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

All Superinterfaces:

Comparable<T>, Writable

All Known Implementing Classes:

BooleanWritable, BytesWritable, ByteWritable, DoubleWritable, FloatWritable, ID, ID, IntWritable, JobID, JobID, LongWritable, MD5Hash, NullWritable, Record, RecordTypeInfo, ShortWritable, TaskAttemptID, TaskAttemptID, TaskID, TaskID, Text, VIntWritable, VLongWritable

@InterfaceAudience.Public
@InterfaceStability.Stable
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.

Note that hashCode() is frequently used in Hadoop to partition keys. It's important that your implementation of hashCode() returns the same result across different instances of the JVM. Note also that the default hashCode() implementation in Object does not satisfy this property.

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 o) {
         int thisValue = this.value;
         int thatValue = o.value;
         return (thisValue < thatValue ? -1 : (thisValue==thatValue ? 0 : 1));
       }

       public int hashCode() {
         final int prime = 31;
         int result = 1;
         result = prime * result + counter;
         result = prime * result + (int) (timestamp ^ (timestamp >>> 32));
         return result
       }
     }
 

Method Summary

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

readFields, write

Methods inherited from interface java.lang.Comparable

compareTo