What's the right way in Java 8 to evaluate if /path/a is a subdirectory of /path?

Question

Pretty much what the title asks.

Say I'm passed a Path of "/tmp/foo/bar" and I want to specifically ensure that that path is a subdirectory of a path "/tmp", what is the most "Java 8"ish way of going about this?

Specifically I am interested in asking "Given two independent paths, /foo and /foo/bar/baz how can I test if /foo/bar/baz is a subdirectory of /foo without recursing the directory tree?" I am not interested in exploring all the sub directories under /foo and finding /foo/bar/baz downstream.

I've been playing with the idea of

@Test
public void test() {
    final Path root = Paths.get("/tmp");
    final Path path0 = Paths.get("/");
    final Path path1 = Paths.get("/opt/location/sub");
    final Path path2 = Paths.get("/tmp/location/sub");

    final Pattern ptrn = Pattern.compile("^[a-zA-Z].*$");

    final Function<Path, String> f = p -> root.relativize(p).toString();

    Assert.assertFalse("root",ptrn.matcher(f.apply(root)).matches());
    Assert.assertFalse("path0",ptrn.matcher(f.apply(path0)).matches());
    Assert.assertFalse("path1",ptrn.matcher(f.apply(path1)).matches());
    Assert.assertTrue("path2",ptrn.matcher(f.apply(path2)).matches());
}

but this feels like I worked right up to the edge of Java 8 and then dove back to old patterns and missed the boat.

Answer 1

boolean startsWith(Path other)

Tests if this path starts with the given path.