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<a href="http://www.bruceeckel.com/javabook.html">Bruce Eckel's Thinking in Java</a>
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<H2 ALIGN=LEFT>
Controlling
layout
<P><A NAME="Index1674"></A><A NAME="Index1675"></A></H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
way that you place components on a form in Java is probably different from any
other GUI system you’ve used. First, it’s all code; there are no
“resources” that control placement of components. Second, the way
components are placed on a form is controlled by a “<A NAME="Index1676"></A>layout
manager” that decides how the components lie based on the order that you <A NAME="Index1677"></A><A NAME="Index1678"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>add( )</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
them. The size, shape, and placement of components will be remarkably different
from one layout manager to another. In addition, the layout managers adapt to
the dimensions of your applet or application window, so if that window
dimension is changed (for example, in the HTML page’s applet
specification) the size, shape, and placement of the components could change.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Both
the <A NAME="Index1679"></A><A NAME="Index1680"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Applet</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and <A NAME="Index1681"></A><A NAME="Index1682"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Frame
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">classes
are derived from <A NAME="Index1683"></A><A NAME="Index1684"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Container</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
whose job it is to contain and display
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Component</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s.
(The
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Container</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is a <A NAME="Index1685"></A><A NAME="Index1686"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Component</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
so it can also react to events.) In
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Container,</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
there’s a method called <A NAME="Index1687"></A><A NAME="Index1688"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>setLayout( )</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that allows you to choose a different layout manager.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
this section we’ll explore the various layout managers by placing buttons
in them (since that’s the simplest thing to do). There won’t be any
capturing of button events since this is just intended to show how the buttons
are laid out.
</FONT><a name="_Toc375545458"></a><a name="_Toc408018694"></a><P></DIV>
<A NAME="Heading410"></A><H3 ALIGN=LEFT>
FlowLayout</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">So
far, all the applets that have been created seem to have laid out their
components using some mysterious internal logic. That’s because the
applet uses a default layout scheme: the <A NAME="Index1689"></A><A NAME="Index1690"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>FlowLayout</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This simply “flows” the components onto the form, from left to
right until the top space is full, then moves down a row and continues flowing
the components.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Here’s
an example that explicitly (redundantly) sets the layout manager in an applet to
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>FlowLayout</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and then places buttons on the form. You’ll notice that with
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>FlowLayout</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
the components take on their “natural” size. A <A NAME="Index1691"></A><A NAME="Index1692"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Button</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
for example, will be the size of its string.
</FONT><P></DIV>
<font color="#990000"><PRE><font color="#009900">//: FlowLayout1.java</font>
<font color="#009900">// Demonstrating the FlowLayout</font>
<font color="#0000ff">import</font> java.awt.*;
<font color="#0000ff">import</font> java.applet.*;
<font color="#0000ff">public</font> <font color="#0000ff">class</font> FlowLayout1 <font color="#0000ff">extends</font> Applet {
<font color="#0000ff">public</font> <font color="#0000ff">void</font> init() {
setLayout(<font color="#0000ff">new</font> FlowLayout());
<font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i < 20; i++)
add(<font color="#0000ff">new</font> Button("Button " + i));
}
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">All
components will be compacted to their smallest size in a
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>FlowLayout</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
so you might get a little bit of surprising behavior. For example, a label will
be the size of its string, so right-justifying it yields an unchanged display.
