BoxLayout
Each of the five layout managers discussed so far is defined in the java.awt package and has been available
since Java 1.0. However, BoxLayout is a more recent addition to Java and is included in the javax.swing
package. BoxLayout is an attempt to provide some of the flexibility of GridBagLayout without the
complexity involved in its use.
A BoxLayout allows you to create either a single row or a single column of components. In other words,
the components you add to a BoxLayout are arranged vertically from top-to-bottom, or horizontally
from left-to-right.
BoxLayout is different from the other layout managers in a number of ways, and uses some properties
defined in Component that the other layout managers ignore. For example, BoxLayout respects a
component's maximum size, and will never make the component larger than the dimensions specified by that
property. In addition, a BoxLayout that arranges its components vertically (or a "vertical BoxLayout") uses
each component's alignment along the X axis, which is available through the getAlignmentX() method in
Component. Similarly, BoxLayout uses the components' alignments along the Y axis (and the
corresponding getAlignmentY() method) when it arranges them horizontally.
BoxLayout is different from the other layout managers in one other important way: it uses a component's
maximum size to determine the amount of space that the component should occupy. In many cases, a
component's maximum size is the same as or is close to its preferred size. However, as we'll see later, some
components have very large maximum size values, which can produce unexpected or undesirable results
when used with a BoxLayout.
Alignment Values, Ascents and Descents
Component alignment values play a major role in determining how components are positioned within a
BoxLayout-managed container, but before we can examine how alignment values are used, it's necessary to
define some terms.
A component's alignment is represented by a float value that can range from 0.0 to 1.0, and you may find it
helpful to think of this number as a percentage value, with 0.0 representing 0% and 1.0 representing 100%. By
default, a component's X and Y alignment values are both set to 0.5. The component's ascent value is
calculated by multiplying one of its dimensions by one of its alignment values. For example, if you're using a
horizontal BoxLayout, you could calculate the preferred height ascent for a component by multiplying the
component's preferred height by its Y alignment value, as in the following equation:
Dimension prefSize = comp.getPreferredSize();
int ascent = (int)(prefSize.height * comp.getAlignmentY());
Similarly, a component's descent value is calculated by subtracting the component's ascent value from the
size that was used to calculate the ascent, as shown below:
int descent = prefSize.height - ascent;
In other words, the sum of the ascent and descent values is equal to the dimension that was used to calculate
them, and they represent the portions of the component that lie on either side of an imaginary line. For
example, suppose that the above code was executed for a component with a preferred height of 400 pixels,
and that the component's Y alignment value is 0.25. The ascent value would be 100 (400 * 0.25 = 100), while
the descent value would be 300 (400 - 100 = 300).
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