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<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Lists and Combo Boxes</title><link rel="stylesheet" href="core.css" type="text/css"/><meta name="generator" content="DocBook XSL Stylesheets V1.74.0"/></head><body><div class="sect1" title="Lists and Combo Boxes"><div class="titlepage"><div><div><h1 class="title"><a id="learnjava3-CHP-17-SECT-4"/>Lists and Combo Boxes</h1></div></div></div><p><a id="idx10975" class="indexterm"/> <a id="idx10984" class="indexterm"/> <a id="idx10993" class="indexterm"/> <a id="idx10999" class="indexterm"/> <code class="literal">JList</code>s and <code class="literal">JComboBox</code>es are a step up on the evolutionary
chain from <code class="literal">JButton</code>s and <code class="literal">JLabel</code>s. Lists let the user choose from a group
of alternatives. They can be configured to force a single selection or
allow multiple choices. Usually, only a small group of choices is
displayed at a time; a scrollbar lets the user move to the choices that
aren’t visible. The user can select an item by clicking on it. She can
expand the selection to a range of items by holding down Shift and
clicking on another item. To make discontinuous selections, the user can
hold down the Control key instead of the Shift key (on a Mac, this is the
Command key).</p><p>A combo box is a crossbreed between a text field and a list. It
displays a single line of text (possibly with an image) and a
downward-pointing arrow on one side. If you click on the arrow, the combo
box opens up and displays a list of choices. You can select a single
choice by clicking on it. After a selection is made, the combo box closes
up; the list disappears, and the new selection is shown in the text
field.</p><p>Like other components in Swing, lists and combo boxes have data
models that are distinct from visual components. The list also has a
selection model that controls how selections can be made on the list
data.</p><p>Lists and combo boxes are similar because they have similar data
models. Each is simply an array of acceptable choices. This similarity is
reflected in Swing, of course: the type of a <code class="literal">JComboBox</code>’s data model is a subclass of the type
used for a <code class="literal">JList</code>’s data model. The next
example demonstrates this relationship.</p><p>The following example creates a window with a combo box, a list, and
a button. The combo box and the list use the same data model. When you
press the button, the program writes out the current set of selected items
in the list (see <a class="xref" href="ch17s03.html#learnjava3-CHP-17-FIG-4" title="Figure 17-4. A combo box and a list using the same data model">Figure 17-4</a>).</p><div class="figure"><a id="learnjava3-CHP-17-FIG-4"/><div class="figure-contents"><div class="mediaobject"><a id="I_17_tt1012"/><img src="httpatomoreillycomsourceoreillyimages1707656.png" alt="A combo box and a list using the same data model"/></div></div><p class="title">Figure 17-4. A combo box and a list using the same data model</p></div><p>Here’s the code for the example:</p><a id="I_17_tt1013"/><pre class="programlisting"> <code class="c1">//file: Lister.java</code>
<code class="kn">import</code> <code class="nn">java.awt.*</code><code class="o">;</code>
<code class="kn">import</code> <code class="nn">java.awt.event.*</code><code class="o">;</code>
<code class="kn">import</code> <code class="nn">javax.swing.*</code><code class="o">;</code>
<code class="kd">public</code> <code class="kd">class</code> <code class="nc">Lister</code> <code class="o">{</code>
<code class="kd">public</code> <code class="kd">static</code> <code class="kt">void</code> <code class="nf">main</code><code class="o">(</code><code class="n">String</code><code class="o">[]</code> <code class="n">args</code><code class="o">)</code> <code class="o">{</code>
<code class="n">JFrame</code> <code class="n">frame</code> <code class="o">=</code> <code class="k">new</code> <code class="n">JFrame</code><code class="o">(</code><code class="s">"Lister v1.0"</code><code class="o">);</code>
<code class="c1">// create a combo box</code>
<code class="n">String</code> <code class="o">[]</code> <code class="n">items</code> <code class="o">=</code> <code class="o">{</code> <code class="s">"uno"</code><code class="o">,</code> <code class="s">"due"</code><code class="o">,</code> <code class="s">"tre"</code><code class="o">,</code> <code class="s">"quattro"</code><code class="o">,</code> <code class="s">"cinque"</code><code class="o">,</code>
