How to close(dispose/free memory) all JInternalFrame in a JDesktopPane?

 I use DesktopManager.closeFrame(JInternalFrame) to close all the JInternalFrame in the JDesktopPane. It does work! But the problem I met is that the memory isn't released(I use the statement "java -verbosegc myclass" to debug). Please tell me what is the problem?? Thank you.

Attn :
JInternalFrame frames[] = MyDesktopPane.getAllFrames();
DesktopManager dm = MyDesktopPane.getDesktopManager();
for (int i = 0 ; i < frames.length ; i ++)
{
  dm.closeFrame(frames[i]);
  frames[i].setClosed(false);
}