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Easy Learn Java: Programming Articles, Examples and Tips - Page 503


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Easy Java Lecture 17: Layout managers - a visual guide part I

Go to all tips in Java Lectures by Anatoliy Malyarenko

Laying out components within a container

by: Anatoliy Malyarenko

Abstract

Contents of the lecture.

  • A visual guide to layout managers.
  • Tips on choosing the layout manager.
  • How to use different layout managers.

BorderLayout

border layout demo

Every content pane is initialised to use a BorderLayout. A BorderLayout places components in up to five areas: top, bottom, left, right, and center. All extra space is placed in the center area.

BoxLayout

The BoxLayout class puts components in a single row or column. It respects the components' requested maximum sizes and also lets you align components.

CardLayout

      

 

The CardLayout class lets you implement an area that contains different components at different times. A CardLayout is often controlled by a combo box, with the state of the combo box determining which panel (group of components) the CardLayout displays. An alternative to using CardLayout is using a tabbed pane, which provides similar functionality but with a pre-defined GUI.

FlowLayout


FlowLayout is the default layout manager for every JPanel. It simply lays out components in a single row, starting a new row if its container isn't sufficiently wide. Both panels in CardLayoutDemo, shown previously, use FlowLayout.

GrigBagLayout

GridBagLayout is a sophisticated, flexible layout manager. It aligns components by placing them within a grid of cells, allowing some components to span more than one cell. The rows in the grid can have different heights, and grid columns can have different widths.

GridLayout

GridLayout simply makes a bunch of components equal in size and displays them in the requested number of rows and columns.

SpringLayout




SpringLayout is a flexible layout manager designed for use by GUI builders. It lets you specify precise relationships between the edges of components under its control. For example, you might define that the left edge of one component is a certain distance (which can be dynamically calculated) from the right edge of a second component.

Setting the layout manager

A layout manager is an object that implements the LayoutManager interface and determines the size and position of the components within a container. Although components can provide size and alignment hints, a container's layout manager has the final say on the size and position of the components within the container.

As a rule, the only containers whose layout managers you need to worry about are JPanels and content panes. Each JPanel object is initialised to use a FlowLayout, unless you specify differently when creating the JPanel. Content panes use BorderLayout by default. If you don't like the default layout manager that a panel or content pane uses, you're free to change it to a different one.

You can set a panel's layout manager using the JPanel constructor. For example:

JPanel panel = new JPanel(new BorderLayout());

After a container has been created, you can set its layout manager using the setLayout method. For example:

Container contentPane = frame.getContentPane();
contentPane.setLayout(new FlowLayout());

Although we recommend that you use layout managers, you can perform layout without them. By setting a container's layout property to null, you make the container use no layout manager. With this strategy, called absolute positioning, you must specify the size and position of every component within that container. One drawback of absolute positioning is that it doesn't adjust well when the top-level container is resized. It also doesn't adjust well to differences between users and systems, such as different font sizes and locales.

Adding components to a container

When you add components to a panel or content pane, the arguments you specify to the add method depend on the layout manager that the panel or content pane is using. For example, BorderLayout requires that you specify the area to which the component should be added, using code like this:

pane.add(aComponent, BorderLayout.PAGE_START);

The how-to section for each layout manager, described later, has details on what, if any, arguments you need to specify to the add method. Some layout managers, such as GridBagLayout and SpringLayout, require elaborate setup procedures. Many layout managers, however, simply place components based on the order they were added to their container.

Providing size and alignment hints

Sometimes you need to customise the size hints that a component provides to its container's layout manager, so that the component will be laid out well. You can do this by specifying one or more of the minimum, preferred, and maximum sizes of the component.

You can invoke the component's methods for setting size hints -- setMinimumSize, setPreferredSize, and setMaximumSize. Or you can create a subclass of the component that overrides the appropriate getter methods -- getMinimumSize, getPreferredSize, and getMaximumSize. Here is an example of making a component's maximum size unlimited:

component.setMaximumSize(new Dimension(Integer.MAX_VALUE, Integer.MAX_VALUE));

Many layout managers don't pay attention to a component's requested maximum size. However, BoxLayout and SpringLayout do.

Besides providing size hints, you can also provide alignment hints. For example, you can specify that the top edges of two components should be aligned. You set alignment hints either by invoking the component's setAlignmentX and setAlignmentY methods, or by overriding the component's getAlignmentX and getAlignmentY methods. Although most layout managers ignore alignment hints, BoxLayout honors them.

Putting space between components

Three factors influence the amount of space between visible components in a container:

The layout manager Some layout managers automatically put space between components; others don't. Some let you specify the amount of space between components. See the how-to page below for each layout manager for information about spacing support.

Invisible components You can create lightweight components that perform no painting, but that can take up space in the GUI. Often, you use invisible components in containers controlled by BoxLayout.

Empty borders No matter what the layout manager, you can affect the apparent amount of space between components by adding empty borders to components. The best candidates for empty borders are components that typically have no default border, such as panels and labels. Some other components might not work well with borders in some look-and-feel implementations, because of the way their painting code is implemented.

to be continued....


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Posted by jalex on Wednesday, May 02, 2007 (21:26:29) (3955 reads)

Assertions do not work in Tomcat and Java!

Go to all tips in JSP, Servlets

Question: Assertions do not work on my Tomcat 5.15 on Linux! I use JDK 1.5_05, used many code examples of this Java feature - assertion, nothing works!!!!!

What tool could I use to find out the reason of the problem? Or just it is Tomcat that does not support assertions?

Answer: Tomcat server supports assertions, but you need explicitly enable assertions in the JVM. You have to find catalina.sh file (on UNIX/Linux/Solaris OS) in Tomcat's "bin" directory and locate a line with "JAVA_OPTS".

This variable is used to pass different options to JVM during start up. For example:

JAVA_OPTS = "-Xms128m -Xmx512m" add options to manage memory.

Be aware that assertions can be enabled/disabled in general, for all classes and packages by switch:


"-ea" or "-enableassertions"

"-da" or "-disableassertions"

If you add a package, sub package or class name after the switch you will limit the area where switch is applied. It works both for enable and disable commands. Append the switch for assertions to the line JAVA_OPTS, for example for your class only:

JAVA_OPTS = "-Xms128m -Xmx512m -ea MyClass "

Do not forget to use the full class name, including package name and sub packages if the class belongs to some package.

One more general notice: you have to use the switch before any class loaded! For example:

java -ea MyClass, not

java MyClass -ea


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Posted by jalex on Monday, April 23, 2007 (21:35:09) (3133 reads)

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