Assign all values in a Set<String> to a Map<String, String> with streams

Question

I have a list1 containing different strings which start with a string from another list (fooBarList).

List<String> list1 = Arrays.asList("FOO1234", "FOO1111", "BAR1", "BARRRRR");
List<String> fooBarList = Array.asList("FOO", "BAR");

I would like to create a Hashmap<String, List<String>> hm which seperates the strings from the list1 depending on what they start with.

Result should look like this:

{FOO=["FOO1234",FOO1111"], BAR=["BAR1", "BARRRRR"]}

the fooBarList defines the different keys.

how do I achieve this with the help of streams? I just don't know how to do the step where I am basically saying: FOO1234 starts with FOO so check if key FOO exists and add it to the list else create a new list. ( I imagine I have to iterate over the fooBarList in the within the stream which just seems wrong to me )

/edit: sure with only 2 values in the fooBarList I could do simple checks if the string starts with "foo" then do this else do that. But what if my list contains 100s of strings.

Answer 1

A simple Collectors::groupingBy should do the trick:

Map<String, List<String>> prefixGrouping = list1.stream()
    .collect(
        Collectors.groupingBy(
            item -> fooBarList.stream().filter(item::startsWith).findFirst().orElse("")
        )
    );

---

Input: list1=["FOO1234", "FOO1111", "BAR1", "BARRRRR", "LOL"], fooBarList=["FOO", "BAR"]

Output: {""=["LOL"], "BAR"=["BAR1", "BARRRRR"], "FOO"=["FOO1234", "FOO1111"]}

---

You're looking for the prefix in the grouping function and if there is no prefix matching, then it will go into the emtpy String bin.