map/reduce 下ArrayWritable 的使用

在  map/reduce的开发中经常使用数组类型, 例如DoubleWritable ,当需要使用一组DoubleWritable的时候,

DoubleWritable[] doublewritable =new DoubleWritable[n]

但是DoubleWritable[] 这种形式的数组并不能作为value在map和reduce中传递. 实际上,map/reduce 的序列类型为ArrayWritable.

当我要使用DoubleWritable类型的数组作为value进行传递时,需要重写一个类继承于ArrayWritable

public class NewArrayWritable extends ArrayWritable{
	public NewArrayWritable() { 
	     super(DoubleWritable.class); 
	   } 

}

NewArrayWritable ntemp=new NewArrayWritable();

ntemp.set(ctemp);

job.setMapOutputValueClass(IntWritable.class);

job.setMapOutputValueClass(NewArrayWritable.class);

注意: 此时程序可能出现wrong value class: class org.apache.hadoop.io.IntWritable is not class org.apache.hadoop.examples.NewArrayWritable 

异常

解决方法: 注释掉主函数中job.setCombinerClass(IntSumReducer.class);

map/reduce 下ArrayWritable 的使用

原文：http://blog.csdn.net/ltianchao/article/details/18555159