要点：
类型比较在hadoop的mapreduce中非常重要，主要用来比较keys;
hadoop中的RawComparator<T>接口继承自java的comparator, 主要用来比较序列化的objects;
- hadoop中的WritableComparator class更全面，提供了两种主要的比较方法，一种是直接比较object，另一种是较serialized representations;
举例来说 比较object: compare(new IntWritable(21), new IntWritable(998)); 比较serialized representations： compare(serialize(new IntWritable(21)), serialize(new IntWritable(998))),
提示：继承关系
1.org.apache.hadoop.io
Interface RawComparator<T>
//description
public interface RawComparator<T>
extends Comparator<T>
//method
int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2)
2.org.apache.hadoop.io
Interface WritableComparable<T>
//description
public interface WritableComparable<T>
extends Writable, Comparable<T>
//method
Methods inherited from interface org.apache.hadoop.io.Writable
readFields, write
3.java.lang.Object
|__ org.apache.hadoop.io.WritableComparator
//description
public class WritableComparator
extends Object
implements RawComparator
//methods
int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2)
int compare(Object a, Object b)
int compare(WritableComparable a, WritableComparable b)
static int compareBytes(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2)
4.java.util
Interface Comparator<T>
//description
public interface Comparator<T>
//methods
int compare(T o1, T o2)
boolean equals(Object obj)
代码：
import java.lang.Byte;
import java.io.DataOutputStream;
import java.io.ByteArrayOutputStream;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.WritableComparator;
import org.apache.hadoop.io.RawComparator;
public class MyIntWritableComparactor {
public static byte[] serialize(IntWritable writable) throws Exception {
ByteArrayOutputStream out = new ByteArrayOutputStream();
DataOutputStream dataOut = new DataOutputStream(out);
writable.write(dataOut);
dataOut.close();
return out.toByteArray();
}
@SuppressWarnings("unchecked")
public static void main(String[] args) throws Exception {
RawComparator<IntWritable> comparator = WritableComparator.get(IntWritable.class);
IntWritable w1 = new IntWritable(13);
IntWritable w2 = new IntWritable(12);
System.out.println("w1: " + w1 + " w2: " + w2);
System.out.println("w1 compare w2 : " + comparator.compare(w1,w2));
byte[] b1 = serialize(w1);
byte[] b2 = serialize(w2);
System.out.println("b1.length: " + b1.length);
System.out.println("b2.length: " + b2.length);
System.out.println("b1.length compare b2.length: " + comparator.compare(b1, 0, b1.length, b2, 0, b2.length));
}
}
编译，运行：
//注意我用的是hadoop2.2 $ source $YARN_HOME/libexec/hadoop-config.sh $ mkdir myclass $ javac -d myclass MyIntWritableCompare.java $ jar -cvf mycompare.jar -C myclass ./ $ export HADOOP_CLASSPATH=$CLASSPATH:mycompare.jar $ yarn MyIntWritableCompare
输出：
$ yarn jar text.jar Text w1: w2: w1 compare w2 : b1.length: b2.length: b1.length compare b2.length: