Listing Immediate Subdirectories Using flatMap

Functional Programming in Java — by Venkat Subramaniam (34 / 96)

👈 Listing Select Files in a Directory | TOC | Watching a File Change 👉

We saw how to list the members of a given directory. Let’s look at the effort to explore the immediate (one level deep) subdirectories in a given directory, first using a rudimentary operation and then, more conveniently, using the flatMap method.

Let’s use the traditional for loop first to iterate over the files in a given directory. If a subdirectory contains any files, we’ll add them to our list; otherwise, we’ll add the subdirectory itself to the list. Finally, we’ll print the total number of files found. Here’s the code — for the hard way.

We fetch the list of files in the current directory and loop through each of the files. For each file, we query for its children and add them, if present, to the list of files. That works, but it comes with the usual culprits: mutability, primitive obsession, imperative, noise.… We can get rid of these using a nice little method called flatMap.

As the name indicates, this method will flatten after mapping. It maps the elements in a collection, much like the map method does. But unlike the map method, where we generally return an element from the lambda expression, we return a Stream instead. The method then flattens the multiple streams, obtained by mapping each element, into one flat stream.

We can use flatMap for various operations, but the problem on hand nicely demonstrates the usefulness of this method. Each subdirectory has a list or stream of files and we’re trying to get a combined (or flattened) list of files in all the subdirectories of the current directory.

Some directories (or files) may be empty and may not have children. In that case, we simply wrap a stream around the no-child directory or file element. In case…

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