Why I should use a JPanel?

Question

What's the difference between:

public class Test {

    public static void main(String[] args) {
        JButton button= new JButton("1");
        button.setVisible(true);
        JPanel panel= new JPanel();
        panel.add(button);
        panel.setVisible(true);
        JFrame frame = new JFrame();
        frame.add(panel);
        frame.setVisible(true);
        frame.pack();

    }
}

and

public class Test {

    public static void main(String[] args) {
        JButton button= new JButton("1");
        button.setVisible(true);
        JFrame frame = new JFrame();
        frame.add(button);
        frame.setVisible(true);
        frame.pack();

    }
}

I know that a JPanel is a container for GUI components but I really don't see the utility of use it. Certainly I'm very wrong but I'm starting with Swing, so... Why I should use a JPanel? What's really the purpose?

user3437460 · Accepted Answer · 2016-03-19T15:07:49.060

Why I should use a JPanel?

You use a JPanel for one or more of the following benefits:

To group components together.
To better organize your components.
To enable us to use multiple layouts and combine their effects. (For example a GridLayout for number pad, a CardLayout for display panel where you can switch drawings).
To break down a large problem into sub-problems. So instead of implementing everything in one big frame, you can concentrate implementing the properties of each individual panels (such as layout, dimension, background image, background color..etc)
For reusability if you have customized panels. A common panel can be used by different classes.
For easy debugging, since you can test each panel individually.
For scalability.
For maintainability.

Usually I perceive a JFrame like a real life painting frame and JPanel like a piece of blank paper. We don't paint directly onto the frame. Instead, we paint on a piece of paper, and insert the paper into the frame. Painting directly on the frame is possible, but no one does that.

The same goes with JFrame and JPanel. We can add components directly to the frame, but usually we add it to the panel, then add the panel to the frame.

@Vinicius You may take a look at my solution. – user3437460 Mar 19 '16 at 15:00 — user3437460, Mar 19 '16 at 15:00
Fantastic analogy with the painting frame and paper! – Ray Oct 06 '21 at 01:58 — Ray, Oct 06 '21 at 01:58

score 3 · Answer 2 · answered Mar 19 '16 at 12:32

There is basically no visual difference in your two examples, but the UI is too poor for exhibiting a difference. A JPanel is exactly what a panel is in real life, something used to display things inside it. It is generally used to contains different elements that you want to display in a given layout.

score 2 · Answer 3 · answered Mar 19 '16 at 12:31

2

The purpose is to group components together.

Look at this web page for example. You could imagine a center panel with the question and answers laid out vertically, a right panel containing yet other panels: one for the blog, one for the job announcement, one for the related questions, and finally a south panel for the footer.

The footer panel can be reused in other pages than question pages.

The layout of the page as a whole is easier to understand and implement if you divide it into independant panels.

answered Mar 19 '16 at 12:31

JB Nizet

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Look at this web page for example. - hmmm which one please :-) – mKorbel Mar 19 '16 at 14:48
This one. The one you're looking at right now while reading this comment. – JB Nizet Mar 19 '16 at 14:50
It also allows you to use different borders and layouts in the different panels that might be combined into a single GUI. [This answer](http://stackoverflow.com/a/5630271/418556) has screenshots of an example of both. – Andrew Thompson Mar 20 '16 at 00:12

score 2 · Answer 4 · answered Mar 19 '16 at 12:33

2

You can put more than one JPanel in the JFrame. JPanel is a conainer for you other components and can grouping it by logical features.

answered Mar 19 '16 at 12:33

Kornelia Kobiela

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score 2 · Answer 5 · answered Mar 19 '16 at 12:33

You can use a JPanel to create a reusable building block of your user interface and maintain a single point of maintenance for all instances of it. Let's say you would create a JFrame containing a diff view for two texts. The left and right view are lets say identical twins. It is way easier to create a panel twice and to add these two panels on the frame instead of adding all subsequent components twice into your frame directly.

Whenever you now have to change the view, let's say you want to add another button, or whatever you can do it at your panel and both - the left and right view will contain the changes immediately.

score 2 · Answer 6 · answered Mar 19 '16 at 12:47

2

Grouping certain components together so you can manipulate groups of them within loops. Maybe to check if all text boxes within a region meet a certain requirement or to destroy them completely.

answered Mar 19 '16 at 12:47

Sifer

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Why I should use a JPanel?

6 Answers6