确保 frame.setVisible(true); 在最后一行

import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
public class Test01 {
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        JFrame frame = new JFrame("Recipes"); 
        //frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(500, 500);
        frame.setLocation(430, 100);
        JPanel panel = new JPanel();
        frame.add(panel);
        JLabel lbl = new JLabel("Select time taken");
        lbl.setVisible(true);
        panel.add(lbl);
        String[] choices = { "10 MINS","15 MINS", "20 MINS","25 MINS","30 MINS"};
        final JComboBox<String> cb = new JComboBox<String>(choices);
        cb.setVisible(true);
        panel.add(cb);
        JButton btn = new JButton("OK");
        panel.add(btn);
        frame.setVisible(true);
    }
}