Csdrhrt

Is there a way to collect both matching and not matching elements of stream in one processing?
Take this example:

final List<Integer> numbers = Arrays.asList( 1, 2, 3, 4, 5 );

final List<Integer> even = numbers.stream().filter( n -> n % 2 == 0 ).collect( Collectors.toList() );

final List<Integer> odd = numbers.stream().filter( n -> n % 2 != 0 ).collect( Collectors.toList() );

Is there a way to avoid running through the list of numbers twice? Something like "collector for matches and collector for no-matches"?

You may do it like so,

Map<Boolean, List<Integer>> oddAndEvenMap = numbers.stream()

        .collect(Collectors.partitioningBy(n -> n % 2 == 0));

final List<Integer> even = oddAndEvenMap.get(true);

final List<Integer> odd = oddAndEvenMap.get(false);

If you have more than 2 groups (instead of odd and even here using %2) for example to group ints in remainder classes %3 you can use a Function:

Function<Integer, Integer> fun =  i -> i%3;

List<Integer> a = Arrays.asList(1,2,3,4,5,6,7,8,9,10);

Map<Integer, List<Integer>> collect = a.stream().collect(Collectors.groupingBy(fun));



System.out.println(collect);

//{0=[3, 6, 9], 1=[1, 4, 7, 10], 2=[2, 5, 8]}

Or imagine you have a list of strings which you want to group by starting char
instead of grouping matching and non-matching (for e.g. starts with a or not)
you could do something like :

Function<String, Character> fun =  s -> s.charAt(0);

List<String> a = Arrays.asList("baz","buzz","azz","ayy","foo","doo");

Map<Character, List<String>> collect = a.stream().collect(Collectors.groupingBy(fun));



System.out.println(collect);

//{a=[azz, ayy], b=[baz, buzz], d=[doo], f=[foo]}