Displaying icon on menu bar in Java Swing

I have an application with a Swing GUI and I would like to add a search-field with a search-button (lupe icon) to the menu bar. However, the lupe icon won't display. Here is my code:

public class Ui_Frame {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(Ui_Frame::createAndShowGUI);
    }
    

    private static void createAndShowGUI() {
         f = new JFrame("Myframe"); 

         ...

         JMenuBar menubar = new JMenuBar();
         Icon lupeIcon = new ImageIcon("Resources/lupe_icon.png");
         JButton j_searchButton = new JButton(lupeIcon);
         menubar.add(j_searchButton);

         ...

         f.setJMenuBar(menubar);
         f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
         f.pack();
         f.setVisible(true);

My project structure is like

Project 
       Src
          Ui_Frame.java
          Resources 
                 lupe_icon.png 

The resulting button shows no icon at all :

I do not get any error message, it compiles without problems. Even when I try to catch any exception from the new ImageIcon(...) I'm not getting any hint at what the error is.

Any ideas as to what Im doing wrong here?

EDIT:

Here is a minimal example:

import javax.swing.*;

public class test {

    private static JFrame f;

    public static void main(String[] args) {
        SwingUtilities.invokeLater(test::createAndShowGUI);
    }

    private static void createAndShowGUI() {

        f = new JFrame("Test");
        JMenuBar menubar = new JMenuBar();

        Icon lupeIcon = new ImageIcon(test.class.getResource("/Resources/lupe_icon.png"));
        JButton j_searchButton = new JButton(lupeIcon);
        menubar.add(j_searchButton);

        f.setJMenuBar(menubar);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.pack();
        f.setSize(500,500);
        f.setVisible(true);
    }
}

As you can see I am now loading the image with class.getResource(...) as was suggested by @AndrewThompson and @SergiyMedvynskyy, but that does not solve the problem. Also I was told that my classes should not be static, but since my Main needs to be static for me to be able to run the program and I have been told that I should start the UI with SwingUtilities.invokeLater(test::createAndShowGUI);
which also forces createAndShowGUI() to be static, I do not know how to make it not static.

For me it's working without mention folder "Resources" in the path,
like that:

Icon lupeIcon = new ImageIcon(test.class.getResource("/lupe_icon.png"));

But, i'm not sure your project structure is correct.

If still doesn't work, try rename folder with small 'r', and maybe also change project structure, this is my structure:

> \src\main\java... (all java files)
>
> \src\main\resources (resources folder, it displayed for me like a package)

Note - i think best is to check why u can't get the image in debug

After switching to Icon lupeIcon = new ImageIcon(test.class.getResource("Resources/lupe_icon.png")); to load the icon instead of new ImageIcon("Resources/lupe_icon.png"); I noticed that I was getting nullpointer-exceptions, despite the image clearly being there. After some googling I found out that the resource folder apparently is not just any folder but has to be marked as the resource root folder, as suggested here.

After marking the folder everything works fine