well found the solution finally:
to eliminate jcomponents:
Container parentobject = object.getParent();
parentobject.remove(object);
parentobject.validate();
parentobject.repaint();
and to update the jtextarea:
areaobject.setText("new text");
so the code will be:
public static void main(String[] args) {
JFrame f = new JFrame("text");
f.getContentPane().setBackground(Color.BLACK);
f.setSize(290,220);
f.setLocation(735,550);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JTextArea area=new JTextArea("some text");
area.setBounds(0,0, 280,100);
JLabel l1,l2;
l1=new JLabel("text");
l1.setBounds(0,107, 55,25);
l2=new JLabel("text");
l2.setBounds(140,107, 55,25);
JTextField t1,t2;
t1=new JTextField();
t1.setBounds(55,107, 75,25);
t2=new JTextField();
t2.setBounds(195,107, 75,25);
JButton b = new JButton("text");
b.setBounds(82, 142, 100, 33);
f.add(b);
f.add(t1); f.add(t2);
f.add(area);
f.add(l1); f.add(l2);
f.setLayout(null);
f.setVisible(true);
b.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
String UN = t1. getText();
String PW = t2. getText();
Container parentl1 = l1.getParent();
parentl1.remove(l1);
parentl1.validate();
parentl1.repaint();
Container parentl2 = l2.getParent();
parentl2.remove(l2);
parentl2.validate();
parentl2.repaint();
Container parentt1 = t1.getParent();
parentt1.remove(t1);
parentt1.validate();
parentt1.repaint();
Container parentt2 = t2.getParent();
parentt2.remove(t2);
parentt2.validate();
parentt2.repaint();
Container parentb = b.getParent();
parentb.remove(b);
parentb.validate();
parentb.repaint();
area.setText("new text"); ................
});