Sort mapreduce output keys in descending order

Add the following class to your current class

public static class ReverseComparator extends WritableComparator {
    
    private static final Text.Comparator TEXT_COMPARATOR = new Text.Comparator();
    public ReverseComparator() {
        super(Text.class);
    }

    @Override
    public int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2) {
       return (-1)* TEXT_COMPARATOR.compare(b1, s1, l1, b2, s2, l2);
    }

    @SuppressWarnings("rawtypes")
    @Override
    public int compare(WritableComparable a, WritableComparable b) {
        if (a instanceof Text && b instanceof Text) {
                return (-1)*(((Text) a).compareTo((Text) b));
        }
        return super.compare(a, b);
    }
}

in new api(mapreduce) add the following to your configuration.

Job.setSortComparator(ReverseComparator.class);

NB. This only works if your key belongs to Text.class else modify the reverse comparator class accordingly