When I add JPanel into full screen JFrame, JPanel sizing does not work and JPanel becomes full screen too, whats problem here and how to fix?

import java.awt.*;
import javax.swing.*;

public class pcb extends JFrame {   
    public static void main(String[] args) {
        JFrame pcb = new JFrame();
        pcb.setUndecorated(true);
        pcb.setBackground(new Color(0, 0, 0, 0)); // transparent jframe
        pcb.setExtendedState(JFrame.MAXIMIZED_BOTH); // full size
        
        JPanel timerFrame = new JPanel();
        timerFrame.setLayout(null);
        timerFrame.setSize(350, 200);
        timerFrame.setBackground(new Color(100, 150, 100, 255));
        timerFrame.setVisible(true);
        pcb.add(timerFrame);
        
        pcb.setVisible(true);
    }
}