Code:

import java.awt.BorderLayout;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;

import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JPanel;

/**
 * @author Velja Radenkovic
 * class: MainForm
 * package:
 */
public class MainForm extends JFrame{

    /*
     * Private fields
     */
    private JPanel mainPanel;
    private JComboBox combo;
    private JButton button;

    /**
     * Default constructor
     */
    public MainForm(){this.initalize();}

    //
    //Entry point
    //
    public static void main(String[] args)
    {
        System.out.println("Program started!");
        MainForm mainFrm = new MainForm();
        mainFrm.pack();
        mainFrm.setSize(800, 600);
        mainFrm.setVisible(true);
    }

    /*
     * Initalize ui
     */
    private void initalize()
    {
        this.setContentPane(this.getMainPanel());
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    /*
     * gets the mainPanel
     */
    private JPanel getMainPanel()
    {
        if(this.mainPanel == null)
        {
            this.mainPanel = new JPanel();
            this.mainPanel.setLayout(new BorderLayout(10, 10));
            this.mainPanel.add(this.getCombo(), BorderLayout.NORTH);
            this.mainPanel.add(this.getButton(), BorderLayout.CENTER);
        }
        return this.mainPanel;
    }

    /*
     * gets the combo
     */
    private JComboBox getCombo()
    {
        if(this.combo == null)
        {
            this.combo = new JComboBox();
            this.combo.addItem("Stavka jedan");
            this.combo.addItem("Stavka dva");
            this.combo.addKeyListener(new KeyAdapter() {


                public void keyReleased(KeyEvent e) {
                    System.out.println("Key released!");
                }


            });
        }
        return this.combo;
    }

    /*
     *
     */
    private JButton getButton()
    {
        if(this.button == null)
        {
            this.button = new JButton("Klikni me klikni");
        }
        return this.button;
    }
}