WebDriver mainWindow = new FirefoxDriver();

Nvigation navigation = mainWindow.navigate();

navigation.to(http://www.baidu.com);

WebElement  bnInMainWindow = mainWindow.findElement(By.name("tj_reg"));

btnInMainWindow.click();

String[] handles = new String[mainWindow.getWindowHandles().size()];

mainWindow.getWindowHandles().toArray(handles);

for(int i = 0;i<handles.length;i++)

{

     System.out.println(handles[i]);

}