@Stable
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
}
}
@InterfaceAudience.Public
@InterfaceStability.Stable
public interface WritableComparable<T> extends Writable, Comparable<T> {
}
--------------
org.apache.hadoop.io.Writable
@Public
@Stable
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;
}
}
@InterfaceAudience.Public
@InterfaceStability.Stable
public interface Writable {
/**
* Serialize the fields of this object to <code>out</code>.
*
* @param out <code>DataOuput</code> to serialize this object into.
* @throws IOException
*/
void write(DataOutput out) throws IOException;
/**
* Deserialize the fields of this object from <code>in</code>.
*
* <p>For efficiency, implementations should attempt to re-use storage in the
* existing object where possible.</p>
*
* @param in <code>DataInput</code> to deseriablize this object from.
* @throws IOException
*/
void readFields(DataInput in) throws IOException;
}