Re: ArrayList grouping?

"Matt Humphrey" <>
Thu, 21 May 2009 08:57:16 -0400
"" <> wrote in message

How can I group values in ArrayList

01.01.2009 , 1 , 10000
02.01.2009, 1, 20000
01.01.2009, 2, 30000

and I want to transform this to two new lists so I can have in the first
arrayList grouped values by distinct date, and third row summed;

01.01.2009, 40000
02.01.2009, 20000

and second list distinct date+no and third row summed;

01.01.2009 , 1, 10000
01.01.2009, 2, 30000
02.01.2009, 1 , 20000

How can i do this transformation ?

If you're asking how to do this only for the example provided, it's fairly
straightforward, but not necessarily too different from the general-purpose
form. I think, though, that you're asking how to do it in general so that
the code does not know anything about the source object. Such a structure
is called "pivot table" and there may be some free software you can use,
although the examples I found involved a heavyweight Java spreadsheet tool
that happened to include pivot tables.

If I really had to do this from scratch, I would define a builder class that
accepts zero or more accessors whose values must be distinct and then a
Number-type accessor method which will be summed. The output would be a
list of DistinctSum objects that each contain the source values (whose
accessor values all match) as well as a sum value. As a design, something

public interface Accessor <T,S> {
  S getValue (T);

public class PivotBuilder <T> {
  public void addKeyAccessor (Accessor <T,?> key);
  // The only requirement on value distinction is that equals is meaningful

  public void setSumAccessor (Accessor <T,? extends Number> sum);

  public List <DistinctSum<? extends T>> compute (List <? extends T>

public class DistinctSum <T> {
  public Set <? extends T> getSources ();
  // Gets summed sources which have matching key values
  public double getSum ();

My use of generics is weak so I'm not sure if I've got it all right. The
sum objects will always have at least one source value--applying the
original accessors will return the distinct source values. In the case of
zero key accessors, there is one sum that accomodates all values. In the
case of no values there are no sums.

You would use it something like:

PivotBuilder pb = new PivotBuilder <Example> ();
pb.addKeyAccessor (new Accessor <Example,Date> () {
  public Date getValue (Example e) {
    return e.getDate ();
pb.addKeyAccessor (new Accessor <Example,Integer> () {
  public Integer getValue (Example e) {
    return e.getItemNumber ();
pb.addSumAccessor (new Accessor <Example,Double> () {
  public Double getValue (Example e) {
    return new Double (e.getValue ());
List <Example> sourceList = ....
List <DistinctSum<Example>> result = pb.compute (sourceList);

Matt Humphrey

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