import java.awt.BorderLayout;
import java.awt.FlowLayout;
import java.util.ArrayList;
import java.util.Arrays;
import javax.swing.JComboBox;
import javax.swing.JFrame;
public class Main extends JFrame {
public Main() {
super();
java.util.List<String> list = new ArrayList<String>();
list.add("A");
list.add("B");
list.add("C");
list.add("D");
Object[] arrayObject = list.toArray();
String[] data = Arrays.copyOf(arrayObject, arrayObject.length,
String[].class); // java 1.6+
JComboBox<String> combo = new JComboBox<>(data);
setLayout(new FlowLayout(FlowLayout.CENTER));
add(combo, BorderLayout.CENTER);
}
public static void main(String[] args) {
Main g = new Main();
g.setVisible(true);
g.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
g.setBounds(100, 100, 300, 300);
}
}