GroupingComparator

GroupingComparator.java

public class GroupingComparator implements RawComparator<PairOfStringInt>{

	private static final Text.Comparator TEXT_COMPARATOR = new Text.Comparator();
	
	@Override
	public int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2) {
		return TEXT_COMPARATOR.compare(b1, s1, l1, b2, s2, l2);
/*
		int firstL1;;
		int firstL2;
		try{
			
			firstL1 = WritableUtils.decodeVIntSize(b1[s1]) + WritableComparator.readVInt(b1, s1);
			firstL2 = WritableUtils.decodeVIntSize(b2[s2]) + WritableComparator.readVInt(b2, s2);
			
			return TEXT_COMPARATOR.compare(b1, s1, firstL1, b2, s2, firstL2);
			
		}
		catch (IOException e) {
			throw new RuntimeException(e);
		}
*/		
		
	}
	
	@Override
	public int compare(PairOfStringInt p1, PairOfStringInt p2) {
		
		return p1.getLeftElement().compareTo(p2.getLeftElement());
		
	}

Your value grouping comparator seems right for the apples example you gave on the ML, but you need your Partitioner to support it by looking up on the same part of your composite key.