Practical NIO with Ben Evans: https://blogs.oracle.com/javamagazine/post/java-path-nio2-directory-extensions-zip

WalkFileTreeDemo.java

import java.io.IOException;
import java.nio.file.AccessDeniedException;
import java.nio.file.FileVisitResult;
import java.nio.file.Files;
import java.nio.file.NoSuchFileException;
import java.nio.file.Path;
import java.nio.file.SimpleFileVisitor;
import java.nio.file.attribute.BasicFileAttributes;

public class WalkFileTreeDemo {

    public static void main(String[] args) throws IOException {
        var startingDir = Path.of("C:", "Users", "szabolcs.z.besenyei");
        var visitor = new SizeCollectorVisitor();
        Files.walkFileTree(startingDir, visitor);
        System.out.format("Visited: %d, Disk space: %f MB%n", visitor.visited, visitor.sum / Math.pow(1024, 2));
    }

    public static class SizeCollectorVisitor extends SimpleFileVisitor<Path> {
        long sum = 0L;
        long visited = 0L;

        @Override
        public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) {
            if (file != null && attrs != null) {
                ++visited;
                sum += attrs.size();
            }
            return FileVisitResult.CONTINUE;
        }

        @Override
        public FileVisitResult visitFileFailed(Path file, IOException exc) throws IOException {
            if (exc instanceof AccessDeniedException) {
                System.err.println("Could not visit directory: " + file.getFileName());
                return FileVisitResult.SKIP_SUBTREE;
            } else if (exc instanceof NoSuchFileException) {
                //System.err.println("No such file: " + file.getFileName());
                return FileVisitResult.CONTINUE;
            } else
                return super.visitFileFailed(file, exc);
        }
    }
}