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<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"><head><title>Buttons and Labels</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="Buttons and Labels"><div class="titlepage"><div><div><h1 class="title"><a id="learnjava3-CHP-17-SECT-1"/>Buttons and Labels</h1></div></div></div><p><a id="idx10973" class="indexterm"/> <a id="idx10983" class="indexterm"/> <a id="idx10991" class="indexterm"/> <a id="idx10998" class="indexterm"/> We’ll start with the simplest components: buttons and labels. Frankly, there isn’t much to say about them. If you’ve seen one button, you’ve seen them all, and you’ve already seen buttons in the applications in <a class="xref" href="ch02.html" title="Chapter 2. A First Application">Chapter 2</a> (<code class="literal">HelloJava3</code> and <code class="literal">HelloJava4</code>). A button generates an <code class="literal">ActionEvent</code> when the user presses it. To receive these events, your program registers an <code class="literal">ActionListener</code>, which must implement the <code class="literal">actionPerformed()</code> method. The argument passed to <code class="literal">actionPerformed()</code> is the event itself.</p><p>There’s one more thing worth saying about buttons, which applies to any component that generates an action event. Java lets us specify an “action command” string for buttons (and other components, like menu items, that can generate action events). The action command is less interesting than it sounds. It is just a <code class="literal">String</code> that serves to identify the component that sent the event. By default, the action command of a <a id="I_indexterm17_id793867" class="indexterm"/><code class="literal">JButton</code> is the same as its label; it is included in action events so that you could use it to figure out which button an event came from. However, you’ll often know this from the context of your event listener.</p><p>To get the action command from an action event, call the event’s <a id="I_indexterm17_id793885" class="indexterm"/><code class="literal">getActionCommand()</code> method. The following code checks whether the user pressed the button labeled <span class="emphasis"><em>Yes</em></span>:</p><a id="I_17_tt1002"/><pre class="programlisting"> <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">e</code><code class="o">){</code> <code class="k">if</code> <code class="o">(</code><code class="n">e</code><code class="o">.</code><code class="na">getActionCommand</code><code class="o">().</code><code class="na">equals</code><code class="o">(</code><code class="s">"Yes"</code><code class="o">)</code> <code class="o">{</code> <code class="c1">//the user pressed "Yes"; do something</code> <code class="o">...</code> <code class="o">}</code> <code class="o">}</code></pre><p><span class="emphasis"><em>Yes</em></span> is a string, not a command per se. You can change the action command by calling the button’s <a id="I_indexterm17_id793916" class="indexterm"/><code class="literal">setActionCommand()</code> method. The following code changes button <code class="literal">myButton</code>’s action command to “confirm:”</p><a id="I_17_tt1003"/><pre class="programlisting"> <code class="n">myButton</code><code class="o">.</code><code class="na">setActionCommand</code><code class="o">(</code><code class="s">"confirm"</code><code class="o">);</code></pre><p>It’s a good idea to get used to setting action commands explicitly; this helps to prevent your code from breaking when you or some other developer internationalizes it or otherwise changes the button’s label. If you rely on the button’s label, your code stops working as soon as that label changes; a French user might see the label <code class="literal">Oui</code> rather than <code class="literal">Yes</code>.</p><p>Swing buttons can have an image in addition to a label. The <code class="literal">JButton</code> class includes constructors that accept an <code class="literal">Icon</code> object, which knows how to draw itself. You can create buttons with captions, images, or both. A handy class called <a id="I_indexterm17_id793974" class="indexterm"/><code class="literal">ImageIcon</code> takes care of loading an image for you and can be used to add an image to a button. The following example shows how this works:</p><a id="I_17_tt1004"/><pre class="programlisting"> <code class="c1">//file: PictureButton.