[FIXED] Compilation error creating HttpHeaders using Collectors.toMap on the HttpServletRequest's headerNames Enumeration

Issue

I'm trying to copy all existing headers from an HttpServletRequest to a Spring HttpHeaders object to use with RestTemplate. This can be done easily in a loop on enumeration, but I'm getting this error while using streams:

HttpHeaders headers = new HttpHeaders();
Enumeration<String> existingHeaders = request.getHeaderNames();

headers.putAll(
 Collections.list(existingHeaders)
.stream()
.collect(
Collectors.toMap(Function.identity(),HttpServletRequest::getHeader))
);

I declared the variable Enumeration<String> for the stream to not consider elements as Object but I'm still getting this compilation error at collect():

The method toMap(Function<? super T,? extends K>, Function<? super T,? extends U>) 
in the type Collectors is not applicable for the arguments
(Function<Object,Object>, HttpServletRequest::getHeader)

Solution

The reason your code does not compile is that its Collectors.toMap(...) call is producing a Map<String, String>, but headers.putAll(...) requires a Map<String, List<String>>. This can be fixed by changing the Collectors.toMap(...) call to produce a compatible map:

HttpHeaders headers = new HttpHeaders();
Enumeration<String> existingHeaders = request.getHeaderNames();

headers.putAll(Collections.list(existingHeaders)
        .stream()
        .collect(Collectors.toMap(Function.identity(), 
                name -> Collections.list(request.getHeaders(name)))));

Since the same HTTP header can have multiple values, HttpHeaders implements Map<String,List<String>> rather than Map<String,String>. Therefore, putAll(map) requires a map with List<String> values, not one with String values.

Answered By - M. Justin