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Trail: Creating a GUI with JFC/Swing
Lesson: Laying Out Components Within a Container

## How to Use BoxLayout

> The Swing packages include a general purpose layout manager named `BoxLayout`. `BoxLayout` either stacks its components on top of each other (with the first component at the top) or places them in a tight row from left to right -- your choice. You might think of it as a full-featured version of `FlowLayout`. Here is an applet that demonstrates using `BoxLayout` to display a centered column of components:

> This is a picture of the applet's GUI. To run the applet, click the picture. The applet will appear in a new browser window.

> By creating one or more lightweight containers that use `BoxLayout`, you can achieve some layouts for which the more complex `GridBagLayout` is often used. `BoxLayout` is also useful in some situations where you might consider using `GridLayout` or `BorderLayout`. One big difference between `BoxLayout` and the existing AWT layout managers is that `BoxLayout` respects each component's maximum size and X/Y alignment. We'll discuss that later.

> The following figure shows a GUI that uses two instances of `BoxLayout`. In the top part of the GUI, a top-to-bottom box layout places a label above a scroll pane. In the bottom part of the GUI, a left-to-right box layout places two buttons next to each other. A `BorderLayout` combines the two parts of the GUI and ensures that any excess space is given to the scroll pane.

> The following code, taken from `ListDialog.java`, lays out the GUI. This code is in the constructor for the dialog, which is implemented as a `JDialog` subclass. The bold lines of code set up the box layouts and add components to them.

> >     JScrollPane listScroller = new JScrollPane(list);
> >     listScroller.setPreferredSize(new Dimension(250, 80));
> >     listScroller.setMinimumSize(new Dimension(250, 80));
> >     listScroller.setAlignmentX(LEFT_ALIGNMENT);
> >     ...
> >     //Lay out the label and scroll pane from top to bottom.
> >     JPanel listPane = new JPanel();
> >     listPane.setLayout(new BoxLayout(listPane, BoxLayout.Y_AXIS));
> >     JLabel label = new JLabel(labelText);
> >     listPane.add(label);
> >     listPane.add(Box.createRigidArea(new Dimension(0,5)));
> >     listPane.add(listScroller);
> >     listPane.setBorder(BorderFactory.createEmptyBorder(10,10,10,10));

> >     //Lay out the buttons from left to right.
> >     JPanel buttonPane = new JPanel();
> >     buttonPane.setLayout(new BoxLayout(buttonPane, BoxLayout.X_AXIS));
> >     buttonPane.setBorder(BorderFactory.createEmptyBorder(0, 10, 10, 10));
> >     buttonPane.add(Box.createHorizontalGlue());
> >     buttonPane.add(cancelButton);
> >     buttonPane.add(Box.createRigidArea(new Dimension(10, 0)));
> >     buttonPane.add(setButton);

> >     //Put everything together, using the content pane's BorderLayout.
> >     Container contentPane = getContentPane();
> >     contentPane.add(listPane, BorderLayout.CENTER);
> >     contentPane.add(buttonPane, BorderLayout.SOUTH);

> The first bold line creates a top-to-bottom box layout and sets it up as the layout manager for `listPane`. The two arguments to the `BoxLayout` constructor are the container that it manages and the axis along with the components will be laid out. The next three bold lines add the label and scroll pane to the container, separating them with a rigid area -- an invisible lightweight component used to add space between components. In this case, the rigid area has no width and puts exactly 5 pixels between the label and scroll pane. Rigid areas are discussed later, in Using Invisible Components as Filler.

> The next chunk of bold code creates a left-to-right box layout and sets it up for the `buttonPane` container. Then the code adds two buttons to the container, using a rigid area to put 10 pixels between the buttons. To make the buttons be placed at the right side of their container, the first component added to the container is glue. This glue is an invisible lightweight component that grows as necessary to absorb any extra space in its container. Glue is discussed in Using Invisible Components as Filler.

> As an alternative to using invisible components, you can sometimes use empty borders to create space around components. For example, the preceding code snippet uses empty borders to put 10 pixels between all sides of the dialog and its contents, and between the two parts of the contents. Borders are completely independent of layout managers. They're simply how Swing components draw their edges. See How to Use Borders for more information.

> The following sections discuss `BoxLayout` in more detail:

> - Box layout features
> - Using invisible components as filler
> - Fixing alignment problems
> - Specifying component sizes
> - The box layout API
> - Examples that use box layouts
> Don't let the length of the `BoxLayout` discussion scare you! You can probably use `BoxLayout` with the information you already have. If you run into trouble or you want to take advantage of `BoxLayout`'s power, read on.

