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Apache Hadoop Common
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/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.hadoop.io;

import java.io.DataOutput;
import java.io.DataInput;
import java.io.IOException;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;

/**
 * A serializable object which implements a simple, efficient, serialization 
 * protocol, based on {@link DataInput} and {@link DataOutput}.
 *
 * Any key or value type in the Hadoop Map-Reduce
 * framework implements this interface.
 * 
 * Implementations typically implement a static read(DataInput)
 * method which constructs a new instance, calls {@link #readFields(DataInput)} 
 * and returns the instance.
 * 
 * Example:
 * 
 *     public class MyWritable implements Writable {
 *       // Some data
 *       private int counter;
 *       private long timestamp;
 *
 *       // Default constructor to allow (de)serialization
 *       MyWritable() { }
 *
 *       public void write(DataOutput out) throws IOException {
 *         out.writeInt(counter);
 *         out.writeLong(timestamp);
 *       }
 *
 *       public void readFields(DataInput in) throws IOException {
 *         counter = in.readInt();
 *         timestamp = in.readLong();
 *       }
 *
 *       public static MyWritable read(DataInput in) throws IOException {
 *         MyWritable w = new MyWritable();
 *         w.readFields(in);
 *         return w;
 *       }
 *     }
 * 

 */
@InterfaceAudience.Public
@InterfaceStability.Stable
public interface Writable {
  /** 
   * Serialize the fields of this object to out.
   * 
   * @param out DataOuput to serialize this object into.
   * @throws IOException
   */
  void write(DataOutput out) throws IOException;

  /** 
   * Deserialize the fields of this object from in.  
   * 
   * For efficiency, implementations should attempt to re-use storage in the 
   * existing object where possible.
   * 
   * @param in DataInput to deseriablize this object from.
   * @throws IOException
   */
  void readFields(DataInput in) throws IOException;
}