Adding JPanel from another class to JPanel in JFrame

I can't get my JFrame from main class to display JPanel from another class. Everything compiles without errors.

JFrameTest.java:

package jframetest;

import java.awt.FlowLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;


public class JFrameTest extends JFrame {

    public JFrameTest() {

        FlowLayout mainLayout = new FlowLayout();
        setSize(320, 480);
        setResizable(false);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(mainLayout);

        JPanel panelMain = new JPanel(mainLayout);

        JButton testButton = new JButton("Test12");
        panelMain.add(testButton);

        JPanelOne panel = new JPanelOne();
        panelMain.add(panel);
        panel.setVisible(true);
        add(panelMain);



    }

    public static void main(String[] arguments) {

        JFrameTest frame = new JFrameTest();
        frame.setVisible(true);


    }
}

JPanelOne.java:

package jframetest;

import java.awt.FlowLayout;
import javax.swing.JButton;
import javax.swing.JPanel;


public class JPanelOne extends JPanel {

    public JPanelOne() {

        FlowLayout layoutPanel = new FlowLayout();
        JPanel panel = new JPanel(layoutPanel);
        JButton button = new JButton("test");
        panel.add(button);
        panel.setVisible(true);

    }

}

Is it good practice to keep diffrent JPanels in their own classes? (Example: Wanting to have JFrame contain few same size JPanels, which will be switched by setting setVisible() to true/false)

EDIT

Thank you for all your answers. Noted. Now back to my question:

Now that I know how to add single GUI elements created in other classes I want to know if it is possible to organize elements with layout manager in one of the classes (maybe in some other container than JPanel), so I can