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
019 package org.apache.hadoop.io;
020
021 import java.io.*;
022
023 import org.apache.hadoop.classification.InterfaceAudience;
024 import org.apache.hadoop.classification.InterfaceStability;
025
026 /** Singleton Writable with no data. */
027 @InterfaceAudience.Public
028 @InterfaceStability.Stable
029 public 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 "optimized" 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