Filter an Arraylist in different Arrays

Question

I want to filter a single array into parts. Suppose I have an ArrayList of String :

 Array1  = ["2015-01-06","2015-04-06",
    "2016-06-06","2016-01-06",
    "2016-05-06","2017-02-06",
    "2017-04-06","2017-03-06",
    "2015-03-06","2016-04-06",
    "2016-02-06","2015-05-06",
    "2015-01-06","2016-06-06"]

I want it to filter in new arrays according to their years, so that the output will look like :

arrayA = ["2015-01-06","2015-04-06","2015-03-06","2015-05-06","2015-01-06"]
arrayB = ["2016-06-06","2016-01-06","2016-05-06","2016-04-06","2016-02-06","2016-06-06"]
arrayC = [""2017-02-06","2017-04-06","2017-03-06""]

The arrays are based on years now. I don't know the proper way for accomplishing this task.

hey @KevinEsche thanks a lot man for responding , yeah it'll be much better but am getting the in this pattern from my source — remy boys
– remy boys, Commented Mar 10, 2016 at 14:32
Why Map<String, String[]> instead of Map<String, List<String>? — beresfordt
– beresfordt, Commented Mar 10, 2016 at 14:33
You'd need to parse the individual array elements to some degree (if year is always 4 digit and at the start, a substring might suffice) and then add the element to a list. Use a map for the individual lists as already suggested. — Thomas
– Thomas, Commented Mar 10, 2016 at 14:37

Tunaki · Accepted Answer · 2016-03-10 16:33:19Z

10

The question was edited to include the Android tag, making this inapplicable for the OP since Android doesn't support Java 8 yet, I'll leave it in case it helps someone not using Android though.

Using the Stream API, you can easily build a Map<Integer, List<String>> where each key is a year. You can use groupingBy(classifier) where the classifier extracts the year from the date. In this case, the date is parsed with LocalDate.parse but you could easily extend that by giving your own formatter.

public static void main(String[] args) {
    String[] array = {"2015-01-06","2015-04-06",
                      "2016-06-06","2016-01-06",
                      "2016-05-06","2017-02-06",
                      "2017-04-06","2017-03-06",
                      "2015-03-06","2016-04-06",
                      "2016-02-06","2015-05-06",
                      "2015-01-06","2016-06-06"};

    Map<Integer, List<String>> map =
        Arrays.stream(array)
              .collect(Collectors.groupingBy(s -> LocalDate.parse(s).getYear()));
              // or a simple s -> Integer.valueOf(s.substring(0, s.indexOf('-'))) in this case
}

You can then access the list you want from that map.

edited Mar 10, 2016 at 16:33

answered Mar 10, 2016 at 14:41

Tunaki

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5 Comments

remy boys Over a year ago

hey @Tunaki thanks a lot for responding man , wasn't expected that much fast and so many responses i tried your answer and am getting some unresolved method exceptions in and in Collectors , Arrays.stream and in index.of i know that i have to use Stream API but i never used it before i tried to find some getting started but didn't get what i'm expecting ? is it what i need for using Stream Api ? github.com/aNNiMON/Android-Java-8-Stream-Example

Tunaki Over a year ago

@remyboys It's an API that is included in Java 8. It's built-in, you don't need another library.

remy boys Over a year ago

okay but i tried to rebuild my project so many times still android studio is not recognising it :( , and my java version is already updated

A_Di-Matteo Over a year ago

but Android doesn't support Java 8, that's why you can't use it I suppose

Tunaki Over a year ago

@remyboys Android doesn't support Java 8 so, yes, it won't work. You should have added the android tag from the start. I edited to reflect that.

Gianni B. · Accepted Answer · 2016-03-10 14:40:55Z

4

This will create an ArrayList each year

ArrayList<String> array1 = new ArrayList<String>();
Map<String, List<String>> mapForYear = new TreeMap<String, List<String>>();
for(String date : array1)
{
    String year = date.substring(0,4);
    if(!mapForYear.containsKey(year))
        mapForYear.put(year, new ArrayList<String>());
    mapForYear.get(year).add(date);
}

answered Mar 10, 2016 at 14:40

Gianni B.

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2 Comments

remy boys Over a year ago

as a suggestion better use HashMap instead of TreeMap :)

Gianni B. Over a year ago

I've used TreeMap because this will garantee you that your Map is sorted by year. If you don't need that you can use HashMap as you want ;)

Andrew · Accepted Answer · 2016-03-10 15:20:40Z

0

That's easy if you use java 8 and Stream API. I wrote the following piece of code:

public List<String> getListByYear(String[] array, int year) {
    return Arrays.stream(array)
                 .filter(s -> s.contains(Integer.toString(year)))
                 .collect(Collectors.toList());
}

As to the question, there is such solution:

for (String s : array) {
    int key = LocalDate.parse(s).getYear();
    List<String> value = map.get(key);
    map.put(key, value == null
             ? new ArrayList<String>() { { add(s); } }
             : new ArrayList<String>(value) { { add(s); } }
    );
}

The same result as @Tunaki provided, but a less pretty. If Java 8 is not supported, it will be as the alternative as the most compact solution.

Output is here:

{
    2016 = [2016-06-06, 2016-01-06, 2016-05-06, 2016-04-06, 2016-02-06, 2016-06-06], 
    2017 = [2017-02-06, 2017-04-06, 2017-03-06],
    2015 = [2015-01-06, 2015-04-06, 2015-03-06, 2015-05-06, 2015-01-06]
}

edited Mar 10, 2016 at 15:20

answered Mar 10, 2016 at 14:37

Andrew

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2 Comments

GhostCat Over a year ago

Not sure how helpful this is. He doesn't want to specify a year - he wants to build a data structures that is already "sliced" for the different years.

remy boys Over a year ago

@Jägermeister yeah man thats true , cause i can't predict the year it could be anything

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