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

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));

Answer 3

You can do this by using java 8 streams, but not in one line. Use two date patterns one for input and other for output
DateTimeFormatter inFormat = DateTimeFormatter.ofPattern("dd/M/yyyy");
DateTimeFormatter outFormat = DateTimeFormatter.ofPattern("yyyy/M/dd");
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);
//Since it is String array need lambda expression to update and return
List<String[]> result = date.stream().map(arr->{
arr[0]=LocalDate.parse(arr[0],inFormat).format(outFormat);
return arr;
}).collect(Collectors.toList());

Answer 4

Why use a stream?
DateTimeFormatter originalFormatter = DateTimeFormatter.ofPattern("d/M/u");
DateTimeFormatter wantedFormatter = DateTimeFormatter.ofPattern("u/M/d");
date.forEach(arr -> {
LocalDate ld = LocalDate.parse(arr[0], originalFormatter);
arr[0] = ld.format(wantedFormatter);
});
To inspect the result:
date.forEach(arr -> System.out.println(Arrays.toString(arr)));
Output:
[2018/2/12, a1, a2]
[2018/3/13, b1, b2]
[2018/4/14, c1, c2]
My code (as well as the code in some of the other answers) modifies your original arrays. Please decide if this is OK. If it is, you shouldn’t really use a stream since they are supposed to be free from side-effects. If you do need the original arrays to remain untouched, a stream is fine (using the same formatters as before):
List<String[]> newDateList = date.stream()
.map(arr -> {
LocalDate ld = LocalDate.parse(arr[0], originalFormatter);
String[] newArr = Arrays.copyOf(arr, arr.length);
newArr[0] = ld.format(wantedFormatter);
return newArr;
})
.collect(Collectors.toList());
newDateList.forEach(arr -> System.out.println(Arrays.toString(arr)));
Output is the same as before.