In order to join (combine) fix file paths we can use Path#resolve() and Path#resolveSibling():.
// using Path#resolve()
Path base1 = Paths.get("D:/learning/books");
Path path1 = base1.resolve("Java.pdf");
// using Path#resolveSibling()
Path base2 = Paths.get("D:/learning/books/Java.pdf");
Path path3 = base2.resolveSibling("PHP.pdf");