org.apache.hadoop.io
Interface WritableComparable<T>

All Superinterfaces:

Comparable<T>, Writable

All Known Implementing Classes:

BooleanWritable, BytesWritable, ByteWritable, DatanodeDescriptor, DatanodeID, DatanodeInfo, DocumentID, DoubleWritable, FloatWritable, ID, IntWritable, JobID, LongWritable, MD5Hash, MultiFileWordCount.WordOffset, NullWritable, Record, RecordTypeInfo, Shard, TaskAttemptID, TaskID, Text, UTF8, VIntWritable, VLongWritable

public interface WritableComparable<T>
extends Writable, Comparable<T>

A Writable which is also Comparable.

WritableComparables can be compared to each other, typically via Comparators. Any type which is to be used as a key in the Hadoop Map-Reduce framework should implement this interface.

Example:

     public class MyWritableComparable implements WritableComparable {
       // 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 int compareTo(MyWritableComparable w) {
         int thisValue = this.value;
         int thatValue = ((IntWritable)o).value;
         return (thisValue < thatValue ? -1 : (thisValue==thatValue ? 0 : 1));
       }
     }
 

Method Summary

Methods inherited from interface org.apache.hadoop.io.Writable

readFields, write

Methods inherited from interface java.lang.Comparable

compareTo