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">PictureButton</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="n">Icon</code> <code class="n">icon</code> <code class="o">=</code> <code class="k">new</code> <code class="n">ImageIcon</code><code class="o">(</code><code class="s">"rhino.gif"</code><code class="o">);</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="n">icon</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">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">"Urp!"</code><code class="o">);</code> <code class="o">}</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="na">add</code><code class="o">(</code> <code class="n">button</code> <code class="o">);</code> <code class="n">frame</code><code class="o">.</code><code class="na">pack</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 example creates an <code class="literal">ImageIcon</code> from the <span class="emphasis"><em>rhino.gif</em></span> file. Then, a <code class="literal">JButton</code> is created from the <code class="literal">ImageIcon</code>. The whole thing is displayed in a <code class="literal">JFrame</code>. This example also shows the idiom of using an anonymous inner class as an <code class="literal">ActionListener</code>.</p><p>There’s even less to be said about <a id="I_indexterm17_id794036" class="indexterm"/><code class="literal">JLabel</code> components. They’re just text strings or images housed in a component. There aren’t any special events associated with labels; about all you can do is specify the text’s alignment, which controls the position of the text within the label’s display area. As with buttons, <code class="literal">JLabel</code>s can be created with <code class="literal">Icon</code>s if you want to create a picture label. The following code creates some labels with different options:</p><a id="I_17_tt1005"/><pre class="programlisting"> <code class="c1">// default alignment (CENTER)</code> <code class="n">JLabel</code> <code class="n">label1</code> <code class="o">=</code> <code class="k">new</code> <code class="n">JLabel</code><code class="o">(</code><code class="s">"Lions"</code><code class="o">);</code> <code class="c1">// left aligned</code> <code class="n">JLabel</code> <code class="n">label2</code> <code class="o">=</code> <code class="k">new</code> <code class="n">JLabel</code><code class="o">(</code><code class="s">"Tigers"</code><code class="o">,</code> <code class="n">SwingConstants</code><code class="o">.</code><code class="na">LEFT</code><code class="o">);</code> <code class="c1">//label with no text, default alignment</code> <code class="n">JLabel</code> <code class="n">label3</code> <code class="o">=</code> <code class="k">new</code> <code class="n">JLabel</code><code class="o">();</code> <code class="c1">// create image icon</code> <code class="n">Icon</code> <code class="n">icon</code> <code class="o">=</code> <code class="k">new</code> <code class="n">ImageIcon</code><code class="o">(</code><code class="s">"rhino.gif"</code><code class="o">);</code> <code class="c1">// create image label</code> <code class="n">JLabel</code> <code class="n">label4</code> <code class="o">=</code> <code class="k">new</code> <code class="n">JLabel</code><code class="o">(</code><code class="n">icon</code><code class="o">);</code> <code class="c1">// assigning text to label3</code> <code class="n">label3</code><code class="o">.</code><code class="na">setText</code><code class="o">(</code><code class="s">"and Bears"</code><code class="o">);</code> <code class="c1">// set alignment</code> <code class="n">label3</code><code class="o">.</code><code class="na">setHorizontalAlignment</code><code class="o">(</code><code class="n">SwingConstants</code><code class="o">.</code><code class="na">RIGHT</code><code class="o">);</code></pre><p>The alignment constants are defined in the <code class="literal">SwingConstants</code> interface.</p><p>We’ve built several labels using a variety of constructors and several of the class’s methods. To display the labels, just add them to a container by calling the container’s <code class="literal">add()</code> method.