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 org.apache.hadoop.classification.InterfaceAudience;
022import org.apache.hadoop.classification.InterfaceStability;
023
024/**
025 * A {@link Writable} which is also {@link Comparable}. 
026 *
027 * <p><code>WritableComparable</code>s can be compared to each other, typically 
028 * via <code>Comparator</code>s. Any type which is to be used as a 
029 * <code>key</code> in the Hadoop Map-Reduce framework should implement this
030 * interface.</p>
031 *
032 * <p>Note that <code>hashCode()</code> is frequently used in Hadoop to partition
033 * keys. It's important that your implementation of hashCode() returns the same 
034 * result across different instances of the JVM. Note also that the default 
035 * <code>hashCode()</code> implementation in <code>Object</code> does <b>not</b>
036 * satisfy this property.</p>
037 *  
038 * <p>Example:</p>
039 * <p><blockquote><pre>
040 *     public class MyWritableComparable implements WritableComparable<MyWritableComparable> {
041 *       // Some data
042 *       private int counter;
043 *       private long timestamp;
044 *       
045 *       public void write(DataOutput out) throws IOException {
046 *         out.writeInt(counter);
047 *         out.writeLong(timestamp);
048 *       }
049 *       
050 *       public void readFields(DataInput in) throws IOException {
051 *         counter = in.readInt();
052 *         timestamp = in.readLong();
053 *       }
054 *       
055 *       public int compareTo(MyWritableComparable o) {
056 *         int thisValue = this.value;
057 *         int thatValue = o.value;
058 *         return (thisValue &lt; thatValue ? -1 : (thisValue==thatValue ? 0 : 1));
059 *       }
060 *
061 *       public int hashCode() {
062 *         final int prime = 31;
063 *         int result = 1;
064 *         result = prime * result + counter;
065 *         result = prime * result + (int) (timestamp ^ (timestamp &gt;&gt;&gt; 32));
066 *         return result
067 *       }
068 *     }
069 * </pre></blockquote></p>
070 */
071@InterfaceAudience.Public
072@InterfaceStability.Stable
073public interface WritableComparable<T> extends Writable, Comparable<T> {
074}