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Reinier Zwitserloot
117 lines
5.9 KiB
HTML
117 lines
5.9 KiB
HTML
<!DOCTYPE html>
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<html><head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<link rel="stylesheet" type="text/css" href="../../logi/reset.css" />
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<link rel="stylesheet" type="text/css" href="../features.css" />
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<link rel="shortcut icon" href="../../favicon.ico" type="image/x-icon" />
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<meta name="description" content="Spice up your java" />
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<title>EXPERIMENTAL - @ExtensionMethod</title>
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</head><body><div id="pepper">
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<div class="minimumHeight"></div>
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<div class="meat">
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<div class="header"><a href="../../index.html">Project Lombok</a></div>
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<h1>@ExtensionMethod</h1>
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<div class="byline">Annoying API? Fix it yourself: Add new methods to existing types!</div>
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<div class="since">
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<h3>Since</h3>
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<p>
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@ExtensionMethod was introduced as experimental feature in lombok v0.11.2.
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</p>
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</div>
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<div class="experimental">
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<h3>Experimental</h3>
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<p>
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Experimental because:
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<ul>
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<li>High-impact on code style</li>
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<li>Really would like to ship with utility methods to expand common classes, but so far lombok doesn't have a good distribution method for such runtime dependencies</li>
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<li>Affects quite a bit of eclipse, and auto-complete e.d. do not work yet in netbeans</li>
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<li>Should @ExtensionMethod be legal on methods? Should it be legal on packages?</li>
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</ul>
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Current status: <em>positive</em> - Currently we feel this feature may move out of experimental status with no or minor changes soon.
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</div>
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<div class="overview">
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<h3>Overview</h3>
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<p>
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You can make a class containing a bunch of <code>public</code>, <code>static</code> methods which all take at least 1
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parameter. These methods will extend the type of the first parameter, as if they were instance methods, using the
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<code>@ExtensionMethod</code> feature.
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</p>
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</p><p>
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For example, if you create <code>public static String toTitleCase(String in) { ... }</code>, you can use the
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<code>@ExtensionMethod</code> feature to make it look like the <code>java.lang.String</code> class has a method named
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<code>toTitleCase</code>, which has no arguments. The first argument of the static method fills the role of <code>this</code>
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in instance methods.
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</p><p>
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All methods that are <code>public</code>, <code>static</code>, and have at least 1 argument whose type is not primitive, are
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considered extension methods, and each will be injected into the namespace of the type of the first parameter as if they were
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instance methods. As in the above example, a call that looks like: <code>foo.toTitleCase()</code> is replaced with
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<code>ClassContainingYourExtensionMethod.toTitleCase(foo);</code>. Note that it is actually not an instant
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<code>NullPointerException</code> if <code>foo</code> is null - it is passed like any other parameter.
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</p><p>
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You can pass any number of classes to the <code>@ExtensionMethod</code> annotation; they will all be searched for
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extension methods. These extension methods apply for any code that is in the annotated class.
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</p><p>
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Lombok does not (currently) have any runtime dependencies which means lombok does not (currently) ship with any useful
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extension methods so you'll have to make your own. However, here's one that might spark your imagination:
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<pre>public class ObjectExtensions {
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public static <T> or(T object, T ifNull) {
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return object != null ? object : ifNull;
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}
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}</pre>
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With the above class, if you add <code>@ExtensionMethod(ObjectExtensions.class)</code> to your class definition, you can write:
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<pre>String x = null;
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System.out.println(x.or("Hello, World!"));</pre>
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The above code will not fail with a <code>NullPointerException</code>; it will actually output <code>Hello, World!</code>
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</p>
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</div>
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<div class="snippets">
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<div class="pre">
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<h3>With Lombok</h3>
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<div class="snippet">@HTML_PRE@</div>
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</div>
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<div class="sep"></div>
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<div class="post">
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<h3>Vanilla Java</h3>
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<div class="snippet">@HTML_POST@</div>
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</div>
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</div>
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<div style="clear: left;"></div>
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<div class="overview confKeys">
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<h3>Supported configuration keys:</h3>
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<dl>
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<dt><code>lombok.extensionMethod.flagUsage</code> = [<code>warning</code> | <code>error</code>] (default: not set)</dt>
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<dd>Lombok will flag any usage of <code>@ExtensionMethod</code> as a warning or error if configured.</dd>
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</dl>
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</div>
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<div class="overview">
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<h3>Small print</h3><div class="smallprint">
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<p>
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Calls are rewritten to a call to the extension method; the static method itself is not inlined. Therefore, the
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extension method must be present both at compile and at runtime.
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</p><p>
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Generics is fully applied to figure out extension methods. i.e. if the first parameter of your extension method is
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<code>List<? extends String></code>, then any expression that is compatible with that will have your extension method,
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but other kinds of lists won't. So, a <code>List<Object></code> won't get it, but a <code>List<String></code> will.
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</p>
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</div>
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</div>
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<div class="footer">
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<a href="index.html">Back to experimental features</a> | <a href="Accessors.html">Previous feature (@Accessors)</a> | <a href="FieldDefaults.html">Next feature (@FieldDefaults)</a><br />
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<a href="../../credits.html" class="creditsLink">credits</a> | <span class="copyright">Copyright © 2009-2015 The Project Lombok Authors, licensed under the <a href="http://www.opensource.org/licenses/mit-license.php">MIT license</a>.</span>
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