001/* 002 * Copyright (c) 2007-2015 Concurrent, 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 }