What is NIO.2 Path & Files API in Java?

Path and Files replaced java.io.File in Java 7. Why File had to go: it reported failure as false. That one difference is most of the reason to migrate. A

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NIO.2 Path & Files API

Use the modern file system API (Path, Files) introduced in Java 7 for file operations.

Path and Files replaced java.io.File in Java 7.

Old File = a paper map (limited, no real-time updates). Path + Files = GPS navigation (detailed, accurate, with real-time traffic/error info).

Key Concepts

    Path p = Path.of("data", "orders.csv");

    String text = Files.readString(p);
    Files.writeString(p, text);
    Files.copy(src, dst, StandardCopyOption.REPLACE_EXISTING);
    Files.createDirectories(p.getParent());

Why File had to go: it reported failure as false.

    new File("x").delete();          // false. Why? No idea.
    Files.delete(path);              // NoSuchFileException, with a message

Filefalse

That one difference is most of the reason to migrate. A failure you cannot diagnose is worse than an exception.

Path manipulation is proper, rather than string concatenation:

    Path base = Path.of("/srv/app");
    base.resolve("logs/app.log");    // /srv/app/logs/app.log
    base.relativize(other);          // the path from base to other
    p.normalize();                   // collapses . and ..
    p.getParent();  p.getFileName();

resolve has a trap worth knowing. If the argument is absolute, it is returned as-is:

resolve

    Path.of("/srv/app").resolve("/etc/passwd");    // /etc/passwd

So resolving user input is a path traversal risk. Normalise and check the result still starts with your base directory.

Walking a tree gives you a stream:

    try (var paths = Files.walk(dir)) {
      paths.filter(Files::isRegularFile).forEach(this::index);
    }

Close that stream. Files.walk, Files.list and Files.lines hold an open directory handle, so a forgotten try-with-resources leaks file descriptors until the process runs out.

Files.walkFiles.listFiles.lines

What else File could not do.

    ATOMIC_MOVE          rename that either happens or does not
    symbolic links       followed or not, your choice
    POSIX permissions    read and set them
    WatchService         be told when a directory changes
    FileVisitor          recursive walk with control at each step

File

And Path is an interface. That is what lets a provider back it with a zip file or an in-memory filesystem, which is how tests avoid touching disk at all.

Path

What the interviewer is probing.1. "Why did Path and Files replace File?" Probing: the concrete failure of the old API. Stalls: "They are newer." Moves up: File reported failure as a boolean, so a failed delete gave you false and no reason; Files throws an exception that names the cause.

2. "What is the risk in base.resolve(userInput)?" Probing: path traversal. Stalls: "None, it joins paths." Moves up: an absolute argument is returned as-is, so user input can escape the base directory; normalise and check the result still starts with the base.

3. "Why must Files.walk be in a try-with-resources?" Probing: the open handle. Stalls: "It is just a stream." Moves up: it holds an open directory handle, so a forgotten close leaks file descriptors until the process runs out.

4. "What can Files do that File could not?" Probing: breadth. Stalls: "Not much." *Moves up:* atomic moves, symbolic link control, POSIX permissions, file attributes, a recursive FileVisitor, and a WatchService for change events.