How to add multiple JPanels to a JScrollPane?

Question

I'm struggling to add multiple JPanel containers to a single JScrollPane in Java swing. My desired output is to have a scroll pane existing on a base panel that is on a JFrame, and to have that scroll pane contain 2 panels, for now.

Basically, just how like a scroll pane can have its view set to a single panel, I want to have it set to 2 panels.

The only things existing on these panels are drawn Graphics on them, no JComponent instances. I've looked at other links like this one: Java Swing: how to add multiple JPanels to a JScrollPane, but I wasn't getting the desired output. Here is my latest attempt, which only shows the scroll pane edge and the panels aren't visible (minimal reproducible):

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

public class Test {
    
    public static void main(String[] args) {
        new Test();
    }
    
    public Test() {
        JFrame frame = new JFrame("Test"); //initialize frame
        frame.setSize(500, 500);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        JPanel base = new JPanel(); //make base panel with SpringLayout
        SpringLayout baseLayout = new SpringLayout();
        base.setLayout(baseLayout);
        
        JPanel panel1 = new JPanel() { //initialize JPanels
            public void paintComponent(Graphics tool) {
                super.paintComponent(tool);
                tool.drawRect(50, 50, 100, 100);
            }
        };
        
        JPanel panel2 = new JPanel() {
            public void paintComponent(Graphics tool) {
                super.paintComponent(tool);
                tool.drawRect(75, 75, 50, 50);
            }
        };
        
        Container cont = new Container(); //create a container to hold panels
        SpringLayout contLayout = new SpringLayout();
        cont.setLayout(contLayout);
        
        contLayout.putConstraint(SpringLayout.WEST, panel1, 0, SpringLayout.WEST, cont); 
        contLayout.putConstraint(SpringLayout.NORTH, panel1, 0, SpringLayout.NORTH, cont);
        cont.add(panel1); //add panel1 to top left corner of container
        
        
        contLayout.putConstraint(SpringLayout.WEST, panel2, 0, SpringLayout.WEST, cont); 
        contLayout.putConstraint(SpringLayout.NORTH, panel2, 0, SpringLayout.NORTH, cont);
        cont.add(panel2); //add panel2 to top left corner of container
        
        cont.setPreferredSize(new Dimension(201, 201));
        
        JScrollPane scroll = new JScrollPane(cont); //add container to scroll pane
        scroll.setPreferredSize(new Dimension(200, 200));
        
        baseLayout.putConstraint(SpringLayout.HORIZONTAL_CENTER, scroll, 0, SpringLayout.HORIZONTAL_CENTER, base); 
        baseLayout.putConstraint(SpringLayout.VERTICAL_CENTER, scroll, 0, SpringLayout.VERTICAL_CENTER, base);
        base.add(scroll); //add scroll pane to center of base JPanel
        
        frame.add(base);
        frame.setVisible(true);
    }
}

Answer 1

Alright so I did more experimenting and I added these few lines and now both of the panels show up correctly within the JScrollPane:

panel1.setPreferredSize(cont.getPreferredSize());
panel2.setPreferredSize(cont.getPreferredSize());
        
panel1.setOpaque(false);
panel2.setOpaque(false);

Not sure why it worked, if someone could explain that would be great. Here is the working code:

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

public class Test {
    
    public static void main(String[] args) {
        new Test();
    }
    
    public Test() {
        JFrame frame = new JFrame("Test"); //initialize frame
        frame.setSize(500, 500);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        JPanel base = new JPanel(); //make base panel with SpringLayout
        SpringLayout baseLayout = new SpringLayout();
        base.setLayout(baseLayout);
        
        JPanel panel1 = new JPanel() { //initialize JPanels
            public void paintComponent(Graphics tool) {
                super.paintComponent(tool);
                tool.drawRect(50, 50, 100, 100);
            }
        };
        
        JPanel panel2 = new JPanel() {
            public void paintComponent(Graphics tool) {
                super.paintComponent(tool);
                tool.drawRect(75, 75, 50, 50);
            }
        };
        
        Container cont = new Container(); //create a container to hold panels
        SpringLayout contLayout = new SpringLayout();
        cont.setLayout(contLayout);
        
        contLayout.putConstraint(SpringLayout.WEST, panel1, 0, SpringLayout.WEST, cont); 
        contLayout.putConstraint(SpringLayout.NORTH, panel1, 0, SpringLayout.NORTH, cont);
        cont.add(panel1); //add panel1 to top left corner of container
        
        contLayout.putConstraint(SpringLayout.WEST, panel2, 0, SpringLayout.WEST, cont); 
        contLayout.putConstraint(SpringLayout.NORTH, panel2, 0, SpringLayout.NORTH, cont);
        cont.add(panel2); //add panel2 to top left corner of container
        
        
        cont.setPreferredSize(new Dimension(201, 201));
        panel1.setPreferredSize(cont.getPreferredSize());
        panel2.setPreferredSize(cont.getPreferredSize());
        
        panel1.setOpaque(false);
        panel2.setOpaque(false);
        
        JScrollPane scroll = new JScrollPane(cont); //add container to scroll pane
        scroll.setPreferredSize(new Dimension(200, 200));
        
        baseLayout.putConstraint(SpringLayout.HORIZONTAL_CENTER, scroll, 0, SpringLayout.HORIZONTAL_CENTER, base); 
        baseLayout.putConstraint(SpringLayout.VERTICAL_CENTER, scroll, 0, SpringLayout.VERTICAL_CENTER, base);
        base.add(scroll); //add scroll pane to center of base JPanel
        
        frame.add(base);
        frame.setVisible(true);
    }
}