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 "optimized" 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