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  public String toString() {
039    return "(null)";
040  }
041
042  @Override
043  public int hashCode() { return 0; }
044  
045  @Override
046  public int compareTo(NullWritable other) {
047    return 0;
048  }
049  public boolean equals(Object other) { return other instanceof NullWritable; }
050  public void readFields(DataInput in) throws IOException {}
051  public void write(DataOutput out) throws IOException {}
052
053  /** A Comparator &quot;optimized&quot; for NullWritable. */
054  public static class Comparator extends WritableComparator {
055    public Comparator() {
056      super(NullWritable.class);
057    }
058
059    /**
060     * Compare the buffers in serialized form.
061     */
062    @Override
063    public int compare(byte[] b1, int s1, int l1,
064                       byte[] b2, int s2, int l2) {
065      assert 0 == l1;
066      assert 0 == l2;
067      return 0;
068    }
069  }
070
071  static {                                        // register this comparator
072    WritableComparator.define(NullWritable.class, new Comparator());
073  }
074}
075