在java中处理流时记录警告消息

Question

I want to log error messages if the map rdsDesksByName.get(e.getKey()) does not have any value for the key. How can I handle this within streams?

Currently I return createDeskWithScope(rdsDesksByName.get(e.getKey()),ds.outOfScope()) , but how can I enhance this function to log an error message if rdsDesksByName does not have any value for the key, otherwise it should process as usual

List<Desk> desks = deskScopesByName.entrySet()
    .stream()
    .flatMap(e -> {
        return deskScopesByName.get(e.getKey())
            .stream()
            .map(ds -> {
                return createDeskWithScope(rdsDesksByName.get(e.getKey()), ds.outOfScope());
            });                     
    })
   .collect(Collectors.toList());

Answer 1

In the flatmapping operation, find an object by the key and use the if-else condition:

• If the object is null (no value for such key), log a message and return Stream.empty() .
• Otherwise, continue processing using createDeskWithScope method.

Here is how the code might look like (untested, but should give you an idea):

List<Desk> desks = deskScopesByName.entrySet()
    .stream()
    .flatMap(entry -> {
        String key = entry.getKey();
        Desk rdsDesk = rdsDesksByName.get(key);
        if (rdsDesk == null) {
            log.warn("There is no Desk present for {}", key);
            return Stream.empty();
        } else {
            return entry.getValue()
                .stream()
                .map(ds -> createDeskWithScope(rdsDesk, ds.outOfScope()));
        }
    })
    .collect(Collectors.toList());

I recommend wrapping the flatmapping operation inside a method.