Refactor a method to using stream API

Question

I am currently told to count all .sql files which are hangin' out on some servers. Manually solving this very basic task is not an option, instead, I wrote some code that makes use of a SimpleFileVisitor<Path> and stores all the sql files found along with its parent path in a Map<Path, List<Path>>.

Now I want to receive the total amount of sql files found independent from their locations. I got it working with an enhanced for loop (almost the classic way):

public int getTotalAmountOfSqlFiles(Map<Path, List<Path>> sqlFilesInDirectories) {
    int totalAmount = 0;
        
    for (Path directory : sqlFilesInDirectories.keySet()) {
        List<Path> sqlFiles = sqlFilesInDirectories.get(directory);
        totalAmount += sqlFiles.size();
    }
        
    return totalAmount;
}

The question is now, how can I do the same using the stream API?

I wasn't able to get compilable code to work which isn't obviously doing the wrong thing.
The following line looks like a good idea to me, but not to the compiler, unfortunately.

totalAmount = sqlFilesInDirectories.entrySet().stream().map(List::size).sum();

The compiler says

Cannot infer type argument(s) for <R> map(Function<? super T,? extends R>

Does anyone know what I am doing wrong (and maybe provide some educated stream-API using solution)?

Answer 1

Not sure why people are involving keySet here when the required sum is only of List size contained in values. Just sum the size of all values.

return sqlFilesInDirectories.values().stream().mapToInt(List::size).sum();

And even the for loop version should be simply this,

for (List<Path> list : sqlFilesInDirectories.values()) {
    totalAmount += list.size();
}

As iterating over keyset and then getting value from map isn't really required and will not be better performance wise.

Answer 2

It's because in stream you're taking whole entries instead of values. This should do it:

totalAmount = sqlFilesInDirectories.values().stream().map(List::size).sum();

Answer 3

Other answers provide the shortest way to sum all entries, but if you need the amount of scripts per Path you can use the following:

Map<Path, Integer> amountOfFilesForPath =
        files.entrySet().stream().collect(Collectors.groupingBy(Map.Entry::getKey,
        Collectors.summingInt(value -> value.getValue().size())));

And you can also get the total value:

int sum = amountOfFilesForPath.values().stream().mapToInt(Integer::intValue).sum();