001/**
002 * Licensed to the Apache Software Foundation (ASF) under one
003 * or more contributor license agreements.  See the NOTICE file
004 * distributed with this work for additional information
005 * regarding copyright ownership.  The ASF licenses this file
006 * to you under the Apache License, Version 2.0 (the
007 * "License"); you may not use this file except in compliance
008 * with the License.  You may obtain a copy of the License at
009 *
010 *     http://www.apache.org/licenses/LICENSE-2.0
011 *
012 * Unless required by applicable law or agreed to in writing, software
013 * distributed under the License is distributed on an "AS IS" BASIS,
014 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015 * See the License for the specific language governing permissions and
016 * limitations under the License.
017 */
018
019package org.apache.hadoop.io;
020
021import java.io.*;
022
023import org.apache.hadoop.classification.InterfaceAudience;
024import org.apache.hadoop.classification.InterfaceStability;
025
026/** Singleton Writable with no data. */
027@InterfaceAudience.Public
028@InterfaceStability.Stable
029public class NullWritable implements WritableComparable<NullWritable> {
030
031  private static final NullWritable THIS = new NullWritable();
032
033  private NullWritable() {}                       // no public ctor
034
035  /** Returns the single instance of this class. */
036  public static NullWritable get() { return THIS; }
037  
038  @Override
039  public String toString() {
040    return "(null)";
041  }
042
043  @Override
044  public int hashCode() { return 0; }
045  
046  @Override
047  public int compareTo(NullWritable other) {
048    return 0;
049  }
050  @Override
051  public boolean equals(Object other) { return other instanceof NullWritable; }
052  @Override
053  public void readFields(DataInput in) throws IOException {}
054  @Override
055  public void write(DataOutput out) throws IOException {}
056
057  /** A Comparator &quot;optimized&quot; for NullWritable. */
058  public static class Comparator extends WritableComparator {
059    public Comparator() {
060      super(NullWritable.class);
061    }
062
063    /**
064     * Compare the buffers in serialized form.
065     */
066    @Override
067    public int compare(byte[] b1, int s1, int l1,
068                       byte[] b2, int s2, int l2) {
069      assert 0 == l1;
070      assert 0 == l2;
071      return 0;
072    }
073  }
074
075  static {                                        // register this comparator
076    WritableComparator.define(NullWritable.class, new Comparator());
077  }
078}
079