code might be less elegant if chained call of cast to type is present

[Kevin222004]

whenever some type casting has been done using .map

    private Set<String> getExternalResourceLocations() {
        return Stream.concat(fileSetChecks.stream(), filters.getFilters().stream())
            .filter(ExternalResourceHolder.class::isInstance)
            .map(ExternalResourceHolder.class::cast)
            .flatMap(resource -> resource.getExternalResourceLocations().stream())
            .collect(Collectors.toSet());
    }

 <mutation unstable="false"> 
   <sourceFile>Checker.java</sourceFile> 
   <mutatedClass>com.puppycrawl.tools.checkstyle.Checker</mutatedClass> 
   <mutatedMethod>getExternalResourceLocations</mutatedMethod> 
   <mutator>org.pitest.mutationtest.engine.gregor.mutators.experimental.NakedReceiverMutator</mutator> 
   <description>replaced call to java/util/stream/Stream::map with receiver</description> 
   <lineContent>.map(ExternalResourceHolder.class::cast)</lineContent> 
 </mutation> 

pitest will suggest the NakedReceiverMutator if we follow pitest in this case then .map() needs to be removed and if we do that then the code cannot be compilable.

we have faced this issue at checkstyle/checkstyle#13913 and checkstyle/checkstyle#13913.
as the solution we have done changes like

    private Set<String> getExternalResourceLocations() {
        return Stream.concat(fileSetChecks.stream(), filters.getFilters().stream())
            .filter(ExternalResourceHolder.class::isInstance)
            .flatMap(resource -> {
                return ((ExternalResourceHolder) resource)
                        .getExternalResourceLocations().stream();
            })
            .collect(Collectors.toSet());
    }