Resize JLabel to fit resized main JPanel

I have two child panels and one parent panel (background). My goal is to resize JLabel icon in firstPanel, when main panel is resized. I tried to use StretchIcon, but it works only when adding JLabel icon to main.
My code now:

    import darrylbu.icon.StretchIcon;
    import javax.imageio.ImageIO;
    import javax.swing.*;
    import java.awt.*;
    import java.awt.image.BufferedImage;
    import java.io.IOException;
    import java.net.URL;

public class Task {
JFrame frame;
JPanel main;

public static void main(String args[]) {
    new Task();
}

public Task() {
    frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setPreferredSize(new Dimension(600, 400));
    main = mainPanel();
    frame.add(main);
    main.add(firstPanel());
    main.add(secondPanel());
    frame.pack();
    frame.setVisible(true);
}

private JPanel mainPanel(){
    JPanel main_panel = new JPanel() {
        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            BufferedImage background = null;
            try {
                background  = ImageIO.read(new URL("https://jpegshare.net/images/17/b0/17b0bad019ea5e37c84a5147a33b0ce7.jpg"));
            } catch (IOException e) {
                e.printStackTrace();
            }
            g.drawImage(background, 0, 0, this.getWidth(), this.getHeight(), this);
        }
    };
    main_panel.setLayout(new OverlayLayout(main_panel));
    main_panel.setLayout(new GridLayout(1, 2));
    return main_panel;
}

private JPanel firstPanel() {
    JPanel first_panel = new JPanel();
    first_panel.setOpaque(false);
    BufferedImage image = null;
    try {
        image = ImageIO.read(new URL("https://jpegshare.net/images/7a/f3/7af3bfc51cb1170be9f5655d643147d7.png"));
    } catch (IOException e) {
        e.printStackTrace();
    }
    StretchIcon img = new StretchIcon(image);
    JLabel icon = new JLabel(img);
    first_panel.add(icon);
    first_panel.setLayout(getLayout());
    return first_panel;
}

private JPanel secondPanel(){
    JPanel second_panel = new JPanel() {
        @Override
        public Dimension getMaximumSize() {
            return new Dimension(300, 400);
        }
        @Override
        public void paintComponent(Graphics g) {
            BufferedImage image = null;
            try {
                image = ImageIO.read(new URL("https://jpegshare.net/images/17/bc/17bc21f519133ca31f857a65c897925a.jpg"));
            } catch (IOException e) {
                e.printStackTrace();
            }
            g.drawImage(image, 0, 0,
                    getWidth() / 2, getHeight() / 2, this);
        }
    };
    second_panel.setLayout(new BoxLayout(second_panel, BoxLayout.Y_AXIS));

    String [] resources = {"flowers", "nyanko", "tutturu~"};
    for(String s : resources){
        JLabel label = new JLabel(0 + " " + s);
        label.setFont(new Font("Serif", Font.BOLD, 18));
        second_panel.add(label);
    }

    return second_panel;
}

private GridBagLayout getLayout() {
    GridBagLayout gridBagLayout = new GridBagLayout();
    gridBagLayout.rowHeights = new int[]{0, 0, 0, 0};
    gridBagLayout.rowWeights = new double[]{0.0, 0.0, 1.0, 1.0};
    gridBagLayout.columnWidths = new int[]{0, 0, 0, 0};
    gridBagLayout.columnWeights = new double[]{0.0, 0.0, 1.0, 1.0};
    return gridBagLayout;
}

}

You can solve this with one resize listener attached to main panel. I assume that you want to resize JLabel exactly how much you resized your main panel. Here is how you can achieve that:

private Dimension main_size = null; //Inital Dimension of main JPanel
main.addComponentListener(new ComponentAdapter() {
		      @Override
		      public void componentResized(ComponentEvent e) {
		    	//This method is called everytime main panel is resized
		    	  if((main_size==null) ||
		    	     (main_size.width==0 && main_size.height==0)) {
		    		  main_size=main.getSize();
		    	  }
		    	  updateJLabelSize(icon,e.getComponent().getSize());
		      }
		    });	      

//Method for updating JLabel size
private void updateJLabelSize(JLabel icon,Dimension newSize) {
		int heightDifference = newSize.height - main_size.height;
		int widthDifference = newSize.width - main_size.width;
		icon.setSize(new Dimension(icon.getWidth() + widthDifference, 
        icon.getHeight()+heightDifference));
		main_size = newSize;
	}