其实很简单，看看这个：

public class YourClass extends EvalFunc<String>{

    @Override
    public String exec(Tuple input) throws IOException {

        DataBag bag = (DataBag)input.get(0);

        Tuple categoryCode = null;

        //Keep the count of every cell in the 
        Tuple auxiliary = TupleFactory.getInstance().newTuple(3);

        int i = 0;
        for(Iterator<Tuple> code=bag.iterator(); code.hasNext();) {
            categoryCode=code.next();
            //You can use append if don't know from the very beginning
            //the size of tuple
            auxiliary.set(i, categoryCode.get(0).toString());
            i+=1;
        }

        return auxiliary.toDelimitedString(",");
    }   
}

最好使用辅助元组来简化操作，然后只使用instance方法 toDelimitedString() ，非常简单。