Проект Monitor2

FileEvent.java

package Monitor;

import java.io.File;
import java.util.Arrays;

public class FileEvent implements IFileEvent {
	@Override
	public void onFileAdded(File fdir) {
        System.out.println("Directory includes files .txt" + Arrays.toString(fdir.list()));
    }

}

IFileEvent.java

package Monitor;

import java.io.File;

public interface IFileEvent {
	void onFileAdded(File fdir);
}

Main.java

package Monitor;

//Написать код для мониторинга каталога. Выводить на экран 
// предупреждение если в каталог добавляется текстовый файл (*.txt).


public class Main {
	public static void main(String[] args) {
        String dir = "d:\\Dir";
		Monitor m = new Monitor(dir, new FileEvent());
		m.start();
	}
}

Monitor.java

package Monitor;

import java.io.File;
import java.io.FilenameFilter;

public class Monitor {
    String dir;
    IFileEvent event;

    public Monitor(String dir, IFileEvent event) {
        this.dir = dir;
        this.event = event;
    }

    public void start() {

        while (true) {

            File fdir = new File(dir);

            if (fdir.exists() && fdir.isDirectory() && event != null) {

                File[] files = fdir.listFiles(new FilenameFilter() {
                    @Override
                    public boolean accept(File file, String name) {
                        return name.endsWith(".txt");
                    }
                });

                if (files.length != 0) {
                    event.onFileAdded(fdir);
                    break;
                }
            }

            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
            }

            System.out.println("Waiting...");
        }
    }
}