<code class="s">"sei"</code><code class="o">,</code> <code class="s">"sette"</code><code class="o">,</code> <code class="s">"otto"</code><code class="o">,</code> <code class="s">"nove"</code><code class="o">,</code> <code class="s">"deici"</code><code class="o">,</code>
<code class="s">"undici"</code><code class="o">,</code> <code class="s">"dodici"</code> <code class="o">};</code>
<code class="n">JComboBox</code> <code class="n">comboBox</code> <code class="o">=</code> <code class="k">new</code> <code class="n">JComboBox</code><code class="o">(</code><code class="n">items</code><code class="o">);</code>
<code class="n">comboBox</code><code class="o">.</code><code class="na">setEditable</code><code class="o">(</code><code class="kc">true</code><code class="o">);</code>
<code class="c1">// create a list with the same data model</code>
<code class="kd">final</code> <code class="n">JList</code> <code class="n">list</code> <code class="o">=</code> <code class="k">new</code> <code class="n">JList</code><code class="o">(</code><code class="n">comboBox</code><code class="o">.</code><code class="na">getModel</code><code class="o">());</code>
<code class="c1">// create a button; when it's pressed, print out</code>
<code class="c1">// the selection in the list</code>
<code class="n">JButton</code> <code class="n">button</code> <code class="o">=</code> <code class="k">new</code> <code class="n">JButton</code><code class="o">(</code><code class="s">"Per favore"</code><code class="o">);</code>
<code class="n">button</code><code class="o">.</code><code class="na">addActionListener</code><code class="o">(</code><code class="k">new</code> <code class="n">ActionListener</code><code class="o">()</code> <code class="o">{</code>
<code class="kd">public</code> <code class="kt">void</code> <code class="nf">actionPerformed</code><code class="o">(</code><code class="n">ActionEvent</code> <code class="n">ae</code><code class="o">)</code> <code class="o">{</code>
<code class="n">Object</code><code class="o">[]</code> <code class="n">selection</code> <code class="o">=</code> <code class="n">list</code><code class="o">.</code><code class="na">getSelectedValues</code><code class="o">();</code>
<code class="n">System</code><code class="o">.</code><code class="na">out</code><code class="o">.</code><code class="na">println</code><code class="o">(</code><code class="s">"-----"</code><code class="o">);</code>
<code class="k">for</code> <code class="o">(</code> <code class="n">Object</code> <code class="n">o</code> <code class="o">:</code> <code class="n">selection</code> <code class="o">)</code>
<code class="n">System</code><code class="o">.</code><code class="na">out</code><code class="o">.</code><code class="na">println</code><code class="o">(</code> <code class="n">o</code> <code class="o">);</code>
<code class="o">}</code>
<code class="o">});</code>
<code class="c1">// put the controls the content pane</code>
<code class="n">Container</code> <code class="n">c</code> <code class="o">=</code> <code class="n">frame</code><code class="o">.</code><code class="na">getContentPane</code><code class="o">();</code> <code class="c1">// unnecessary in 5.0+</code>
<code class="n">JPanel</code> <code class="n">comboPanel</code> <code class="o">=</code> <code class="k">new</code> <code class="n">JPanel</code><code class="o">();</code>
<code class="n">comboPanel</code><code class="o">.</code><code class="na">add</code><code class="o">(</code><code class="n">comboBox</code><code class="o">);</code>
<code class="n">c</code><code class="o">.</code><code class="na">add</code><code class="o">(</code><code class="n">comboPanel</code><code class="o">,</code> <code class="n">BorderLayout</code><code class="o">.</code><code class="na">NORTH</code><code class="o">);</code>
<code class="n">c</code><code class="o">.</code><code class="na">add</code><code class="o">(</code><code class="k">new</code> <code class="n">JScrollPane</code><code class="o">(</code><code class="n">list</code><code class="o">),</code> <code class="n">BorderLayout</code><code class="o">.</code><code class="na">CENTER</code><code class="o">);</code>
