Files and paths

Modern Java has two main tools for files, both from the java.nio.file package. Path is a path to a file or folder — just an address that may not even exist. Files is a set of static methods that do things with files: create, copy, delete, read.

A path

A path is created with Path.of. You can pass the parts separately — Java joins them with the separator used by the operating system:

void main() {
    Path path = Path.of("docs", "2026", "report.txt");
 
    IO.println(path.getFileName());
    IO.println(path.getParent().getFileName());
}

Output:

report.txt
2026

The path docs/2026/report.txt is relative: it starts from the current folder the program was launched from. An absolute path starts from the root of the drive; toAbsolutePath() returns it. All examples in this lesson use relative paths and create the files they need themselves.

Does the file exist

A Path on its own knows nothing about the disk. To check a file you need the Files class:

void main() throws IOException {
    Path path = Path.of("hello.txt");
    IO.println(Files.exists(path));
 
    Files.writeString(path, "Hello, €!");
    IO.println(Files.exists(path));
    IO.println(Files.size(path) + " bytes");
}

Output:

false
true
11 bytes

Files.writeString creates the file and writes a string into it. The size is 11 bytes: in UTF-8 Latin letters take one byte each, while the euro sign takes three.

Folders

Files.createDirectories creates a folder along with all missing parent folders. Files.isDirectory and Files.isRegularFile tell a folder from an ordinary file:

void main() throws IOException {
    Path dir = Path.of("photos", "summer");
    Files.createDirectories(dir);
 
    Path photo = dir.resolve("sea.jpg");
    Files.writeString(photo, "...");
 
    IO.println(photo);
    IO.println(Files.isDirectory(dir));
    IO.println(Files.isRegularFile(photo));
}

Output (on Windows the separator is \):

photos/summer/sea.jpg
true
true

The resolve method appends one more part to a path — handier than gluing strings by hand.

Copying, moving, deleting

void main() throws IOException {
    Path original = Path.of("notes.txt");
    Files.writeString(original, "Plan for the week");
 
    Path copy = Files.copy(original, Path.of("notes-copy.txt"), StandardCopyOption.REPLACE_EXISTING);
    Path moved = Files.move(copy, Path.of("archive.txt"), StandardCopyOption.REPLACE_EXISTING);
 
    IO.println(Files.exists(copy));
    IO.println(Files.readString(moved));
 
    Files.delete(moved);
    IO.println(Files.exists(moved));
}

Output:

false
Plan for the week
false

Without StandardCopyOption.REPLACE_EXISTING, copy and move throw an exception if the target file already exists. Files.deleteIfExists deletes a file without an error if it isn't there.

Listing files

Files.list returns a stream of the paths inside a folder. The order of files depends on the operating system, so it's better to sort the result. The stream keeps the folder open, so it is closed with try-with-resources:

void main() throws IOException {
    Path dir = Files.createDirectories(Path.of("inbox"));
    for (String name : List.of("b.txt", "a.txt", "c.log")) {
        Files.writeString(dir.resolve(name), name);
    }
 
    try (Stream<Path> files = Files.list(dir)) {
        files.map(p -> p.getFileName().toString())
                .filter(name -> name.endsWith(".txt"))
                .sorted()
                .forEach(IO::println);
    }
}

Output:

a.txt
b.txt

To go through all nested folders too, use Files.walk — it works the same way but goes deeper.

Common mistake

Files.delete doesn't delete a folder that isn't empty — you have to delete everything inside first:

void main() throws IOException {
    Path dir = Files.createDirectories(Path.of("tmp"));
    Files.writeString(dir.resolve("data.txt"), "1");
 
    Files.delete(dir);
}

Output:

Exception in thread "main" java.nio.file.DirectoryNotEmptyException: tmp
	at Main.main(Main.java:5)

Delete the contents, then the folder itself. For a tree of folders, walk it with Files.walk in reverse order so that files are deleted before their folders.

Thanks for reading!

A path

Does the file exist

Folders

Copying, moving, deleting

Listing files

Common mistake

Thanks for reading!