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--- courses:mapreduce-tutorial:step-29 [2012/02/05 18:23]
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+++ courses:mapreduce-tutorial:step-29 [2012/02/05 18:49]
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@@ Line 1: / Line 1: @@
 ====== MapReduce Tutorial : Custom sorting and grouping comparators. ======
-====== Sorting comparator ======
+====== Fast sorting comparator ======
 The keys are sorted before processed by a reducer, using a
-[[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/io/RawComparator.html|Raw comparator]].
+[[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/io/RawComparator.html|Raw comparator]]. The default comparator uses the [[compareTo]] method provided by the key type, which is a subclass of [[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/io/WritableComparable.html|WritableComparable]]. Consider for example the following ''IntPair'' type:
+<code java>
+public static class IntPair implements WritableComparable<IntPair> {
+  private int first = 0;
+  private int second = 0;
+  public void set(int left, int right) { first = left; second = right; }
+  public int getFirst() { return first; }
+  public int getSecond() { return second; }
+  public void readFields(DataInput in) throws IOException {
+    first = in.readInt();
+    second = in.readInt();
+  }
+  public void write(DataOutput out) throws IOException {
+    out.writeInt(first);
+    out.writeInt(second);
+  }
+  public int compareTo(IntPair o) {
+    if (first != o.first) return first < o.first ? -1 : 1;
+    else return second < o.second ? -1 : second == o.second ? 0 : 1;
+  }
+}
+</code>
+If we would like in a Hadoop job to sort the ''IntPair'' using the first element only, we can provide a ''RawComparator'' and set it using [[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/mapreduce/Job.html#setSortComparatorClass(java.lang.Class)|job.setSortComparatorClass]]:
 ====== Grouping comparator ======

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