import javax.swing.*;
import java.awt.*;
import java.io.BufferedReader;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

final class Window {
    private final JFrame frame;
    private final JLabel fileTextLbl;

    public Window() {
        frame = new JFrame();
        frame.setSize(800, 600);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null);
        frame.setLayout(new FlowLayout());

        fileTextLbl = new JLabel("hallo");
        frame.add(fileTextLbl);

        readFile();
    }

    public void show() {
        frame.setVisible(true);
    }

    private void readFile() {
        ScheduledExecutorService executor = Executors.newScheduledThreadPool(1);
        executor.scheduleAtFixedRate(() -> {
            try (BufferedReader br = Files.newBufferedReader(Paths.get("test.txt"))) {
                fileTextLbl.setText(br.readLine());
            } catch(IOException ioe) {}
        }, 0, 3, TimeUnit.SECONDS);
    }
}

public class Start {
    public static void main(String[] args) {
        new Window().show();
    }
}