files and paths

import java.nio.file.*;
 
void main() {
    Path p = Path.of("does/not/exist.txt");    // fine. it's just a name
    IO.println(p.getFileName() + " " + Files.exists(p));
    IO.println(p.toAbsolutePath().startsWith(System.getProperty("user.dir")));
}

exist.txt false
true

this one needs a real JVM (files, the network or a database), so it doesn't run in the browser. copy it into java Main.java to try it.

import java.nio.file.*;
import java.util.List;
 
void main() throws Exception {
    Path dir = Files.createTempDirectory("demo");
    Path todo = dir.resolve("todo.txt");
 
    IO.println(Files.exists(todo));
    Files.writeString(todo, "buy milk\nship it\n");
    Files.writeString(todo, "call mom\n", StandardOpenOption.APPEND);
 
    List<String> lines = Files.readAllLines(todo);
    IO.println(lines + " " + Files.size(todo) + " bytes");
 
    Path copy = Files.copy(todo, dir.resolve("todo.bak"));
    IO.println(Files.mismatch(todo, copy));          // -1 means identical
 
    try (var files = Files.list(dir)) {
        IO.println(files.map(p -> p.getFileName().toString()).sorted().toList());
    }
    Files.delete(copy); Files.delete(todo); Files.delete(dir);
}

false
[buy milk, ship it, call mom] 26 bytes
-1
[todo.bak, todo.txt]

this one needs a real JVM (files, the network or a database), so it doesn't run in the browser. copy it into java Main.java to try it.

import java.io.File;
import java.nio.file.*;
 
void main() throws Exception {
    Path dir = Files.createTempDirectory("full");
    Files.writeString(dir.resolve("a.txt"), "hi");
 
    IO.println("File.delete: " + new File(dir.toString()).delete());
    try {
        Files.delete(dir);
    } catch (Exception e) {
        IO.println("Files.delete: " + e.getClass().getSimpleName());
    }
    try {
        Files.delete(dir.resolve("nope.txt"));
    } catch (Exception e) {
        IO.println("Files.delete: " + e.getClass().getSimpleName());
    }
    Files.delete(dir.resolve("a.txt"));
    Files.delete(dir);
}

File.delete: false
Files.delete: DirectoryNotEmptyException
Files.delete: NoSuchFileException

this one needs a real JVM (catching a runtime exception), so it doesn't run in the browser. copy it into java Main.java to try it.

import java.io.IOException;
import java.nio.file.*;
import java.util.Comparator;
import java.util.stream.Stream;
 
void main() throws IOException {
    Path root = Files.createTempDirectory("site");
    Files.createDirectories(root.resolve("src/main"));
    Files.createDirectories(root.resolve("docs"));
    Files.writeString(root.resolve("src/main/App.java"), "class App {}");
    Files.writeString(root.resolve("src/main/Util.java"), "class Util {}");
    Files.writeString(root.resolve("docs/readme.md"), "# hi");
    Files.writeString(root.resolve("build.gradle"), "");
 
    try (Stream<Path> all = Files.walk(root)) {
        all.map(root::relativize).map(Path::toString).sorted().forEach(p -> IO.println("[" + p + "]"));
    }
 
    try (Stream<Path> java = Files.find(root, 10, (p, attrs) -> p.toString().endsWith(".java"))) {
        IO.println("java files: " + java.count());
    }
 
    // delete the whole tree: children before parents, so walk in reverse order
    try (Stream<Path> all = Files.walk(root)) {
        all.sorted(Comparator.reverseOrder()).forEach(p -> {
            try { Files.delete(p); } catch (IOException e) { throw new RuntimeException(e); }
        });
    }
    IO.println("deleted: " + !Files.exists(root));
}

[]
[build.gradle]
[docs]
[docs/readme.md]
[src]
[src/main]
[src/main/App.java]
[src/main/Util.java]
java files: 2
deleted: true

this one needs a real JVM (files, the network or a database), so it doesn't run in the browser. copy it into java Main.java to try it.

import java.nio.file.*;
import java.util.concurrent.TimeUnit;
import static java.nio.file.StandardWatchEventKinds.*;
 
void main() throws Exception {
    Path dir = Files.createTempDirectory("inbox");
    try (WatchService watcher = FileSystems.getDefault().newWatchService()) {
        dir.register(watcher, ENTRY_CREATE, ENTRY_DELETE);
 
        Files.createFile(dir.resolve("order-42.json"));
 
        WatchKey key = watcher.poll(10, TimeUnit.SECONDS);
        for (WatchEvent<?> e : key.pollEvents()) {
            IO.println(e.kind() + " " + e.context());
        }
        IO.println("still valid: " + key.reset());
    }
    Files.delete(dir.resolve("order-42.json"));
    Files.delete(dir);
}

ENTRY_CREATE order-42.json
still valid: true

this one needs a real JVM (files, the network or a database), so it doesn't run in the browser. copy it into java Main.java to try it.