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  }