2nd solution to Naresh Jain's challenge

gistfile1.java

int calculateAveragePreviousPercentageComplete() { return sumAndAverageTheValueOfField(PREVIOUS_PERCENTAGE_COMPLETE); }
int calculateAverageCurrentPercentageComplete() { return sumAndAverageTheValueOfField(CURRENT_PERCENTAGE_COMPLETE); }
int calculateAverageProgressPercentage() { return sumAndAverageTheValueOfField(PROGRESS_PERCENTAGE); }	

int sumAndAverageTheValueOfField( Functor<StudentActivityByAlbum, Integer> fieldGetter){
    return addAll(collect( activities, fieldGetter)) / activities.size(); }

<T> int addAll(Iterable<Integer> items){ return reduce( items, PLUS); }

static final Functor<StudentActivityByAlbum, Integer> PREVIOUS_PERCENTAGE_COMPLETE = new Functor<StudentActivityByAlbum, Integer>(){
    @Override public Integer execute(StudentActivityByAlbum value) { return value.getPreviousPercentageCompleted(); }};
static final Functor<StudentActivityByAlbum, Integer>  CURRENT_PERCENTAGE_COMPLETE =new Functor<StudentActivityByAlbum, Integer>(){
    @Override public Integer execute(StudentActivityByAlbum value) { return value.getPercentageCompleted(); }};
static final Functor<StudentActivityByAlbum, Integer>  PROGRESS_PERCENTAGE =new Functor<StudentActivityByAlbum, Integer>(){
    @Override public Integer execute(StudentActivityByAlbum value) { return value.getProgressPercentage(); }};	
    
static final Functor2<Integer, Integer, Integer> PLUS  = new Functor2<Integer, Integer, Integer>(){
        @Override public Integer execute(Integer accumulator, Integer value) { return accumulator + value; }};