Functional Programming in Java, Second Edition: Chapter 3: Add an example of a filter negation

Community PragProg Customers
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1

The filter stream operation appears on several places in Chapter 2 and Chapter 3, so it’s difficult to say where to put this, but there should be an example of “filtering by a negated predicate”, maybe on page 60:

The code presented:

        Files.list(Paths.get("."))
                        .filter(Files::isDirectory)
                        .forEach(System.out::println);

But what if we want to have everything EXCEPT directories while using the method reference?

This would of course work:

        Files.list(Paths.get("."))
                .filter(file -> !Files.isDirectory(file))
                .forEach(System.out::println);

But nicer is this application of “predicate composition”

        Files.list(Paths.get("."))
                .filter(Predicate.not(file -> Files.isDirectory(file)))
                .forEach(System.out::println);

which simplifies to

        Files.list(Paths.get("."))
                .filter(Predicate.not(Files::isDirectory))
                .forEach(System.out::println);