public class MedianeLongueurLignesEtape2Mapper
    extends Mapper<IntWritable, LongWritable,
                   NullWritable, LongWritable>
{
    // même clé pour tout le monde
    private NullWritable cleI = NullWritable.get();

    @Override
    public void map(IntWritable cleE, LongWritable valeurE, Context context)
            throws IOException, InterruptedException
    {
        context.write(cleI, valeurE);
    }
}