What is the relation between ContentPane and JPanel?

I found one example in which buttons are added to panels (instances of JPanel) then panels are added to the the containers (instances generated by getContentPane()) and then containers are, by the construction, included into the JFrame (the windows).

I tried two things:

1. I got rid of the containers. In more details, I added buttons to a panel (instance of JPanel) and then I added the panel to the windows (instance of JFrame). It worked fine.

2. I got rid of the panels. In more details, I added buttons directly to the container and then I added the container to the window (instance of JFrame).

So, I do not understand two things.

1. Why do we have two competing mechanism to do the same things?

2. What is the reason to use containers in combination with the panels (JPanel)? (For example, what for we include buttons in JPanels and then we include JPanels in the Containers). Can we include JPanel in JPanel? Can we include a container in container?

ADDED:

Maybe essence of my question can be put into one line of code:

frame.getContentPane().add(panel);

What for we put getContentPane() in between? I tried just frame.add(panel); and it works fine.

ADDED 2:

I would like to add some code to be more clear about what I mean. In this example I use only JPane:

import java.awt.*;
import javax.swing.*;
public class HelloWorldSwing {
    public static void main(String[] args) {
        JFrame frame = new JFrame("HelloWorldSwing");
        JPanel panel = new JPanel();
        panel.setLayout(new BorderLayout());        
        panel.add(new JButton("W"), BorderLayout.NORTH);
        panel.add(new JButton("E"), BorderLayout.SOUTH);
        frame.add(panel);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }
}

And in this example I use only Content Pane:

import java.awt.*;
import javax.swing.*;
public class HelloWorldSwing {
    public static void main(String[] args) {
    JFrame frame = new JFrame("HelloWorldSwing");
    Container pane = frame.getContentPane();
    pane.setLayout(new BorderLayout()); 
    pane.add(new JButton("W"), BorderLayout.NORTH);
    pane.add(new JButton("E"), BorderLayout.SOUTH);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setVisible(true);
    }
}

Both work fine! I just want to know if between these two ways to do things one is better (safer).

It's not two competing mechanisms - a JPanel is a Container (just look at the class hierarchy at the top of the JPanel javadocs). JFrame.getContentPane() just returns a Container to place the Components that you want to display in the JFrame. Internally, it's using a JPanel (by default - you can change this by calling setContentPane()) As for why it's returning a Container instead of a JPanel - it's because you should program to an interface, not an implementation - at that level, all that you need to care about is that you can add Components to something - and even though Container is a class rather than an interface - it provides the interface needed to do exactly that.

As for why both JFrame.add() and JFrame.getContentPane().add() both do the same thing - JFrame.add() is overridden to call JFrame.getContentPane().add(). This wasn't always the case - pre-JDK 1.5 you always had to specify JFrame.getContentPane().add() explicitly and JFrame.add() threw a RuntimeException if you called it, but due to many complaints, this was changed in JDK 1.5 to do what you'd expect.

Good question. I found it helpful to understand that "Swing provides three generally useful top-level container classes: JFrame, JDialog, and JApplet. ... As a convenience, the add method and its variants, remove and setLayout have been overridden to forward to the contentPane as necessary."—Using Top-Level Containers

I believe the reason is because Swing was built off of AWT, and Container is a top level AWT object. It really isn't the greatest design choice, though, since you generally don't want to mix AWT (heavyweight) objects with Swing (lightweight).

I think the best way to handle it is to always cast the contentPane to a JPanel.

JPanel contentPanel = (JPanel)aFrame.getContentPane();

It is all written in the last version API doc that JFrame.add() (nowerdays) is enough.

You may compare to older Java versions here.