How to loop through stream groups in Java to perform operations on Strings in each group

Question

I have a sorted ArrayList A and used streams to group by the substring(3,7). I'm using a large dataset, so I don't know all the different substring(3,7) there are.

ArrayList A for example looks something like this (but with a lot more data): ooo122ppp, aaa122b333, zzz122bmmm, ccc9o9i333, mmm9o9i111, qqqQmQm888, 777QmQmlll, vvvjjj1sss

I need to loop through each group so that I can do something to that grouped data. I've tried for loops, if statements, etc, but can't figure it out. I've tried this, but I get an error regarding the for loop. How am I able to loop through each group I have to perform operations on the Strings in the group?

Collection<List<String>> grouped = A.stream().collect(groupingBy(ex -> ex.substring(3,7))).values();
for(int g=0; g<grouped.forEach(); g++) {
   //do something 
}

Answer 1

You can use the forEach method on Collection . It should not be confused with a regular for loop.

grouped.forEach(group -> {
   group.forEach(str -> {
      //do something
   });
});

Answer 2

Are you looking for something like this?

    List<String> stringList = List.of("ooo122ppp", "aaa122b333", "zzz122bmmm", "ccc9o9i333", "mmm9o9i111", "qqqQmQm888", "777QmQmlll", "vvvjjj1sss");
    Map<String, List<String>> collection = stringList.stream().collect(Collectors.groupingBy(ex -> ex.substring(3, 7)));
    for (Map.Entry<String, List<String>> entry : collection.entrySet()) {
        System.out.println("group <" + entry.getKey() + "> strings " + entry.getValue());
    }

Output

group <jjj1> strings [vvvjjj1sss]
group <122b> strings [aaa122b333, zzz122bmmm]
group <122p> strings [ooo122ppp]
group <9o9i> strings [ccc9o9i333, mmm9o9i111]
group <QmQm> strings [qqqQmQm888, 777QmQmlll]

Otherwise please try to better explain the requirement;)