</FONT><a name="_Toc375545459"></a><a name="_Toc408018695"></a><P></DIV>
<A NAME="Heading411"></A><H3 ALIGN=LEFT>
BorderLayout</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
layout manager has the concept of four border regions and a center area. When
you add something to a panel that’s using a <A NAME="Index1693"></A><A NAME="Index1694"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>BorderLayout</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
you must use an
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>add( )</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method that takes a
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object as its first argument, and that string must specify (with proper
capitalization) “<A NAME="Index1695"></A><A NAME="Index1696"></A>North”
(top), “<A NAME="Index1697"></A><A NAME="Index1698"></A>South”
(bottom), “<A NAME="Index1699"></A><A NAME="Index1700"></A>East”
(right), “<A NAME="Index1701"></A><A NAME="Index1702"></A>West”
(left), or “Center.” If you misspell or mis-capitalize, you
won’t get a compile-time error, but the applet simply won’t do what
you expect. Fortunately, as you will see shortly, there’s a much-improved
approach in Java 1.1<A NAME="Index1703"></A>.</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Here’s
a simple example:
</FONT><P></DIV>
<font color="#990000"><PRE><font color="#009900">//: BorderLayout1.java</font>
<font color="#009900">// Demonstrating the BorderLayout</font>
<font color="#0000ff">import</font> java.awt.*;
<font color="#0000ff">import</font> java.applet.*;
<font color="#0000ff">public</font> <font color="#0000ff">class</font> BorderLayout1 <font color="#0000ff">extends</font> Applet {
<font color="#0000ff">public</font> <font color="#0000ff">void</font> init() {
<font color="#0000ff">int</font> i = 0;
setLayout(<font color="#0000ff">new</font> BorderLayout());
add("North", <font color="#0000ff">new</font> Button("Button " + i++));
add("South", <font color="#0000ff">new</font> Button("Button " + i++));
add("East", <font color="#0000ff">new</font> Button("Button " + i++));
add("West", <font color="#0000ff">new</font> Button("Button " + i++));
add("Center", <font color="#0000ff">new</font> Button("Button " + i++));
}
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">For
every placement but “Center,” the element that you add is
compressed to fit in the smallest amount of space along one dimension while it
is stretched to the maximum along the other dimension. “Center,”
however, spreads out along both dimensions to occupy the middle.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>BorderLayout</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is the default layout manager for applications and dialogs.
</FONT><a name="_Toc375545460"></a><a name="_Toc408018696"></a><P></DIV>
<A NAME="Heading412"></A><H3 ALIGN=LEFT>
GridLayout</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">A
<A NAME="Index1704"></A><A NAME="Index1705"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>GridLayout</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
allows you to build a table of components, and as you add them they are placed
left-to-right and top-to-bottom in the grid. In the constructor you specify the
number of rows and columns that you need and these are laid out in equal
proportions.
</FONT><P></DIV>
<font color="#990000"><PRE><font color="#009900">//: GridLayout1.java</font>
<font color="#009900">// Demonstrating the FlowLayout</font>
<font color="#0000ff">import</font> java.awt.*;
<font color="#0000ff">import</font> java.applet.*;
<font color="#0000ff">public</font> <font color="#0000ff">class</font> GridLayout1 <font color="#0000ff">extends</font> Applet {
<font color="#0000ff">public</font> <font color="#0000ff">void</font> init() {
setLayout(<font color="#0000ff">new</font> GridLayout(7,3));
<font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i < 20; i++)
add(<font color="#0000ff">new</font> Button("Button " + i));
}
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
this case there are 21 slots but only 20 buttons. The last slot is left empty;
no “balancing” goes on with a
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>GridLayout</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><a name="_Toc375545461"></a><a name="_Toc408018697"></a><P></DIV>
<A NAME="Heading413"></A><H3 ALIGN=LEFT>
CardLayout</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
<A NAME="Index1706"></A><A NAME="Index1707"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>CardLayout</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
allows you to create the rough equivalent of a “tabbed dialog,”
which in more sophisticated environments has actual file-folder tabs running
across one edge, and all you have to do is press a tab to bring forward a
different dialog. Not so in the AWT: The
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>CardLayout</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is simply a blank space and you’re responsible for bringing forward new
cards. (The JFC/Swing library contains tabbed panes that look much better and
take care of all the details for you.)
</FONT><P></DIV>
<A NAME="Heading414"></A><H4 ALIGN=LEFT>
Combining
layouts
<P><A NAME="Index1708"></A><A NAME="Index1709"></A></H4>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
example will combine more than one layout type, which seems rather difficult at
first since only one layout manager can be operating for an applet or
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