001/* 002 * Copyright (c) 2016-2017 Chris K Wensel <chris@wensel.net>. All Rights Reserved. 003 * Copyright (c) 2007-2017 Xplenty, Inc. All Rights Reserved. 004 * 005 * Project and contact information: http://www.cascading.org/ 006 * 007 * This file is part of the Cascading project. 008 * 009 * Licensed under the Apache License, Version 2.0 (the "License"); 010 * you may not use this file except in compliance with the License. 011 * You may obtain a copy of the License at 012 * 013 * http://www.apache.org/licenses/LICENSE-2.0 014 * 015 * Unless required by applicable law or agreed to in writing, software 016 * distributed under the License is distributed on an "AS IS" BASIS, 017 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 018 * See the License for the specific language governing permissions and 019 * limitations under the License. 020 */ 021 022package cascading.pipe.assembly; 023 024import java.beans.ConstructorProperties; 025 026import cascading.operation.Identity; 027import cascading.pipe.Each; 028import cascading.pipe.Pipe; 029import cascading.pipe.SubAssembly; 030import cascading.tuple.Fields; 031 032/** 033 * Class Retain is a {@link SubAssembly} that will discard all incoming fields except those given on the constructor. 034 * <p> 035 * Use this pipe to narrow a stream of tuples, removing unused data. 036 * 037 * @see Discard 038 */ 039public class Retain extends SubAssembly 040 { 041 @ConstructorProperties({"previous", "retainFields"}) 042 public Retain( Pipe previous, Fields retainFields ) 043 { 044 super( previous ); 045 046 if( retainFields == null ) 047 throw new IllegalArgumentException( "retainFields may not be null" ); 048 049 setTails( new Each( previous, retainFields, new Identity(), Fields.RESULTS ) ); 050 } 051 }