Hadoop: LongWritable cannot be cast to org.apache.hadoop.io.IntWritable

写MR Job的时候遇到一个坑爹的异常：

LongWritable cannot be cast to org.apache.hadoop.io.IntWritable

当写Map的时候，key的默认输入就是LongWritable。

因为LongWritable指代Block中的数据偏移量。

所以把它强行转换成Text当然就Error了。。

public static class TempMapper extends Mapper<LongWritable, Text, IntWritable, FloatWritable>{

  @Override
  protected void map(<span style="background-color: #ffff00;">LongWritable key</span>, Text value, Context context)
                throws IOException, InterruptedException {
       //code for getting date and temperature
       String temp = columns.get(3);
       context.write(new IntWritable(year), new FloatWritable(Float.valueOf(temp)));
  }
}

坑啊。。