如何 map 列表<string>其中每个字符串都是逗号分隔的字符串到 List<string> 以逗号分隔加入？</string></string>

Question

tags -- ["tag1", "tag2, tag3"]

expected output -- ["tag1","tag2","tag3"]

Current code:

List<String> tagsProcessed = new ArrayList<>();
for(String t : tags) {
    tagsProcessed.addAll(Stream.of(t.split(",")).map(String::trim).collect(Collectors.toList()));
}

How to rewrite it with streams?

Answer 1

one solution of many:

var delimiter = Pattern.compile(",");
tags.stream()
    .flatMap(delimiter::splitAsStream)
    .map(String::trim)
    .forEach(tagsProcessed::add);

---

or, avoiding (hiding!) the trim call (but without removing white spaces at start and end of input strings):

var delimiter = Pattern.compile("\\s*,\\s*");
tags.stream()
    .flatMap(delimiter::splitAsStream)
    .forEach(tagsProcessed::add);

^{I prefer first one, clearly showing the call to trim}

---

all the above can also be changed to (directly) use the result of Collectors.toList() , but beware of kind of returned List . From documentation:

There are no guarantees on the type, mutability, serializability, or thread-safety of the List returned; if more control over the returned List is required, use toCollection(Supplier).

Using it with addAll is not a problem (but similar to using forEach as in above code); also not a problem if the returned list is not being changed.

Answer 2

Use flatMap , which can map one element to any number of elements, represented as a stream. In this case, we map each tag to the array of tags produced by split .

List<String> tagsProcessed = tags.stream()
    .flatMap(tag -> Arrays.stream(tag.split(", ")))
    .collect(Collectors.toList());

Answer 3

You can also use:

List<String> tagsProcessed = tags.stream()
        .map(s -> s.trim().split(",\\s*"))
        .flatMap(Arrays::stream)
        .collect(Collectors.toList());

Answer 4

List<String> tags = Arrays.asList("1 ,2   ,3 ,4  ", "5 ,6 ,7 ,8 ", "  9,  10");
List<String> tagsProcessed =
    tags.stream() //Stream the list of String as is
        .flatMap(tag -> Arrays.stream(tag.split(","))) // flatMap to avoid Stream<Stream<String>>, this produces Stream<String>
        .map(String::trim) //trim as in requirement
        .collect(Collectors.toList());