@InterfaceAudience.Public @InterfaceStability.Stable public interface WritableComparable<T> extends Writable, Comparable<T>
Writable
which is also Comparable
.
WritableComparable
s can be compared to each other, typically
via Comparator
s. 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 } }
readFields, write
compareTo
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