Review

Creates swing UI component on the main thread.

Use invokeLater() to displatch UI creation to the EDT.

• missing/condition/environment

CreateUI.java

main(String[])

class CreateUI {
	public static void main(String[] args) {
    JFrame f = new JFrame("Main Window");
    // add components
    f.pack(); 
    f.setVisible(true);
	}

}

public class RunOnEDT {
	public static void main(String[] args) {
		SwingUtilities.invokeLater(new Runnable() {
            public void run() {
            	JFrame f = new JFrame("Main Window");
            	// add components
            	f.pack(); 
            	f.setVisible(true); 
            }
        });
	}
}