Action no botão

package Suporte;

import java.awt.*;
import java.awt.event.*;
import java.util.ArrayList;

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


public class combo extends javax.swing.JFrame  {

    JComboBox combo = null;
    MyComboModel model = null;
    JButton btOK = null;

    public combo() {

        setTitle("Assunto");
        setLayout(new GridLayout(1, 3));
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        final ArrayList<String> ast = new ArrayList<String>();
        ast.add("Monitor");
        ast.add("Teclado");
        ast.add("Mouse");
        ast.add("Rede");
        ast.add("Internet");
        ast.add("Telefonia fixa");
        ast.add("Telefonia Movel");
        ast.add("Office");
        ast.add("Impressora");
        ast.add("Suporte Brassuco");
        ast.add("DataSul");
        ast.add("Est. Comercial");
        ast.add("Exactus");
        ast.add("IntraSystem");
        ast.add("InfoFisc");

        model = new MyComboModel(ast);

        combo = new JComboBox(model); //Aqui ele cria o combo...
        btOK = new JButton("OK"); //Aqui ele cria o botão...


        
        getContentPane().add(combo); //Aqui ele adiciona o combo...
        add(btOK); //Aqui ele adiciona o botão...


        pack();


    }
}

class MyComboModel extends DefaultComboBoxModel  {

    public ArrayList<String> ast = null;

    public MyComboModel(ArrayList<String> ast) {
        this.ast = ast;
    }

    @Override
    public Object getElementAt(int index) {
        return this.ast.get(index);
    }

    @Override
    public Object getSelectedItem() {
        return super.getSelectedItem();
    }

    @Override
    public int getSize() {
        return this.ast.size();
    }

}