HoverButton.java

import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.Timer;
import java.util.TimerTask;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.SwingUtilities;

public class HoverButton {

    private JFrame frame;


    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                HoverButton test = new HoverButton();
                test.createUI();
            }
        });
    }

    private void createUI() {
        frame = new JFrame();
        JLabel label = new JLabel("Test");
        label.addMouseListener(new MouseAdapter() {

            @Override
            public void mouseEntered(MouseEvent me) {
                startTimer();
            }
        });

        frame.getContentPane().add(label);
        frame.pack();
        frame.setVisible(true);
    }

    private void startTimer() {
        TimerTask task = new TimerTask() {

            @Override
            public void run() {
                SwingUtilities.invokeLater(new Runnable() {

                    @Override
                    public void run() {
                        JOptionPane.showMessageDialog(frame, "Test");
                    }
                });
            }
        };

        Timer timer = new Timer(true);
        timer.schedule(task, 500);
    }
}