Introducing Containers and Layout Managers

by David Wall

July 9, 2002 —

Creating windows in Java isn't a difficult process, really. You just
declare an instance of the JFrame object, specify some points that
define the locations of the window's upper-left and lower-right corners
on the screen (and hence its size), tell what happens to the contents of
the frame when it's closed, and then make the frame visible.

Windows aren't too great unless they contain user interface elements,
such as buttons and labels, but writing your programs to run on many
different kinds of computers them isn't as easy as it sounds. Not only
must you address the considerable assortment of video resolutions and
other low-level designs, but also an assortment of interface appearances
that vary among operating environments.

Enter the Layout Manager
A layout manager is a set of rules that is used to determine how
interface elements are laid out, visually, in a container (such as a
JFrame window). As is the cases with most things in Java, layout
managers exist as objects that can be instantiated.

Here's a program that uses the FlowLayout layout manager to orient a
button in a JFrame window:

import javax.swing.*;
import java.awt.*;

public class ButtonWindow
{

static JFrame myWindow = new JFrame("Greetings, IT World Readers!");

public static void main(String[] args)
{
myWindow.setBounds(100, 150, 500, 150);
myWindow.setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);

FlowLayout manager = new FlowLayout();
Container buttonHolder = myWindow.getContentPane();
buttonHolder.setLayout(manager);

buttonHolder.add(new JButton("Press Me"));

myWindow.setVisible(true);
}

}

There are four interesting lines in this program. This one...

FlowLayout manager = new FlowLayout();

..creates a new object of type FlowLayout. This one...

Container buttonHolder = myWindow.getContentPane();

...distinguishes the container aspect of myWindow from the rest of the
object. This line...

buttonHolder.setLayout(manager);

...associates the FlowLayout object with the container. And finally,
this line...

buttonHolder.add(new JButton("Press Me"));

...builds a button into the container where (because of the association
we just made) its position is determined by the rules of the FlowLayout
object called manager.

You might want to modify that program to include a whole bunch of
buttons, and note how they fit into the window. Here's a variation to
try. If you want to right-justify your elements as they flow into the
container, try replacing this line...

FlowLayout manager = new FlowLayout();

...with this one:

FlowLayout manager = new FlowLayout(FlowLayout.RIGHT);

ITworld