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 org.apache.hadoop.classification.InterfaceAudience;
022    import 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
073    public interface WritableComparable<T> extends Writable, Comparable<T> {
074    }