import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
 
public class frame {
	JFrame frame = new JFrame("Test");
	Container c = frame.getContentPane();
	JPanel panel = new JPanel();
	JTextField textfield = new JTextField(20);
	JButton button = new JButton("Button");
 
	public void openFrame() {
		frame.addWindowListener(new WindowAdapter() {
			public void windowClosing(WindowEvent e) {
				System.exit(0);
			}
		});
		c.add(panel,BorderLayout.CENTER);
		panel.setLayout(new BorderLayout());
		panel.add(textfield, BorderLayout.NORTH);
		panel.add(button, BorderLayout.CENTER);
				
		textfield.addCaretListener(new CaretListener() {
			public void caretUpdate(CaretEvent e) {
				if (textfield.getCaretPosition() == 0 && textfield.getText().equals("")) {
					button.setEnabled(false);
				}else button.setEnabled(true);
			}
		});
	
		button.setEnabled(false);
		frame.pack();
		frame.show();
	}
		
	public static void main(String args[]) {
		frame newframe = new frame();
		newframe.openFrame();
	}
}