### Box Layout Features

> As we said before, a `BoxLayout` arranges components either from top to bottom or from left to right. As it arranges components, the box layout takes the components' alignments and minimum, preferred, and maximum sizes into account. In this section, we'll talk about top-to-bottom (Y axis) layout. The same concepts apply to left-to-right layout. You simply substitute X for Y, height for width, and so on.

> When a `BoxLayout` lays out components from top to bottom, it tries to size each component at the component's preferred height. If the amount of vertical space is not ideal, the box layout tries to adjust each components' height so that the components fill the available amount of space. However, the components might not fit exactly, since `BoxLayout` respects each component's requested minimum and maximum heights. Any extra space appears at the bottom of the container.

> A top-to-bottom box layout tries to make all of its container's components equally wide -- as wide as the largest preferred width. If the container is forced to be wider than that, then the box layout tries to make all the components as wide as the container. If the components aren't all the same width (due to restricted maximum size or to any of them having strict left or right alignment), then X alignment comes into play.

> The X alignments affect not only the components' positions relative to each other, but also the location of the components (as a group) within their container. The following figures illustrate alignment of components that have restricted maximum widths.

> In the first figure, all three components have an X alignment of 0.0 (`Component.LEFT_ALIGNMENT`). This means that the components' left sides should be aligned. Furthermore, it means that all three components are positioned as far left in their container as possible.

> In the second figure, all three components have an X alignment of 0.5 (`Component.CENTER_ALIGNMENT`). This means that the components' centers should be aligned, and that the components should be positioned in the horizontal center of their container.

> In the third figure, the components have an X alignment of 1.0 (`Component.RIGHT_ALIGNMENT`). You can guess what that means for the components' alignment and position relative to their container.

> You might be wondering what happens when the components have both restricted maximum sizes and different X alignments. The next figure shows an example of this:

> As you can see, the left side of the component with an X alignment of 0.0 (`Component.LEFT_ALIGNMENT`) is aligned with the center of the component that has the 0.5 X alignment (`Component.CENTER_ALIGNMENT`), which is aligned with the right side of the component that has an X alignment of 1.0 (`Component.RIGHT_ALIGNMENT`). Mixed alignments like this are further discussed in Fixing Alignment Problems.

> What if none of the components has a maximum width? Well, if all the components have identical X alignment, then all components are made as wide as their container. If the X alignments are different, then any component with an X alignment of 0.0 (left) or 1.0 (right) will be smaller. All components with an intermediate X alignment (such as center) will be as wide as their container. Here are two examples:

> To get to know BoxLayout better, you can run your own experiments with BoxLayoutDemo.

> > Try this:
> > 1. Compile and run BoxLayoutDemo. The source files are `BoxLayoutDemo.java` and `BLDComponent.java`.
> >    See Getting Started with Swing if you need help compiling or running this application. You'll see a window like the one above that contains three rectangles. Each rectangle is an instance of `BLDComponent`, which is a `JComponent` subclass.
> > 2. Click inside one of the rectangles.
> >    This is how you change the rectangle's X alignment.
> > 3. Click the check box at the bottom of the window.
> >    This turns off restricted sizing for all the rectangles.
> > 4. Make the window taller.
> >    This makes the rectangles' container larger than the sum of the rectangles' preferred sizes. The container is a `JPanel` that has a red outline, so that you can tell where the container's edges are.

### Using Invisible Components as Filler

> Each component controlled by a box layout butts up against its neighboring components. If you want to have space between components, you can either add an empty border to one or both components, or insert invisible components to provide the space. You can create invisible components with the help of the `Box` class.

> The `Box` class provides an inner class, `Box.Filler` that provides invisible components. The `Box` class provides convenience methods to help you create common kinds of filler. The following table gives details about creating invisible components with `Box` and `Box.Filler`.

> Type | Size Constraints | How to Create
> --- | --- | ---
> rigid area |  |
>     Box.createRigidArea(size)
> glue | horizontal |  |
>     Box.createHorizontalGlue()
> vertical |  |
>     Box.createVerticalGlue()
> custom `Box.Filler` | (as specified) |
>     new Box.Filler(minSize, prefSize,
>                    maxSize)

> Here's how you generally use each type of filler:

> Rigid area
>   Use this when you want a fixed-size space between two components. For example, to put 5 pixels between two components in a left-to-right box, you can use this code:

>   >     container.add(firstComponent);
>   >     container.add(Box.createRigidArea(new Dimension(5,0)));
>   >     container.add(secondComponent);

>   Note: The `Box` class provides another kind of filler for putting fixed space between components: a vertical or horizontal strut. Unfortunately, struts have unlimited maximum heights or widths (for horizontal and vertical struts, respectively). This means that if you use a horizontal box within a vertical box, for example, the horizontal box can sometimes become too tall. For this reason, we recommend that you use rigid areas instead of struts.

