does subwindow cause browser memory leak?

hi,I wrote a test application and found that everytime I close the sub-window, the browser's memory did not decrease. will you please tell me why is that? thank you!

package com.example.topnewform;

import java.util.HashMap;

import com.vaadin.Application;
import com.vaadin.ui.Button;
import com.vaadin.ui.Window;
import com.vaadin.ui.Button.ClickEvent;
import com.vaadin.ui.Window.CloseEvent;

public class TopnewformApplication extends Application {

/**
*
*/
private static final long serialVersionUID = -8681400824585178761L;
@Override
public void init() {



final Window mainWindow = new Window("Topnewform Application");
Button btn = new Button();
btn.setCaption("add");
btn.addListener(new Button.ClickListener() {

/**
*
*/
private static final long serialVersionUID = -6562244480021067904L;

public void buttonClick(ClickEvent event) {
// TODO Auto-generated method stub
for (int i = 0; i < 20; i++) {
Window wnd = new Window();
wnd.addListener(new Window.CloseListener() {

/**
*
*/
private static final long serialVersionUID = -1324839086947657951L;

public void windowClose(CloseEvent e) {
// TODO Auto-generated method stub
mainWindow.removeWindow(e.getWindow());
System.out.println(e.getWindow() + " removed2!");
}

});
wnd.setModal(false);
wnd.setImmediate(false);
mainWindow.addWindow(wnd);
System.out.println(wnd + " added!");
wnd = null;
}
}

});
Button btn1 = new Button();
btn1.setCaption("remove");
btn1.addListener(new Button.ClickListener() {

/**
*
*/
private static final long serialVersionUID = 2342008830092155491L;

public void buttonClick(ClickEvent event) {

while (!mainWindow.getChildWindows().isEmpty())
{
Window w = mainWindow.getChildWindows().iterator().next();
mainWindow.removeWindow(w);
System.out.println(w + " removed!");
w = null;
}
}

});


mainWindow.addComponent(btn);
mainWindow.addComponent(btn1);
setMainWindow(mainWindow);
}

}