What is the relation between ContentPane and JPanel?
Solution 1
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 Component
s 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 Component
s 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.
Solution 2
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
Solution 3
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();
Solution 4
It is all written in the last version API doc that JFrame.add() (nowerdays) is enough.
You may compare to older Java versions here.
Solution 5
interesting: jframe.setBackground(color)
does not work for me, but jframe.getContentPane().setBackground(color)
works.
Roman
Updated on November 27, 2020Comments
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Roman over 3 years
I found one example in which buttons are added to panels (instances of
JPanel
) then panels are added to the the containers (instances generated bygetContentPane()
) and then containers are, by the construction, included into theJFrame
(the windows).I tried two things:
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 ofJFrame
). It worked fine.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.
Why do we have two competing mechanism to do the same things?
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 includeJPanel
inJPanel
? 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 justframe.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).
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Nate about 14 yearsIt's not mixing AWT and Swing components... see my answer
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Roman about 14 years@Nate, what do you mean by " Internally, it's using a JPanel". You mean that Container is using a JPanel? What do you mean by that? I though that JPanel is a subclass of Container. And what can I change by "setContentPane"? OK. You say that Container is a class (and I always considered Container as a class). But JPane is also a class. Why we cannot use just JPane or just Container? I was able to do the same things by using only JPane. I was also able to do the same using only Containers.
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Nate about 14 years
JFrame
is internally using aJPanel
object to back thegetContentPane()
method. This is why Zachary's answer works. The actualJPanel
reference is buried in theJFrame.rootPane
field. You would callsetContentPane()
on theJFrame
- for example, you could change line 10 in your first example fromframe.add(panel)
toframe.setContentPane(panel)
.Container
is a class.JPanel
is a class (which is a subclass ofContainer
). You're confusing the type of the reference with the type of the object the reference references. Think aboutContainer container = new JPanel()
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Lucio about 10 years@Nate the source (pragmaticjava.blogspot.com.ar/2008/08/…) has been removed.
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Nate about 10 years@Lucio Thanks! Changed the link to a question here on StackOverflow about the same thing.