Mapper output doubled in combiner

Question

The mapper outputs the value only once. But when i checked the combiner is having the value twice. This is really strange to me.

Pls help.

Module of map code from where output is sent:

for(int i = 0; i<alcmapi.size(); i++)
{
    int section = 0;

    for(int j = 0; j<alcmapj.size(); j++)
    {
        if(!alcmapi.get(i).getid().equals(alcmapj.get(j).getid()))
        {
            int fi = Integer.parseInt(alcmapi.get(i).getField());
            int fj = Integer.parseInt(alcmapj.get(j).getField());

            ArrayList<CustomMap> al = new ArrayList<CustomMap>();
            al.add(new CustomMap(alcmapi.get(i).getid(), fi));
            al.add(new CustomMap(alcmapj.get(j).getid(), fj));


            if(fi<fj)
            {
                section = fi;
            }
            else
            {
                section = fj;
            }
            Collections.sort(al);
            {
                output.collect(new Text(t+" "+al.get(0).getid()), new Text(al.get(1).getid()+"  "+section));         

            }
        }
    }
}

In combiner:

I see that the value corresponding to a key appears doubled when I check here:

while(values.hasNext()){
    String val = values.next().toString();
    System.out.println("val:"+val);
}

Thanks in advance!

Answer 1

Looks like you test all elements against all other elements. For example, if elements 3 and 5 meet the criteria then you'd write out the pair once for i=3, j=5 and once again for i=5, j=3.

I suggest you change your inner loop to:

for(int j = i+1; j<alcmapj.size(); j++)