import  javax.swing.*;
 import  javax.swing.JScrollPane;
 import  java.awt.BorderLayout;
 import  javax.swing.JTree;
 import  java.io.*;
 import  javax.swing.JPanel;
 import  java.util.*;

 public   class  Ordner  extends  JPanel {
	 private  File dir =  null ;

	 private  JScrollPane jScrollPane =  null ;

	 private  JPanel jPanel =  null ;

	 private  JTree jTree =  null ;

	 private  Thread thread =  null ;

	 private  JFrame frame =  null ;

	/**
	 * This is the default constructor
	 */
	 public  Ordner(JFrame frame) {
		 super ();
		 this .frame = frame;
		initialize();
	}

	/**
	 * This method initializes this
	 * 
	 * @return void
	 */
	 private   void  initialize() {
		 this .setLayout( new  BorderLayout());
		 this .setSize( 300 ,  200 );
		dir =  new  File(
				"/usr/incomming");
		 this .add(getJScrollPane(), java.awt.BorderLayout.CENTER);

	}

	 void  update() {
		thread = Thread.currentThread();
		 try  {
			 while  (true) {
				thread.sleep( 3000 );
				 this .jTree= null ;
				 this .updateUI();
				 this .repaint();
				 this .frame.pack();
				
				Iterator i = Arrays.asList(dir.list()).iterator();
				 while  (i.hasNext())
					System.out.print(i.next() + "\t");
				System.out.println();

			}
		}  catch  (Exception e) {
			System.err.println(e);
		}
	}

	/**
	 * This method initializes jScrollPane
	 * 
	 * @return javax.swing.JScrollPane
	 */
	 private  JScrollPane getJScrollPane() {
		 if  (jScrollPane ==  null ) {
			jScrollPane =  new  JScrollPane();
			jScrollPane.setViewportView(getJPanel());
		}
		 return  jScrollPane;
	}

	/**
	 * This method initializes jPanel
	 * 
	 * @return javax.swing.JPanel
	 */
	 private  JPanel getJPanel() {
		 if  (jPanel ==  null ) {
			jPanel =  new  JPanel();
			jPanel.setLayout( new  BorderLayout());
			jPanel.add(getJTree(), java.awt.BorderLayout.NORTH);
		}
		 return  jPanel;
	}

	/**
	 * This method initializes jTree
	 * 
	 * @return javax.swing.JTree
	 */
	 private  JTree getJTree() {
		 if  (jTree ==  null ) {
			jTree =  new  JTree(dir.list());
		}
		 return  jTree;
	}

	 public   static   void  main(String[] args) {
		JFrame frame =  new  JFrame();
		Ordner ordner =  new  Ordner(frame);
		frame.getContentPane().add(ordner);
		frame.setSize( 100 ,  100 );
		frame.setLocation( 200 ,  200 );
		frame.setVisible(true);
		frame.setDefaultCloseOperation( 3 );
		frame.pack();

		ordner.update();

	}

}

 import  javax.swing.*;
 import  java.io.*;
 import  java.util.*;

 public   class  Ordner  extends  JPanel {
  private  File dir =  null ;
  private  JTree jTree =  null ;
  private  Thread thread =  null ;
  private  JFrame frame =  null ;

 /**
 * This is the default constructor
 */
  public  Ordner(JFrame frame) {
           this .frame = frame;
         frame.setSize( 300 ,  200 );
         frame.setVisible(true);
         frame.setDefaultCloseOperation( 3 );
        frame.getContentPane().add( this );
           frame.pack();
 
 //здесь можно выбрать любую папку и потом менять ее содержимое
          dir =  new  File( "/usr/incomming");
         jTree= new  JTree(dir.list());
              this .add(jTree);
 
           thread = Thread.currentThread();
          try  {
              while  (true) {
                             thread.sleep( 2000 );
                              this .remove(jTree);
                            jTree =  new  JTree(dir.list());
                             this .add(jTree);
 
                           //ничего из этого не работает:
                         //this.updateUI();
                        //this.repaint();
                        //this.validate();
                           //frame.repaint();
                         frame.pack();
 
 //этот блок кода только для контроля
                        Iterator i = Arrays.asList(dir.list()).iterator();
                                  while  (i.hasNext())
                                            System.out.print(i.next() + "\t");
                               System.out.println();

              }
            }  catch  (Exception e) {  System.err.println(e);}
 }

  public   static   void  main(String[] args) {
                      new  Ordner( new  JFrame());
 }

}//end of klass

 this .remove(jTree);
jTree =  new  JTree(dir.list());
 this .add(jTree);

String s = "Some value";
JButton btn =  new  JButton (s);
 this .add (btn, ....)
.....
s = "Some other value";