How to show JFrame in JInternalFrame?

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Java

netbeans

I have a Frame called frmAbout, I want to show it in a JInternalFrame in another Frame called frmMain, how can I do it?
I've tried this:
javax.swing.JInternalFrame f1 = new javax.swing.JInternalFrame("شاشة الموظفين", false);
f1.setBounds(0, 0, 500, 500);
f1.setVisible(true);
add(f1);

but it doesn't worked!