private NullWritable cleI = NullWritable.get();
private VarianceWritable valeurI = new VarianceWritable();

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