data.
    035 *
    036 * @see Discard
    037 */
    038public class Retain extends SubAssembly
    039  {
    040  @ConstructorProperties({"previous", "retainFields"})
    041  public Retain( Pipe previous, Fields retainFields )
    042    {
    043    super( previous );
    044
    045    if( retainFields == null )
    046      throw new IllegalArgumentException( "retainFields may not be null" );
    047
    048    setTails( new Each( previous, retainFields, new Identity(), Fields.RESULTS ) );
    049    }
    050  }