CardLayout's Show Method Not Doing Anything

So I just started practicing CardLayout, but I can't seem to get it to work. I look at over 10 sites, and I seem to be doing the same thing as them, but for some reason my version isn't working. The next() method is working fine, but calling show() doesn't do anything.

private static CardLayout cards;
private static JPanel panel;

private Main() {
    super("Card Layout");
    setBounds(0, 0, 500, 500);
    setDefaultCloseOperation(3);

    panel = new JPanel();

    add(panel);

    cards = new CardLayout();

    JPanel p1 = new JPanel();
    p1.setBackground(Color.WHITE);

    JPanel p2 = new JPanel();
    p2.setBackground(Color.BLACK);

    JPanel p3 = new JPanel();
    p3.setBackground(Color.RED);

    JPanel p4 = new JPanel();
    p4.setBackground(Color.BLUE);

    panel.add("white", p1);
    panel.add("black", p2);
    panel.add("red", p3);
    panel.add("blue", p4);

    panel.setLayout(cards);

    // cards.next(panel); This works fine for me.
    cards.show(panel, "red");

    setVisible(true);
}

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

There are many problems with the code you posted, but the reason it is not working is simply the wrong order of your code. Before adding components to the JPanel, you need to set its layout manager. Also, you are invoking the wrong add() method. Only add the JPanel with the CardLayout to its parent container after you have added its contained JPanels.

Apart from that, the code you posted doesn't compile because class Main obviously extends JFrame but that is missing in the code you posted.

Here is your code corrected.

import java.awt.CardLayout;
import java.awt.Color;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class Main extends JFrame {
    private static CardLayout cards;
    private static JPanel panel;

    private Main() {
        super("Card Layout");
        setBounds(0, 0, 500, 500);
        setDefaultCloseOperation(3);

        panel = new JPanel();
        cards = new CardLayout();
        panel.setLayout(cards);

        JPanel p1 = new JPanel();
        p1.setBackground(Color.WHITE);

        JPanel p2 = new JPanel();
        p2.setBackground(Color.BLACK);

        JPanel p3 = new JPanel();
        p3.setBackground(Color.RED);

        JPanel p4 = new JPanel();
        p4.setBackground(Color.BLUE);

        panel.add(p1, "white");
        panel.add(p2, "black");
        panel.add(p3, "red");
        panel.add(p4, "blue");

        // cards.next(panel); This works fine for me.
        cards.show(panel, "red");
        add(panel);

        setVisible(true);
    }

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

I recommend the tutorial Creating a GUI With JFC/Swing