In Java, you can create a custom GUI for a JComboBox by providing a custom cell renderer and/or a custom editor.

Here is an example that demonstrates how to use a custom cell renderer to display an icon and text in a JComboBox:

import java.awt.Component;
import javax.swing.DefaultListCellRenderer;
import javax.swing.ImageIcon;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JList;
import javax.swing.JPanel;

public class CustomComboBoxExample {
    public static void main(String[] args) {
        // Create a list of items to display in the combo box
        String[] items = {"Item 1", "Item 2", "Item 3", "Item 4"};

        // Create a custom cell renderer to display an icon and text
        JComboBox<String> comboBox = new JComboBox<>(items);
        comboBox.setRenderer(new CustomCellRenderer());

        // Create a panel to hold the combo box
        JPanel panel = new JPanel();
        panel.add(comboBox);

        // Create a frame to hold the panel
        JFrame frame = new JFrame("Custom ComboBox Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(panel);
        frame.pack();
        frame.setVisible(true);
    }

    private static class CustomCellRenderer extends DefaultListCellRenderer {
        private final ImageIcon icon = new ImageIcon("icon.png");

        @Override
        public Component getListCellRendererComponent(JList<?> list, Object value, int index, boolean isSelected, boolean cellHasFocus) {
            JLabel label = (JLabel) super.getListCellRendererComponent(list, value, index, isSelected, cellHasFocus);
            label.setIcon(icon);
            label.setText(value.toString());
            return label;
        }
    }
}

In this example, we create a JComboBox instance and set a custom cell renderer by calling the setRenderer() method and passing in an instance of our CustomCellRenderer class. This renderer overrides the getListCellRendererComponent() method of the DefaultListCellRenderer class to display an icon and text for each item in the combo box.

We then create a panel and add the combo box to it. Finally, we create a frame and add the panel to it.

You can also create a custom editor to allow the user to input values directly into the JComboBox. To do this, you would create a class that implements the ComboBoxEditor interface and set it using the setEditor() method.