http://d.hatena.ne.jp/irof/20111203/p1

build.sbt

libraryDependencies ++= Seq(
  "org.functionaljava" % "functionaljava" % "3.0"
)

functionalJavaばーじょん.java

import fj.*;
import fj.data.List;
import fj.function.Integers;

public final class Main{
  static final class Data {
    Data(final String code,final String name,final int value){
      this.code  = code;
      this.name  = name;
      this.value = value;
    }
    final String code;
    final String name;
    final int value;
    @Override public final String toString(){
      return code + " " + name + " " + value;
    }
  }

  static final List<Data> dataList = List.list(
    new Data("A01","hoge",100),
    new Data("A01","piyo",200),
    new Data("A02","hoge",300),
    new Data("A03","hoge",400),
    new Data("A03","piyo",500)
  );

  static final List<Data> summary(final List<Data> list) {
    return list.group(
      Equal.equal(
        new F<Data,F<Data,Boolean>>(){
          @Override public F<Data,Boolean> f(final Data a){
            return new F<Data,Boolean>(){
              @Override public Boolean f(final Data b){
                return a.code.equals(b.code);
              }
            };
          }
        }
      )
    ).map(
      new F<List<Data>,Data>(){
        @Override public Data f(final List<Data> a){
          final Integer s = Integers.sum(
            a.map(
              new F<Data,Integer>(){
                @Override public Integer f(final Data b){
                  return b.value;
                }
              }
            )
          );
          return new Data(a.head().code,a.head().name,s);
        };
      }
    );
  }

  public static void main(String[] args){
    summary(dataList).foreach(
      new F<Data,Unit>(){
        @Override public Unit f(final Data a){
          System.out.println(a);
          return Unit.unit();
        }
      }
    );
  }

}

scalaばーじょん.scala

Welcome to Scala version 2.9.1.final (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_29).
Type in expressions to have them evaluated.
Type :help for more information.

scala> case class Data(code:String,name:String,value:Int)
defined class Data

scala> val dataList = List(
     |   Data("A01","hoge",100),
     |   Data("A01","piyo",200),
     |   Data("A02","hoge",300),
     |   Data("A03","hoge",400),
     |   Data("A03","piyo",500)
     | )
dataList: List[Data] = List(Data(A01,hoge,100), Data(A01,piyo,200), Data(A02,hoge,300), Data(A03,hoge,400), Data(A03,piyo,500))

scala> dataList.groupBy(_.code).map{case (_,v) => v.head.copy(value = v.map{_.value}.sum) }
res0: scala.collection.immutable.Map[String,Int] = Map(A03 -> 900, A01 -> 300, A02 -> 300)