Hadoop 2.2映射文件创建失败

我使用Hadoop 2.2

我的代码创建了一个映射文件编写器，如下所示

Configuration conf = new Configuration();  
Path path = new Path(filename);  
writer = new MapFile.Writer(conf,path,  
    SequenceFile.Writer.keyClass(key.getClass()),
    SequenceFile.Writer.valueClass(value.getClass()),
    SequenceFile.Writer.compression(SequenceFile.CompressionType.NONE));

public class ReportingKeyWritable extends BinaryComparable implements
        WritableComparable<BinaryComparable> {

public class ReportingValueWritable implements Writable

我的密钥类声明如下

Configuration conf = new Configuration();  
Path path = new Path(filename);  
writer = new MapFile.Writer(conf,path,  
    SequenceFile.Writer.keyClass(key.getClass()),
    SequenceFile.Writer.valueClass(value.getClass()),
    SequenceFile.Writer.compression(SequenceFile.CompressionType.NONE));

public class ReportingKeyWritable extends BinaryComparable implements
        WritableComparable<BinaryComparable> {

public class ReportingValueWritable implements Writable

构造函数失败，出现以下异常

java.lang.IllegalArgumentException:必须设置key类或comparator选项
位于org.apache.hadoop.io.MapFile$Writer.（MapFile.java:246）
位于DataLoader.main（DataLoader.java:66）

它可以正常工作：

writer = new MapFile.Writer(conf,path,  
MapFile.Writer.keyClass(key.getClass()),
MapFile.Writer.valueClass(value.getClass()),
MapFile.Writer.compression(SequenceFile.CompressionType.NONE));