Writable (Hadoop 1.2.1 API)
Date: 2013-07-22

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## org.apache.hadoop.io
Interface Writable

All Known Subinterfaces:
  InputSplit, WritableComparable<T>

All Known Implementing Classes:
  AbstractDelegationTokenIdentifier, AbstractMapWritable, AccessControlList, ArrayWritable, BlockLocation, BloomFilter, BooleanWritable, BytesWritable, ByteWritable, ClusterMetrics, ClusterStatus, CombineFileSplit, CombineFileSplit, CompositeInputSplit, CompressedWritable, Configuration, ContentSummary, Counter, CounterGroup, Counters, Counters, Counters.Counter, Counters.Group, CountingBloomFilter, Credentials, DataDrivenDBInputFormat.DataDrivenDBInputSplit, DBInputFormat.DBInputSplit, DBInputFormat.DBInputSplit, DBInputFormat.NullDBWritable, DBInputFormat.NullDBWritable, DelegationKey, DelegationTokenIdentifier, DocumentAndOp, DocumentID, DoubleWritable, DynamicBloomFilter, FileChecksum, FileSplit, FileSplit, FileStatus, Filter, FloatWritable, FsPermission, GenericWritable, ID, ID, IntermediateForm, IntWritable, JobConf, JobID, JobID, JobProfile, JobQueueInfo, JobSplit.SplitMetaInfo, JobStatus, JobTokenIdentifier, JvmTask, Key, LineDocTextAndOp, LongWritable, MapTaskCompletionEventsUpdate, MapWritable, MD5Hash, MD5MD5CRC32FileChecksum, MultiFileSplit, MultiFileWordCount.WordOffset, NullWritable, ObjectWritable, PermissionStatus, QueueAclsInfo, Record, RecordTypeInfo, RetouchedBloomFilter, SecondarySort.IntPair, SequenceFile.Metadata, Shard, SleepJob.EmptySplit, SortedMapWritable, TaggedMapOutput, Task, TaskAttemptID, TaskAttemptID, TaskCompletionEvent, TaskID, TaskID, TaskReport, TaskStatus, TaskTrackerStatus, Text, Token, TokenIdentifier, TupleWritable, TwoDArrayWritable, TypedBytesWritable, UTF8, VersionedWritable, VIntWritable, VLongWritable

    public interface Writable

A serializable object which implements a simple, efficient, serialization protocol, based on `DataInput` and `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 `readFields(DataInput)` and returns the instance.

Example:

>          public class MyWritable implements Writable {
>            // Some data
>            private int counter;
>            private long timestamp;

>            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;
>            }
>          }

Method Summary
`void` | `readFields(DataInput in)` Deserialize the fields of this object from `in`.
`void` | `write(DataOutput out)` Serialize the fields of this object to `out`.

Method Detail

### write

    void write(DataOutput out)
               throws IOException

  Serialize the fields of this object to `out`.

  Parameters:
    `out` - `DataOuput` to serialize this object into.
  Throws:
    `IOException`

### readFields

    void readFields(DataInput in)
                    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.

  Parameters:
    `in` - `DataInput` to deseriablize this object from.
  Throws:
    `IOException`

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