</p><p>You can set other label characteristics, such as changing their font or color, using the methods of the <code class="literal">Component</code> class, <code class="literal">JLabel</code>’s distant ancestor. For example, you can call <code class="literal">setFont()</code> and <code class="literal">setBackground()</code> on a label, as with any other component.</p><p>Given that labels are so simple, why do we need them at all? Why not find a way to draw a text string directly on the container object? Remember that a <code class="literal">JLabel</code> is a <code class="literal">JComponent</code>. That means that labels have the normal complement of methods for setting fonts and colors that we mentioned earlier as well as the ability to be persistently and sensibly managed by a layout manager. Therefore, they’re much more flexible than a text string drawn procedurally at an arbitrary location within a container. Speaking of layouts—if you use the <a id="I_indexterm17_id794141" class="indexterm"/><code class="literal">setText()</code> method to change the text of your label, the label’s preferred size may change. But the label’s container automatically lays out its components when this happens so you don’t have to worry about it.</p><div class="sect2" title="HTML Text in Buttons and Labels"><div class="titlepage"><div><div><h2 class="title"><a id="learnjava3-CHP-17-SECT-2"/>HTML Text in Buttons and Labels</h2></div></div></div><p><a id="idx10978" class="indexterm"/> A neat feature of Swing is that it can interpret HTML-formatted text in <code class="literal">JLabel</code> and <code class="literal">JButton</code> labels. The following example shows how to create a button with some HTML-formatted text:</p><a id="I_17_tt1006"/><pre class="programlisting"> <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">"<html>"</code> <code class="o">+</code> <code class="s">"S<font size=-1>MALL<font size=+0> "</code> <code class="o">+</code> <code class="s">"C<font size=-1>APITALS"</code><code class="o">);</code></pre><p>Older versions of Java may not render complex HTML very well. But as of JDK 1.4, most basic HTML features are supported, including crazy things such as images and tables.</p><p><a class="xref" href="ch17s01.html#learnjava3-CHP-17-FIG-1" title="Figure 17-1. Button using HTML table">Figure 17-1</a> uses an HTML table to arrange its text.</p><div class="figure"><a id="learnjava3-CHP-17-FIG-1"/><div class="figure-contents"><div class="mediaobject"><a id="I_17_tt1007"/><img src="httpatomoreillycomsourceoreillyimages1707653.png.jpg" alt="Button using HTML table"/></div></div><p class="title">Figure 17-1. Button using HTML table</p></div><p><a class="xref" href="ch17s01.html#learnjava3-CHP-17-FIG-2" title="Figure 17-2. Button using HTML img tag">Figure 17-2</a> uses an HTML image tag to display an image.</p><div class="figure"><a id="learnjava3-CHP-17-FIG-2"/><div class="figure-contents"><div class="mediaobject"><a id="I_17_tt1008"/><img src="httpatomoreillycomsourceoreillyimages1707654.png" alt="Button using HTML img tag"/></div></div><p class="title">Figure 17-2. Button using HTML img tag</p></div><p>The code for the two figures looks like this:<a id="I_indexterm17_id794255" class="indexterm"/><a id="I_indexterm17_id794262" class="indexterm"/><a id="I_indexterm17_id794269" class="indexterm"/><a id="I_indexterm17_id794276" class="indexterm"/><a id="I_indexterm17_id794283" class="indexterm"/></p><a id="I_17_tt1009"/><pre class="programlisting"> <code class="n">String</code> <code class="n">html</code><code class="o">=</code> <code class="s">"<html><table border=1>"</code> <code class="o">+</code><code class="s">"<tr><td>One</td><td>Two</td></tr>"</code> <code class="o">+</code><code class="s">"<tr><td>Three</td><td>Four</td></tr>"</code> <code class="o">+</code><code class="s">"</table>"</code><code class="o">;</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="n">html</code><code class="o">);</code> <code class="n">String</code> <code class="n">html2</code><code class="o">=</code> <code class="s">"<html><h3>Learning Java</h3>"</code> <code class="o">+</code><code class="s">"<img src=\"http://www.oreilly.com/catalog/covers/learnjava3.s.gif\">"</code><code class="o">;</code> <code class="n">Jbutton</code> <code class="n">button2</code> <code class="o">=</code> <code class="k">new</code> <code class="n">JButton</code><code class="o">(</code><code class="n">html2</code><code class="o">);</code></pre></div></div></body></html>