> Glue
>   Use this to specify where excess space in a layout should go. Think of it as semi-wet glue -- stretchy and expandable, yet taking up no space unless you pull apart the components that it's sticking to. For example, by putting horizontal glue between two components in a left-to-right box, you make any extra space go between those components, instead of to the right of all the components. Here's an example of making the space in a left-to-right box go between two components, instead of to the right of the components:

>   >     container.add(firstComponent);
>   >     container.add(Box.createHorizontalGlue());
>   >     container.add(secondComponent);

> Custom `Box.Filler`
>   Use this to specify a component with whatever minimum, preferred, and maximum sizes you want. For example, to create some filler in a left-to-right layout that puts at least 5 pixels between two components and ensures that the container has a minimum height of 100 pixels, you could use this code:

>   >     container.add(firstComponent);
>   >     Dimension minSize = new Dimension(5, 100);
>   >     Dimension prefSize = new Dimension(5, 100);
>   >     Dimension maxSize = new Dimension(Short.MAX_VALUE, 100);
>   >     container.add(new Box.Filler(minSize, prefSize, maxSize));
>   >     container.add(secondComponent);

### Fixing Alignment Problems

> Two types of alignment problems sometimes occur with `BoxLayout`:
> - A group of components all have the same alignment, but you want to change their alignment to make them look better. For example, instead of having the centers of a group of left-to-right buttons all in a line, you might want the bottoms of the buttons to be aligned. Here's an example:

> - Two or more components controlled by a `BoxLayout` have different default alignments, which causes them to be mis-aligned. For example, as the following shows, if a label and a panel are in a top-to-bottom box layout, the label's left edge is, by default, aligned with the center of the panel.

> In general, all the components controlled by a top-to-bottom `BoxLayout` object should have the same X alignment. Similarly, all the components controlled by a left-to-right `Boxlayout` should generally have the same Y alignment. You can set a `JComponent`'s X alignment by invoking its `setAlignmentX` method. An alternative available to all components is to override the `getAlignmentX` method in a custom subclass of the component class. Similarly, you set the Y alignment of a component by invoking the `setAlignmentY` method or by overriding `getAlignmentY`.

> Here is an example, taken from `AlignmentDemo`, of changing the Y alignments of two buttons so that the buttons' bottoms are aligned:

> >     button1.setAlignmentY(Component.BOTTOM_ALIGNMENT);
> >     button2.setAlignmentY(Component.BOTTOM_ALIGNMENT);

> By default, most components have center X and Y alignment. However, buttons, combo boxes, labels, and menu items have a different default X alignment value: `LEFT_ALIGNMENT`. The previous picture shows what happens if you put a left-aligned component such as a label together with a center-aligned component in a container controlled by a top-to-bottom `BoxLayout`.

> The `AlignmentDemo` program gives examples of fixing mismatched alignment problems. Usually, it's as simple as making an offending button or label be center aligned. For example:

> >     label.setAlignmentX(Component.CENTER_ALIGNMENT);

### Specifying Component Sizes

> As we mentioned before, `BoxLayout` pays attention to a component's requested minimum, preferred, and maximum sizes. While you're fine tuning the layout, you might need to adjust these sizes.

> Sometimes the need to adjust the size is obvious. For example, a button's maximum size is generally the same as its preferred size. If you want the button to be drawn wider when additional space is available, then you need to change its maximum size.

> Sometimes, however, the need to adjust size is not so obvious. You might be getting unexpected results with a box layout, and you might not know why. In this case, it's usually best to treat the problem as an alignment problem first. If adjusting the alignments doesn't help, then you might have a size problem. We'll discuss this further a bit later.

> > Note: Although `BoxLayout` pays attention to a component's maximum size, most layout managers do not. For example, if you put a button in the south part of a `BorderLayout`, the button will probably be wider than its preferred width, no matter what the button's maximum size is. `BoxLayout`, on the other hand, never makes a button wider than its maximum size.

