List<String> to Map<String,String> with stream, with String splitting

Question

I have List with Strings:

List<String> cookiesUpdate = Arrays.asList("A=2" , "B=3");

I want to convert it to Map:

{
  "A": "2",
  "B": "3"
}

Code:

Map<String, String> cookies = cookiesUpdate.stream()
     .collect(Collectors.toMap(String::toString, String::toString));

How to write those splitters above? If compiler thinks key String is Object.

.split("=")[0];
.split("=")[1];

Nowhere Man · Accepted Answer · 2021-03-26 21:43:47Z

6

Split should be done by "=" (or "\\s*=\\s" to exclude whitespaces around =)

Update Also it is better to provide limit argument to String::split to split at the first occurrence of "=", thanks @AndrewF for suggestion!

Fix toMap collector to use the first element of the array as key and the last as the value; a merge function may be needed if several namesake cookies are possible

Map<String, String> map = cookies.stream()
    .map(ck -> ck.split("\\s*=\\s*", 2)) // Stream<String[]>
    .filter(arr -> arr.length > 1) // ignore invalid cookies
    .collect(Collectors.toMap(arr -> arr[0], arr -> arr[1], (v1, v2) -> v1));

If there are multiple cookies with the same name, it may be worth to collect them into Set<String> thus keeping all unique values. For this, Collectors.groupingBy and Collectors.mapping:

Map<String, Set<String>> map2 = cookies.stream()
    .map(ck -> ck.split("\\s*=\\s*", 2)) // Stream<String[]>
    .filter(arr -> arr.length > 1) // ignore invalid cookies
    .collect(Collectors.groupingBy(
        arr -> arr[0], 
        Collectors.mapping(arr -> arr[1], Collectors.toSet())
    ));

edited Mar 26, 2021 at 21:43

answered Mar 26, 2021 at 18:58

Nowhere Man

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3 Comments

AndrewF Over a year ago

Consider String.split( /* ... */ , 2) so that any equals signs in the value will not result in an array with length > 2 and truncated value in the final result.

user4910279 Over a year ago

String.split("\\s*=\\s*", 2) will result in a compilation error Cannot make a static reference to the non-static method split(String, int) from the type String. And you don't need , (v1, v2) -> v1.

Nowhere Man Over a year ago

@saka1029, you're right, it should be ck.split, I fixed that. The merge function may be needed to handle possible duplicates of cookie names to prevent failing after java.lang.IllegalStateException: Duplicate key a (attempted merging values b and c) if the test data are: List<String> cookies = Arrays.asList("a=b", "a=c", "p=qqq");

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