Using Optional with Streams in Java

Question

I was trying to refactor an old code to use streams, and my first approach was this:

public void run() throws IOException { Files.list(this.source) .filter(Images::isImage) .map(Image::new) .filter(image -> image.isProportional(this.height, this.width)) .map(image -> image.resize(this.height, this.width)) .forEach(image -> Images.write(image, this.destination)); }

This is not compiling since new Image() and Images.write() throws IOExceptions.

Wrapping those exceptions with UncheckedIOException wouldn't do the trick as I don't want to stop other images to be processed if one of them fails.

So I ended writing 2 private methods:

private Optional<Image> createImage(Path imagePath) { try { return Optional.of(new Image(imagePath)); } catch (IOException e) { return Optional.empty(); } } private void write(Image image) { try { Images.write(image, this.destination); } catch (IOException e) { // log error } }

createImage() returns an Optional since this seems sensible. However after this my code got really ugly:

public void run() throws IOException { Files.list(source) .filter(Images::isImage) .map(this::createImage) .filter(image -> image.isPresent() && image.get().isProportional(this.height, this.width)) .map(image -> image.get().resize(this.height, this.width)) .forEach(this::write); }

Is there a way to avoid using get() and isPresent() on that code?

Thanks!

srborlongan · Accepted Answer · 2014-06-17 02:24:45Z

One of the nice things about Optionals is that applying filtering, mapping and flat-mapping functions on them only trigger when Optional::isPresent is true, so:

public void run() throws IOException { Files.list(source) .filter(Images::isImage) .map(this::createImage) // turns every non-proportional Optional<Image> into empty optionals .map(image -> image.filter(i -> i.isProportional(this.height, this.width))) // resizes every proportional Optional<Image>, while doing nothing on the empties .map(image -> image.map(i -> i.resize(this.height, this.width))) // applies the writing consumer for each non-empty Optional<Image> .forEach(image -> image.ifPresent(this::write)); }

Another way is to only call Optional::isPresent and Optional::get in separate Stream transformations:

public void run() throws IOException { Files.list(source) .filter(Images::isImage) .map(this::createImage) // filter out the empty optionals .filter(Optional::isPresent) // replace every optional with its contained value .map(Optional::get) .filter(image -> image.isProportional(this.height, this.width)) .map(image -> image.resize(this.height, this.width)) .forEach(this::write); }

Yet another way (which I refuse to recommend as a primary solution because of its relative weirdness) is to change the static image creation method into a Stream generator, instead of an Optional generator, to take advantage of flatMap:

private Stream<Image> createImage(Path imagePath) { try { return Stream.of(new Image(imagePath)); } catch (IOException e) { return Stream.empty(); } } public void run() throws IOException { Files.list(source) .filter(Images::isImage) // inserts into the stream the resulting image (empty streams are handled seamlessly) .flatMap(this::createImage) .filter(image -> image.isProportional(this.height, this.width)) .map(image -> image.resize(this.height, this.width)) .forEach(this::write); }

On second thought, go with this solution; it seems to be simpler, and since the static method is private anyway, less screaming would occur from end-users, other developers, and random people with access to decent Java 8 decompilers (http://www.benf.org/other/cfr/).

Ruben · Accepted Answer · 2019-06-27 12:56:28Z

From Java9, you can use flatMap and Optional::stream to filter empty Optionals:

public void run() throws IOException { Files.list(source) .filter(Images::isImage) .map(this::createImage) .flatMap(Optional::stream) .filter(image -> image.isProportional(this.height, this.width)) .map(image -> image.resize(this.height, this.width)) .forEach(this::write); }

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