<code class="n">c</code><code class="o">.</code><code class="na">add</code><code class="o">(</code><code class="n">button</code><code class="o">,</code> <code class="n">BorderLayout</code><code class="o">.</code><code class="na">SOUTH</code><code class="o">);</code>
<code class="n">frame</code><code class="o">.</code><code class="na">setSize</code><code class="o">(</code><code class="mi">200</code><code class="o">,</code> <code class="mi">200</code><code class="o">);</code>
<code class="n">frame</code><code class="o">.</code><code class="na">setDefaultCloseOperation</code><code class="o">(</code> <code class="n">JFrame</code><code class="o">.</code><code class="na">EXIT_ON_CLOSE</code> <code class="o">);</code>
<code class="n">frame</code><code class="o">.</code><code class="na">setVisible</code><code class="o">(</code><code class="kc">true</code><code class="o">);</code>
<code class="o">}</code>
<code class="o">}</code></pre><p>The combo box is created from an array of strings. This is a
convenience—behind the scenes, the <a id="I_indexterm17_id794870" class="indexterm"/><code class="literal">JComboBox</code> constructor
creates a data model from the strings you supply and sets the <code class="literal">JComboBox</code> to use that data model. Our list is
then created using the data model of the combo box. This works because
<a id="I_indexterm17_id794887" class="indexterm"/><code class="literal">JList</code> expects to use a
<code class="literal">ListModel</code> for its data model, and the
<code class="literal">ComboBoxModel</code> used by the <code class="literal">JComboBox</code> is a subclass of <code class="literal">ListModel</code>.</p><p>The button’s action event handler simply prints out the selected
items in the list, which are retrieved with a call to <a id="I_indexterm17_id794924" class="indexterm"/><code class="literal">getSelectedValues()</code>. This
method actually returns an object array, not a string array. List and
combo box items, like many other things in Swing, are not limited to text.
You can use images, drawings, or some combination of text and images. We
simply print the result as a string.</p><p>You might expect that selecting one item in the combo box would
select the same item in the list. In Swing components, selection is
controlled by a selection model. The combo box and the list have distinct
selection models; after all, you can select only one item from the combo
box, while it’s possible to select multiple items from the list. Thus,
while the two components share a data model, they have separate selection
models.</p><p>We’ve made the combo box editable. By default, it would not be
editable: the user could choose only one item in the drop-down list. With
an editable combo box, the user can type in a selection, as if it were a
text field. Noneditable combo boxes are useful if you just want to offer a
limited set of choices; editable combo boxes are handy when you want to
accept any input but offer some common choices.</p><p>There’s a great class tucked away in the last example that deserves
some recognition. It’s <a id="I_indexterm17_id794958" class="indexterm"/><code class="literal">JScrollPane</code>. In <code class="literal">Lister</code>, you’ll notice that we created one when
we added the <code class="literal">List</code> to the main window.
<code class="literal">JScrollPane</code> simply wraps itself around
another <code class="literal">Component</code> and provides
scrollbars as necessary. The scrollbars show up if the contained <code class="literal">Component</code>’s preferred size (as returned by
<code class="literal">getPreferredSize()</code>) is greater than the
size of the <code class="literal">JScrollPane</code> itself. In the
previous example, the scrollbars show up whenever the size of the <code class="literal">List</code> exceeds the available space.</p><p>You can use <code class="literal">JScrollPane</code> to wrap
any <code class="literal">Component</code>, including components
with drawings, images, or complex user interface panels. We’ll discuss
<code class="literal">JScrollPane</code> in more detail later in
this chapter, and we’ll use it frequently with the text components in
<a class="xref" href="ch18.html" title="Chapter 18. More Swing Components">Chapter 18</a>.<a id="I_indexterm17_id795042" class="indexterm"/><a id="I_indexterm17_id795049" class="indexterm"/><a id="I_indexterm17_id795056" class="indexterm"/><a id="I_indexterm17_id795063" class="indexterm"/></p></div></body></html>