Lambda Java 8 - How to convert while streaming a String List to a Date List

Question

Is there a way to also convert (while streaming and collecting) a List of Strings to a new List of Dates?

My attempt:

I defined a SimpleDateFormat and tried to use that in the follwing code, but I can't get it working all together.

SimpleDateFormat simpleDateFormat = new SimpleDateFormat("dd.mm.yyyy");

ArrayList<Date> dateList = stringList.stream()
              .filter(s -> s.startsWith("<"))
              .collect(Collectors.toList());

Solution

Based on the hints to use map, I got a working solution for now that looks like this. Of course any optimization on this is welcomed!

int year = 2016;
int month = 2;
int day = 15;


final List<LocalDate> dateListEarlier = dateList.stream()
        .filter(s -> s.startsWith("<"))
        .map(s -> s.substring(1).trim())
        .map(s -> LocalDate.parse(s, formatter))
        .sorted()
        .filter(d -> d.isAfter(LocalDate.of(year, month, day)))
        .collect(Collectors.toList());

Might be a good idea to actually convert your String using this defined simpleDateFormat. You filter some stuff and that's all. Here are several examples about type conversion in streams. — Tom
– Tom, Commented Oct 14, 2016 at 15:53

Saravana · Accepted Answer · 2016-10-14 16:23:14Z

10

Since you are using Java8, you could utilize the new DateTime API

    List<String> dateStrings = Arrays.asList("12.10.2016", "13.10.2016", "14.10.2016");
    DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd.MM.yyyy");
    List<LocalDate> localDates = dateStrings.stream().map(date -> LocalDate.parse(date, formatter)).collect(Collectors.toList());
    System.out.println(localDates);

Ouptut

[2016-10-12, 2016-10-13, 2016-10-14]

answered Oct 14, 2016 at 16:23

Saravana

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1 Comment

crcr Over a year ago

why does this not work for the below list?

Tobb · Accepted Answer · 2016-10-14 16:11:31Z

2

Use the map method.

final SimpleDateFormat simpleDateFormat = new SimpleDateFormat("dd.mm.yyyy");

final List<Date> dateList = strings.stream()
                                   .filter(s -> s.startsWith("<"))
                                   .map(s -> { 
                                          try {
                                             simpleDateFormat.parse(s));
                                          } catch (final ParseException e) {
                                             throw new RuntimeException("Parse failed", e);
                                          }
                                    }).collect(Collectors.toList());

edited Oct 14, 2016 at 16:11

answered Oct 14, 2016 at 16:00

Tobb

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4 Comments

Andreas Over a year ago

Error 1: cannot convert from List<Date> to ArrayList<Date>. Error 2: Unhandled exception type ParseException

Andreas Over a year ago

Still won't compile.

Saravana Over a year ago

toList returns List of objects not ArrayList

Tobb Over a year ago

That's what I get for copying OP ;)

Darshan Mehta · Accepted Answer · 2016-10-14 23:04:10Z

0

All you need to do is to write a function that maps String to Date object, see below:

public static void main(String[] args) {
    SimpleDateFormat simpleDateFormat = new SimpleDateFormat("dd.MM.yyyy");
    List<String> stringList = new ArrayList<>();
    stringList.add("10.10.2014");

    Function<String, Date> function = new Function<String, Date>() {

        @Override
        public Date apply(String s) {
            try {
                return simpleDateFormat.parse(s);
            } catch (ParseException e) {
                throw new RuntimeException(e);
            }
        }
    };

    List<Date> collect = stringList.stream().map(function).collect(Collectors.<Date>toList());

    System.out.println(collect);
}

edited Oct 14, 2016 at 23:04

answered Oct 14, 2016 at 16:01

Darshan Mehta

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3 Comments

Andreas Over a year ago

return null when encountering bad data is likely a bad idea.

Andreas Over a year ago

Since you're using Java 8 features, is there any reason you're creating an anonymous class, rather than a lambda expression block?

Darshan Mehta Over a year ago

Corrected null. Also, there is no specific reason. I preferred an anonymous class just for the readability.

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