Is there a way to change dateFormat with java 8 stream?

Question

I want to change date format from "dd/mm/yyyy" to "yyyy/mm/dd" with one line in java8 stream

List<String[]> date = new ArrayList<>();

String[] a= {"12/2/2018","a1","a2"};
String[] b= {"13/3/2018","b1","b2"};
String[] c= {"14/4/2018","c1","c2"};

date.add(a)`
date.add(b);
date.add(c);

I expect the output is

{{"2018/2/12","a1","a2"},{"2018/2/13","b1","b2"},{"2018/2/14","c1","c2"}}

Answer 1

I hope you mean yyyy/MM/dd coz m is for minutes and M for month...

consider a Map from the stream API

public static void main(String[] args) {
    List<String[]> date = new ArrayList<>();
    String[] a= {"12/2/2018","a1","a2"};
    String[] b= {"13/3/2018","b1","b2"};
    String[] c= {"14/4/2018","c1","c2"};

    date.add(a);
    date.add(b);
    date.add(c);


    List<String[]> even = date.stream().map(
            s -> {
                DateTimeFormatter formatter = DateTimeFormatter.ofPattern("d/M/yyyy");
                LocalDate localDate = LocalDate.parse(s[0], formatter);
                DateTimeFormatter formatterNew = DateTimeFormatter.ofPattern("yyyy/MM/dd");
                return new String[]{formatterNew.format(localDate), s[1],s[2]};
            }
    ).collect(Collectors.toList());
    even.forEach(x-> System.out.println(Arrays.toString(x)));
}

that will print out

[2018/02/12, a1, a2]

[2018/03/13, b1, b2]

[2018/04/14, c1, c2]

Answer 2

You can not do this without iterating over all items.

For your simple case dd/mm/yyyy to yyyy/mm/dd you can just use this:

date.forEach(i -> {
    String[] parts = i[0].split("/");
    i[0] = parts[2] + "/" + parts[1] + "/" + parts[0];
});

Using java time api you can use this:

DateTimeFormatter toFormat = DateTimeFormatter.ofPattern("yyyy/M/d");
DateTimeFormatter fromFormat = DateTimeFormatter.ofPattern("d/M/yyyy");
date.forEach(i -> i[0] = LocalDate.parse(i[0], fromFormat).format(toFormat));