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    
021    package cascading.util;
022    
023    import java.util.Iterator;
024    
025    /**
026     * Class SingleValueIterator is a convenience to creating an {@link Iterator} that returns one value for use
027     * with interfaces that only accept Iterator instances.
028     * <p/>
029     * If given a {@code null} value, it will be assumed this Iterator instance has no value, and {@link #hasNext()}
030     * will return false.
031     */
032    public class SingleValueIterator<Value> implements Iterator<Value>
033      {
034      public static final SingleValueIterator EMPTY = new SingleValueIterator();
035    
036      private boolean hasValue = true;
037      protected Value value;
038    
039      public SingleValueIterator()
040        {
041        this.hasValue = false;
042        }
043    
044      public SingleValueIterator( Value value )
045        {
046        this.hasValue = value != null;
047        this.value = value;
048        }
049    
050      @Override
051      public boolean hasNext()
052        {
053        return hasValue;
054        }
055    
056      @Override
057      public Value next()
058        {
059        if( !hasValue )
060          throw new IllegalStateException( "no value available" );
061    
062        try
063          {
064          return value;
065          }
066        finally
067          {
068          hasValue = false;
069          }
070        }
071    
072      @Override
073      public void remove()
074        {
075        throw new UnsupportedOperationException( "unimplimented" );
076        }
077    
078      public void reset( Value value )
079        {
080        this.hasValue = value != null;
081        this.value = value;
082        }
083      }