java 以下代码想要实现点击按钮换图片，为什么不行呢？

import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JPanel;

class PanelTest extends JPanel
{
String path = "" ;
public PanelTest (String path)
{
this.path = path ;
repaint () ;
}
public PanelTest ()
{
path = "image/Background1.jpg" ;
}
public void paintComponent (Graphics g)
{
super.paintComponent(g) ;
g.drawImage(getImage(), 0, 0, this) ;
}
public Image getImage()
{
ImageIcon icon = new ImageIcon (PanelTest.class.getResource(path)) ;
return icon.getImage() ;
}
}

public class FrameTest extends JFrame
{
JMenuBar bar ;
JMenu menu ;
JMenuItem item ;
public FrameTest ()
{
bar = new JMenuBar () ;
menu = new JMenu ("test") ;
item = new JMenuItem ("test") ;

    item.addActionListener(new ActionListener()
    {

        @Override
        public void actionPerformed(ActionEvent e) 
        {
            new PanelTest ("image/Background2.jpg") ;
        }

    });
    menu.add(item) ;
    bar.add(menu) ;
    bar.setBounds(0, 0, 400, 30) ;
    this.add(bar) ;
    this.add(new PanelTest()) ;
    this.setVisible(true) ;
    this.setSize(400,400) ;
}
public static void main (String [] args)
{
    new FrameTest () ;
}

}

1.一个主类jframe里面加一个类jpanel，然后点击frame里面的jmenuitem让jpanel从新的地址得到一张新图片然后重绘，目的是这样，但是为什么不行呢？代码加上包名后直接复制就可以运行，在该包下有一个名为image的文件夹，里面放着图片。
2.这个java文件中有两个类，那么在运行的时候，只生成一个线程就是主线程对吧？

 import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JPanel;

class PanelTest extends JPanel {
    String path = "";

    public PanelTest(String path) {
        this.path = path;
    }

    public PanelTest() {
        path = "image/Background2.jpg";
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawImage(getImage(), 0, 0, this);
    }

    public Image getImage() {
        ImageIcon icon = new ImageIcon(PanelTest.class.getResource(path));
        return icon.getImage();
    }
}

public class FrameTest extends JFrame {
    JMenuBar bar;
    JMenu menu;
    JMenuItem item;
    PanelTest panelTest;

    public FrameTest() {
        bar = new JMenuBar();
        menu = new JMenu("test");
        item = new JMenuItem("test");
        item.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                remove(panelTest);
                add(new PanelTest("image/Background2.jpg"));
                invalidate();
                repaint();
                setVisible(true);
            }

        });
        menu.add(item);
        bar.add(menu);
        bar.setBounds(0, 0, 400, 30);
        this.add(bar);
        panelTest=new PanelTest();
        this.add(panelTest);
        this.setVisible(true);
        this.setSize(400, 400);
    }

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

new PanelTest ("image/Background2.jpg") ; 你这只new一个对象，没有添加到jframe上