Create a collection of sub collection from a given Collection (Group By) in Java

Question

I have an array list of dates spanning for several years i.e. ArrayList<Date> dateList I have an object that represents a grid of months for each year i.e.

public class DateTileGrid
{
    private int mCurrentYear;
    private ArrayList<Date> mDateTiles;
        ... // GETTERS AND SETTERS
}

I want to create an ArrayList<DateTileGrid> that will contain the relevant dates for each year.

I know how to do it with loop (nasty nested loops) but I hope there is a cleaner way to achieve that.

I think @Murali method works best for you, as you need to add a year filter to this (unless you are able to filter the dates and group them at a lower level) — nsawaya
– nsawaya, Commented Aug 10, 2013 at 10:24

Murali · Accepted Answer · 2013-08-10 10:20:12Z

2

Probably this is what you need until java 8 closures comes

HashMap<Integer, DateTileGrid>  dataGridMap = new HashMap<>();
for (Date date: dateList) {
    int year = date.getYear(); //Deprecated.. use something better
    DateTileGrid dataGrid = dataGridMap.get(year);
    if(dataGrid == null){
        dataGrid = new DateTileGrid();
        dataGrid.setCurrentYear(year);
        dataGrid.setDateTiles(new ArrayList<Date>());
        dataGridMap.put(year, dataGrid);
    }
    dataGrid.getDateTiles().add(date);
}
//Here is your result
ArrayList<DateTileGrid> result = new ArrayList<>(dataGridMap.values());

answered Aug 10, 2013 at 10:20

Murali

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