The example in the java doc shows that the compareTo() method uses the
type of the class instead of the
Object type. However the ComparableWritable class does not take any
template and therefore it cannot
set the tempate for the class Comparable. Is that a mistake?
* 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));
* }
* }
If i try to do what it example shows it does not compile:
[javac] /Users/steph/Work/Rinera/TRUNK/vma/hadoop/parser-hadoop/
apps/src/com/rinera/hadoop/weblogs/SummarySQLKey.java:13:
com.rinera.hadoop.weblogs.SummarySQLKey is not abstract and does not
override abstract method compareTo(java.lang.Object) in
java.lang.Comparable
[javac] public class SummarySQLKey
If i don't use the type but instead use the Object type for compareTo()
i get a RuntimeException:
java.lang.RuntimeException: java.lang.InstantiationException:
com.rinera.hadoop.weblogs.SummarySQLKey
at
org.apache.hadoop.io.WritableComparator.newKey(WritableComparator.java:
75)
at
org.apache.hadoop.io.WritableComparator.<init>(WritableComparator.java:
63)
at
org.apache.hadoop.io.WritableComparator.get(WritableComparator.java:42)
at
org.apache.hadoop.mapred.JobConf.getOutputKeyComparator(JobConf.java:
645)
at org.apache.hadoop.mapred.MapTask
$MapOutputBuffer.<init>(MapTask.java:313)
at org.apache.hadoop.mapred.MapTask.run(MapTask.java:174)
at org.apache.hadoop.mapred.LocalJobRunner
$Job.run(LocalJobRunner.java:157)
Caused by: java.lang.InstantiationException:
com.rinera.hadoop.weblogs.SummarySQLKey
at java.lang.Class.newInstance0(Class.java:335)
at java.lang.Class.newInstance(Class.java:303)
at
org.apache.hadoop.io.WritableComparator.newKey(WritableComparator.java:
73)
... 6 more
