How can I use Java 8 streams to sort an ArrayList of objects by a primitive int member?

Question

Here is an example class. I know the simplest thing would be to change the members from primitive type int to object Integer and use stream/lambda/sorted, but there may be reasons to only have a primitive type int such as space. How could I use the streams API to sort a List<DateRange> by int member startRange?

List<DateRange> listToBeSorted = new ArrayList<DateRange>();


static private class DateRange
{
    private int startRange ;
    private int endRange ;
    public int getStartRange() {
        return startRange;
    }
    public void setStartRange(int startRange) {
        this.startRange = startRange;
    }
    public int getEndRange() {
        return endRange;
    }
    public void setEndRange(int endRange) {
        this.endRange = endRange;
    }
}

note compareTo will not work with primitive types. It requires an Object — user6811616, Sep 01 '18 at 13:50
there is at least a 16 byte overhead to having two Integers instead of two primitive int members — user6811616, Sep 01 '18 at 14:34

score 5 · Accepted Answer · answered Sep 01 '18 at 14:09

5

You may do it like so,

List<DateRange> sortedList = listToBeSorted.stream()
    .sorted(Comparator.comparingInt(DateRange::getStartRange))
    .collect(Collectors.toList());

answered Sep 01 '18 at 14:09

Ravindra Ranwala

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score 3 · Answer 2 · answered Sep 02 '18 at 03:22

3

I know you asked for a way to do it with streams, but if you are OK with sorting the original list in-place, you don't need streams for this. Just use the List.sort method:

listToBeSorted.sort(Comparator.comparingInt(DateRange::getStartRange));

answered Sep 02 '18 at 03:22

fps

33,623
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2

Much more efficient. – Boann Sep 02 '18 at 03:53
2

Both of these are helpful thanks. In the case above I purposely made a copy of the List because I am iterating through the sorted list while modifying the original list. – user6811616 Sep 03 '18 at 13:44

How can I use Java 8 streams to sort an ArrayList of objects by a primitive int member?

2 Answers2