setText in JTextFields eines anderen Threads?

// HauptKlasse

public class MainClass extends JFrame
{
    private JTextField textField;
    private static Thread t;

    public boolean isRunning()
    {
        return isRunning;
    }

    public void showMessage(String message)
    {
        textField.setText(message);
    }

    public MainClass(boolean runConstructor)
    {
        if (runConstructor)
        {
            JPanel panel = new JPanel();

            textField = new JTextField("");
            panel.add(textField);

            setContentPane(panel);
        }
    }

    public static void main(String[] args)
    {
        MainClass frame = new MainClass(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(500, 500);
        frame.setVisible(true);

        t = new Thread(new Concurrent());
        t.start();
    }
}

// Nebenläufiger Programmteil

public class Concurrent implements Runnable
{
    public void run()
    {
        try
        {
            MainClass clientObject = new MainClass(false);

            int i = 0;

            while(clientObject.isRunning())
            {
                clientObject.showMessage(String.valueOf(i));
                Thread.sleep(1000);
                i++;
            }

        } catch (Exception ex) {ex.printStackTrace();}
    }
}