You can't add a JFrame to a JApplet. The closest thing to using other windows in an applet is that you can pop up messages in dialog windows using for example, JOptionPane.showMessageDialog.
Applets are limited for security reasons. If you are trying to reuse code intended for use in a JFrame, try binding your code to JPanel. This way, you can add the JPanel to your JFrame for your application. You can also add the JPanel to the JApplet for the applet version.
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Applets are limited for security reasons. If you are trying to reuse code intended for use in a JFrame, try binding your code to JPanel.
This way, you can add the JPanel to your JFrame for your application.
You can also add the JPanel to the JApplet for the applet version.