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 < 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 >>> 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 }