Sorting list of objects by date property

Question

I have an object that contains two LocalDate properties:

public class SomeObject {
    private LocalDate startDate;
    private LocalDate endDate;
}

Constructor and stuff ommitted for brevity. I want to sort a list of these objects by their startdate and then assign the startdate of the next object to the previous object's enddate. To clarify, I start with a list of these objects:

SomeObject object1 = new SomeObject(LocalDate.parse("2015-01-01"), null);
SomeObject object2 = new SomeObject(LocalDate.parse("2014-01-01"), null);
SomeObject object3 = new SomeObject(LocalDate.parse("2016-01-01"), null);

List<SomeObject> list = Arrays.asList(object1, object2, object3);

And after sorting it should return this:

for (SomeObject object : list) {
    System.out.println(object.startDate.toString() + " " + object.endDate.toString() );
}

2014-01-01 2015-01-01
2015-01-01 2016-01-01
2016-01-01 null

Each list will only contain 3 or 4 of these objects at most, but the code might have to process tens of thousands of these lists, so I'm looking for an efficient way to do this.

Answer 1

You can use Collections.sort with a Comparator. In Java 8 with Lambdas it looks like this:

    Collections.sort(list, (x, y) -> x.startDate.compareTo(y.startDate));

    for (int i = 0; i < (list.size() - 1); i++) {
        list.get(i).endDate = list.get(i + 1).startDate;
    }