method toArray() not working on a list

I'm trying to convert a list to an array, but I get an error and I can't figure out why. I'm taking the current hour and in a for loop I get the rest of the hours of today and put it in a list. When i try to change it to an array I get an error. I try to do this because later I use that array in a JCombobox

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.GregorianCalendar;
import java.util.Calendar;

public class Frame extends JFrame implements ActionListener{
    //JFrame elements
    private JButton btnSetTime;
    private JLabel lblTitle;
    private JComboBox comboTime;

    //Standard elements
    private Timer tillPopup, tillShutdown;

    Calendar calendar = new GregorianCalendar();
    int hour = (calendar.get(Calendar.HOUR_OF_DAY));
    List times = createDropdown(hour);
    // Convert ArrayList to array which can be used in the combobox
    String[] dropdownElements = times.toArray();

    String[] a = {"a","b"};
    public Frame(){
        setLayout(new FlowLayout());

        //Labels
        lblTitle = new JLabel("Deze applicatie sluit u computer automatisch af om het energieverbruik te verminderen.");

        //Combobox
        comboTime = new JComboBox(a);
        comboTime.setSelectedIndex(0);

        //Button
        btnSetTime = new JButton("Zet afsluittijd");

        //Timers
        //tillPopup = new Timer(this);
        //tillShutdown = new Timer(this);

        //Add elements to Frame
        add(lblTitle);
        add(comboTime);
        add(btnSetTime);

        //Add actionListeners
        btnSetTime.addActionListener(this);

        setSize(500,300);
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }
    private List createDropdown(int hour){
        List availableHours = new List();
        for(int i = hour; i <=24; i++){
            if (i != 24){
                availableHours.add(i + ":00");
            }
            else if(i == 24){
                availableHours.add("00:00");
            }
        }
        return availableHours;
    }
    public void actionPerformed(ActionEvent e){
        if (e.getSource() == btnSetTime){
            Object popupTime = comboTime.getSelectedItem();
                System.out.println(popupTime);

        }
    }
}

I get the following error:

Frame.java:21: error: cannot find symbol  
String[] dropdownElements = times.toArray();  
                                 ^  
    symbol:   method toArray()  
    location: variable times of type List

How do I change the list I get back from the method createDropdown to an array and why is what I did wrong?