Java 8 nested null check for a string in a map in a list

问题

I need to do a series of null checks ( nested null-checks ) to get an array of strings like below

String[] test;
if(CollectionUtils.isNotEmpty(checkList)){
    if(MapUtils.isNotEmpty(checkList.get(0))){
        if(StringUtils.isNotBlank(checkList.get(0).get("filename"))){
            test = checkList.get(0).get("filename").split("_");
        }
    }
}

Is there a better way, maybe using Java8 Optional, to perform these kind of nested checks? I unsuccessfully tried to use Optional with flatmap / map.

回答1:

You could use a long chain of Optional and Stream operations to transform the input step by step into the output. Something like this (untested):

String[] test = Optional.ofNullable(checkList)
    .map(Collection::stream)
    .orElseGet(Stream::empty)
    .findFirst()
    .map(m -> m.get("filename"))
    .filter(f -> !f.trim().isEmpty())
    .map(f -> f.split("_"))
    .orElse(null);

I'd strongly encourage you to stop using null lists and maps. It's a lot better to use empty collections rather than null collections, that way you don't have to have null checks all over the place. Furthermore, don't allow empty or blank strings into your collections; filter them out or replace them with null early, as soon as you're converting user input into in-memory objects. You don't want to have to insert calls to trim() and isBlank() and the like all over the place.

If you did that you could simplify to:

String[] test = checkList.stream()
    .findFirst()
    .map(m -> m.get("filename"))
    .map(f -> f.split("_"))
    .orElse(null);

Much nicer, no?

回答2:

Don't nest the ifs, but just unwrap and invert them:

String[] defaultValue = // let this be what ever you want

if(checkList == null || checkList.isEmpty()) {
    return defaultValue;
}

Map<String, String> map = checkList.get(0);
if(map == null || map.isEmpty()) {
    return defaultValue;
}

String string = map.get("filename");
if(string == null || string.trim().isEmpty()) {
    return defaultValue;
}

return string.split("_");

Though this only works when you wrap this extraction logic in a method:

public static String[] unwrap(List<Map<String, String>> checkList) {
    ...
}

回答3:

If checkList is null, it will throw null pointer exception on CollectionUtils.isNotEmpty(checkList). Also use in-built empty checker. Better you should code

        if (null != checkList && !checkList.isEmpty() 
                && null != checkList.get(0) && !checkList.get(0).isEmpty()
                && StringUtils.isNotBlank(checkList.get(0).get("filename"))) {

            test = checkList.get(0).get("filename").split("_");

        }

来源：https://stackoverflow.com/questions/56238423/java-8-nested-null-check-for-a-string-in-a-map-in-a-list

标签

java

java-8

nested

optional

null-check