001    /*
002     * Copyright (c) 2007-2014 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    
021    package cascading.pipe.assembly;
022    
023    import java.beans.ConstructorProperties;
024    
025    import cascading.operation.Identity;
026    import cascading.pipe.Each;
027    import cascading.pipe.Pipe;
028    import cascading.pipe.SubAssembly;
029    import 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     */
038    public 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      }