invalid method declaration, return type required, on a GUI

So im trying to make a simple GUI that goes up by one when you click a button in it. I get this error, however, when i try and run the test GUI: Error. Here is the code

import javax.swing.JFrame;
import javax.swing.BorderFactory;
import javax.swing.JPanel;


public class Main {

public GUI() {

  JFrame frame = new JFrame();
  JPanel panel = new JPanel();
  panel.setBorder(borderFactory.createEmptyBorder(30, 30, 10, 30));
  panel.setLayout(new GridLayout(0, 1));

  frame.add(panel, BorderLayout.CENTER);
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  frame.setTitle("Clicks");
  frame.pack();
  frame.setVisible(true);

}

  public static void main(String[] args) {

    new GUI();

  }
}

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