Formatted TextArea doesnt Work

hey, ive got a little problem with my formatted TextArea.
i would like to make a TextArea which cant be left empty and cant contain any spaces, ONLY numbers.
i´ve tried it and its not possible anymore to put anything else than numbers, but it shouldnt be possible to delete the numbers without replacing them. How can i do that?
Here is my code:

geschwindigkeit.addActionListener(
                new ActionListener(){
                   public void actionPerformed(ActionEvent e)
                   {
                     JPanel erklärung = new JPanel(new GridLayout(2,1));
                      JPanel erklärung2 = new JPanel(new GridLayout(1,2));
                     JFrame fenster = new JFrame("Geschwindigkeit");
                     
                     try {//Damit nur Zahlen möglich sind
			MaskFormatter formatter = new MaskFormatter("**");
			formatter.setValidCharacters("0123456789");
			tf = new JFormattedTextField(formatter);
			
		

                     JTextArea textArea = new JTextArea();

                     fenster.add(erklärung);
                     textArea.setEditable(false);
                     textArea3.setEditable(true);
                     textArea.setBackground(Color.ORANGE);
                     textArea3.setBackground(Color.WHITE);
                     textArea.setFont(new Font("Times New Roman",1,22)); //Größe einstellen
                     textArea.append(" Geschwindigkeit festlegen");
                     textArea3.append("");
                     JButton eingabe = new JButton ("Eingabe");
                     eingabe.setPreferredSize(new Dimension(100,25));
                     tf.setText("");
                     erklärung2.add(tf);
                     erklärung2.add(eingabe);
                     erklärung.add(textArea);
                     erklärung.add(erklärung2);
                     fenster.setSize(400, 100);
                     fenster.setLocation(320, 500);
                     fenster.setVisible(true);
                     fenster.setResizable(false);
                     fenster.pack();

                     eingabe.addActionListener(
                        new ActionListener(){
                            public void actionPerformed(ActionEvent e)
                            {
                                if (tf.getText() == " "){
                                    geschwindigkeit2 = 0;
                                }
                                else{
                                geschwindigkeit2 = Integer.parseInt(tf.getText());
                              }
                            }
                    });
                    } catch (Exception e2) {
		}
                }
            });

And the Runtime-Error message:

Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: For input string: "  "
	at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
	at java.lang.Integer.parseInt(Integer.java:481)
	at java.lang.Integer.parseInt(Integer.java:527)
	at FirstGUI$2$1.actionPerformed(FirstGUI.java:459)
	at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
	at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
	at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
	at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
	at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
	at java.awt.Component.processMouseEvent(Component.java:6504)
	at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
	at java.awt.Component.processEvent(Component.java:6269)
	at java.awt.Container.processEvent(Container.java:2229)
	at java.awt.Component.dispatchEventImpl(Component.java:4860)
	at java.awt.Container.dispatchEventImpl(Container.java:2287)
	at java.awt.Component.dispatchEvent(Component.java:4686)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
	at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
	at java.awt.Container.dispatchEventImpl(Container.java:2273)
	at java.awt.window.dispatchEventImpl(window.java:2713)
	at java.awt.Component.dispatchEvent(Component.java:4686)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:707)
	at java.awt.EventQueue.access$000(EventQueue.java:101)
	at java.awt.EventQueue$3.run(EventQueue.java:666)
	at java.awt.EventQueue$3.run(EventQueue.java:664)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
	at java.awt.EventQueue$4.run(EventQueue.java:680)
	at java.awt.EventQueue$4.run(EventQueue.java:678)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:677)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:211)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:128)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:90)