I am a beginner, and i need help with my code that creates GUI. In main function, i use default constructor for class GUI that creates a frame with menu. Then i want the main function to wait with executing the rest of the code (which will start a simulation) until user presses START JButton in the GUI frame. How can I do that without using any loop? Below is my code, kinda long because of the GroupLayout that i use

package simulation;
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import java.util.HashMap;


public class GUI extends JFrame {
    //by default map set to riverside
    public String mapName = "riverside";
    //user number of creatures input is stored in a hashmap
    HashMap<String, Integer> mapOfCreatures = new HashMap<>();


public GUI()
{
    this.setTitle("Simulation Of Evolution");

    this.setDefaultCloseOperation(3);
    setVisible(true);
    this.setSize(500, 500);
    this.setLocation(300, 300);
    initComponents();

}

public void initComponents()
{
    biomes.add(island);
    biomes.add(riverside);
    biomes.add(seaside);
    island.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            mapName = "island";
        }
    });
    riverside.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            mapName = "riverside";
        }
    });
    seaside.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            mapName = "seaside";
        }
    });
    button.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {

            mapOfCreatures.put("wolves", (Integer)l1.getValue());
            mapOfCreatures.put("humans", (Integer)l2.getValue());
            mapOfCreatures.put("cockroach", (Integer)l3.getValue());
            mapOfCreatures.put("dinos", (Integer)l4.getValue());
            mapOfCreatures.put("bird", (Integer)l5.getValue());
            mapOfCreatures.put("fish", (Integer)l6.getValue());

            System.out.println(mapName);
            dispose();
        }
    });
    //this.getContentPane().setBackground(Color.BLUE);

    GroupLayout layout = new GroupLayout(getContentPane());
    this.getContentPane().setLayout(layout);

    /*here is the grouplayout, it is long so i deleted it here/*

//components
JRadioButton island = new JRadioButton("Island");
JRadioButton riverside = new JRadioButton("Riverside");
JRadioButton seaside = new JRadioButton("Seaside");
ButtonGroup biomes = new ButtonGroup();
JButton button = new JButton("START");
JLabel label1 = new JLabel("Please choose number of creatures in the beginning of simulation:");
JLabel label2 = new JLabel("Please choose a biome:");
JLabel label3 = new JLabel("Wolves");
JLabel label4 = new JLabel("Humans");
JLabel label5 = new JLabel("Cockroach");
JLabel label6 = new JLabel("Dinos");
JLabel label7 = new JLabel("Bird");
JLabel label8 = new JLabel("Fish");
JSpinner l1 = new JSpinner(new SpinnerNumberModel(1, 1, 300, 1));
JSpinner l2 = new JSpinner(new SpinnerNumberModel(1, 1, 300, 1));
JSpinner l3 = new JSpinner(new SpinnerNumberModel(1, 1, 300, 1));
JSpinner l4 = new JSpinner(new SpinnerNumberModel(1, 1, 300, 1));
JSpinner l5 = new JSpinner(new SpinnerNumberModel(1, 1, 300, 1));
JSpinner l6 = new JSpinner(new SpinnerNumberModel(1, 1, 300, 1));

**
public static void main(String[] args) {
new GUI();
/here will be rest of code, which should be executed when user presses START Jbutton/
}**

}

My teacher said i shouldnt use any loops,  maybe try callbacks or thread but i have no idea how to implement it