> You can change the minimum, preferred, and maximum sizes in two ways:

> - By invoking the appropriate `setXxxSize` method (which is defined by the `JComponent` class). For example:

>   >     comp.setMinimumSize(new Dimension(50, 25));
>   >     comp.setPreferredSize(new Dimension(50, 25));
>   >     comp.setMaximumSize(new Dimension(Short.MAX_VALUE,
>   >                                       Short.MAX_VALUE));

> - By overriding the appropriate `getXxxSize` method. For example:

>   >     ...//in a subclass of a component class:
>   >     public Dimension getMaximumSize() {
>   >         size = getPreferredSize();
>   >         size.width = Short.MAX_VALUE;
>   >         return size;
>   >     }

> If you're running into trouble with a box layout and you've ruled out alignment problems, then the trouble might well be size-related. For example, if the container controlled by the box layout is taking up too much space, then one or more of the components in the container probably needs to have its maximum size restricted.

> > Help!: Please send us mail (with "swing" in the subject) if you discover other common box layout problems, especially those that are size related.

> You can use two techniques to track down size trouble in a box layout:
> - Add a garish line border to the outside of the Swing components in question. This lets you see what size they really are. For example:

>   >     comp.setBorder(BorderFactory.createCompoundBorder(
>   >                        BorderFactory.createLineBorder(Color.red),
>   >                        comp.getBorder()));

> - Use good old `System.out.println` to print the components' minimum, preferred, and maximum sizes, and perhaps their bounds.

### The Box Layout API

> The following tables list the commonly used `BoxLayout` and `Box` constructors and methods. The API for using box layouts falls into these categories:

> - Creating `BoxLayout` objects
> - Creating space fillers
> - Other useful methods

> Creating `BoxLayout` Objects
> Constructor or Method | Purpose
> --- | ---
> `BoxLayout(Container, int)` | Creates a `BoxLayout` instance that controls the specified `Container`. The integer argument specifies whether the container's components should be laid out left to right (`BoxLayout.X_AXIS`) or top to bottom (`BoxLayout.Y_AXIS`).
> `Box(int)` | Creates a `Box` -- a lightweight container that uses a `BoxLayout` with the specified alignment (`BoxLayout.X_AXIS` or `BoxLayout.Y_AXIS`). Note that a `Box` is not a `JComponent` -- it's implemented as a subclass of `Container`. This makes it as lightweight as possible, but it lacks `JComponent` features such as borders. If you want a simple `JComponent` as a container, use `JPanel`.
> `static Box createHorizontalBox()` | Creates a `Box` that lays out its components from left to right.
> `static Box createVerticalBox()` | Creates a `Box` that lays out its components from top to bottom.

> Creating Space Fillers
> Constructor or Method | Purpose
> --- | ---
> `Component createRigidArea(Dimension)` | Create a rigid lightweight component.
> `Component createHorizontalGlue() Component createVerticalGlue() Component createGlue()` | Create a glue lightweight component. Horizontal glue and vertical glue can be very useful.
> `Component createHorizontalStrut() Component createVerticalStrut()` | Create a "strut" lightweight component. We recommend using rigid areas instead of struts.
> `Box.Filler(Dimension, Dimension, Dimension)` | Creates a lightweight component with the specified minimum, preferred, and maximum sizes (with the arguments specified in that order). See the custom `Box.Filler` discussion, earlier in this section, for details.

> Other Useful Methods
> Method | Purpose
> --- | ---
> `void changeShape(Dimension, Dimension, Dimension)` (in `Box.Filler`) | Change the minimum, preferred, and maximum sizes of the recipient `Box.Filler` object. The layout changes accordingly.

### Examples that Use Box Layouts

> The following table lists some of the many examples that use box layouts.

> Example | Where Described | Notes
> --- | --- | ---
> `BoxWindow` | This page | Uses a box layout to create a centered column of components.
> `AlignmentDemo` | This page | Demonstrates how to fix common alignment problems. Uses two image files: `images/middle.gif` and `images/geek-cght.gif`.
> `BoxLayoutDemo` | This page | Lets you play with alignments and maximum sizes.
> `ListDialog` | This page | A simple yet realistic example of using both a top-to-bottom box layout and a left-to-right one. Uses horizontal glue, rigid areas, and empty borders. Also sets the X alignment of a component.
> `InternalFrameEventDemo` | How to Write an Internal Frame Listener | Uses a top-to-bottom layout to center buttons and a scroll pane in an internal frame.
> `MenuGlueDemo` | Customizing Menu Layout | Shows how to right-align a menu in the menu bar, using a glue component.
> `MenuLayoutDemo` | Customizing Menu Layout | Shows how to customize menu layout by changing the menu bar to use a top-to-bottom box layout, and the popup menu to use a left-to-right box layout.
> `ConversionPanel.java` | Layout Management and Borders | Aligns two components in different box-layout-controlled containers by setting the components' widths to be the same, and their containers' widths to be the same.

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