file list example by FileVisitor.

Main.java

import java.io.File;
import java.io.FilenameFilter;
import java.io.IOException;
import java.nio.file.FileVisitOption;
import java.nio.file.FileVisitResult;
import java.nio.file.FileVisitor;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.attribute.BasicFileAttributes;
import java.util.ArrayList;
import java.util.EnumSet;
import java.util.List;

public class Main {

    public static void main(String[] args) {
        if (args.length == 0) {
            System.exit(-1);
        }
        String root = args[0];

        printMemoryInfo();
        do1(root);
        // do2(root);
        printMemoryInfo();
    }

    private static void do1(String root) {
        Path path = Paths.get(root);
        try {
            FileVisitorImple fv = new FileVisitorImple();
            Files.walkFileTree(path, EnumSet.noneOf(FileVisitOption.class), 1, fv);
            System.out.println("file count " + fv.getFiles().size());
            for (String f : fv.getFiles()) {
                // System.out.println(f);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static void do2(String root) {
        File dir = new File(root);
        File[] files = dir.listFiles(new FilenameFilter() {
            @Override
            public boolean accept(File dir, String name) {
                return true;
            }
        });

        System.out.println("file count " + files.length);
        for (File f : files) {
            // System.out.println(f);
        }
    }

    private static void printMemoryInfo() {
        System.out.println(Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory());
    }
}

class FileVisitorImple implements FileVisitor<Path> {

    private List<String> files = new ArrayList<>();

    public List<String> getFiles() {
        return files;
    }

    @Override
    public FileVisitResult postVisitDirectory(Path dir, IOException exc) throws IOException {
        return FileVisitResult.CONTINUE;
    }

    @Override
    public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
        String fileName = file.getFileName().toString();
        files.add(fileName);
        return FileVisitResult.CONTINUE;
    }

    @Override
    public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
        return FileVisitResult.CONTINUE;
    }

    @Override
    public FileVisitResult visitFileFailed(Path file, IOException exc) throws IOException {
        return FileVisitResult.TERMINATE;
    }
}