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<title>EXPERIMENTAL - @Builder</title>
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<div class="header"><a href="../../index.html">Project Lombok</a></div>
<h1>@Builder</h1>
<div class="byline">... and Bob's your uncle: No-hassle fancy-pants APIs for object creation!</div>
<div class="since">
<h3>Since</h3>
<p>
@Builder was introduced as experimental feature in lombok v0.12.0.
</p>
</div>
<div class="experimental">
<h3>Experimental</h3>
<p>
Experimental because:
<ul>
<li>New feature - community feedback requested.</li>
</ul>
Current status: <em>sure thing</em> - This feature will move to the core package soon.
</div>
<div class="overview">
<h3>Overview</h3>
<p>
The <code>@Builder</code> annotation produces complex builder APIs for your classes.
</p><p>
<code>@Builder</code> lets you automatically produce the code required to have your class be instantiable with code such as:<br />
<code>Person.builder().name("Adam Savage").city("San Francisco").worksAt("Mythbusters").build();</code>
</p><p>
<code>@Builder</code> can be placed on a class, or on a constructor, or on a static method. While the "on a class" and "on a constructor"
mode are the most common use-case, <code>@Builder</code> is most easily explained with the "static method" use-case.
</p><p>
A static method annotated with <code>@Builder</code> (from now on called the <em>target</em>) causes the following 7 things to be generated:<ul>
<li>An inner static class named <code><em>Foo</em>Builder</code>, with the same type arguments as the static method (called the <em>builder</em>).</li>
<li>In the <em>builder</em>: One private non-static non-final field for each parameter of the <em>target</em>.</li>
<li>In the <em>builder</em>: A package private no-args empty constructor.</li>
<li>In the <em>builder</em>: A 'setter'-like method for each parameter of the <em>target</em>: It has the same type as that parameter and the same name.
It returns the builder itself, so that the setter calls can be chained, as in the above example.</li>
<li>In the <em>builder</em>: A <code>build()</code> method which calls the static method, passing in each field. It returns the same type that the
<em>target</em> returns.</li>
<li>In the <em>builder</em>: A sensible <code>toString()</code> implementation.</li>
<li>In the class containing the <em>target</em>: A <code>builder()</code> method, which creates a new instance of the <em>builder</em>.</li>
</ul>
Each listed generated element will be silently skipped if that element already exists (disregarding parameter counts and looking only at names). This
includes the <em>builder</em> itself: If that class already exists, lombok will simply start injecting fields and methods inside this already existing
class, unless of course the fields / methods to be injected already exist. You may not put any other method (or constructor) generating lombok annotation
on a builder class though; for example, you can not put <code>@EqualsAndHashCode</code> on the builder class.
</p><p>
Now that the "static method" mode is clear, putting a <code>@Builder</code> annotation on a constructor functions similarly; effectively,
constructors are just static methods that have a special syntax to invoke them: Their 'return type' is the class they construct, and their
type parameters are the same as the type parameters of the class itself.
</p><p>
Finally, applying <code>@Builder</code> to a class is as if you added <code>@AllArgsConstructor(access = AccessLevel.PACKAGE)</code> to the class and applied the
<code>@Builder</code> annotation to this all-args-constructor. This only works if you haven't written any explicit constructors yourself. If you do have an
explicit constructor, put the <code>@Builder</code> annotation on the constructor instead of on the class.
</p><p>
The name of the builder class is <code><em>Foobar</em>Builder</code>, where <em>Foobar</em> is the simplified, title-cased form of the return type of the
<em>target</em> - that is, the name of your type for <code>@Builder</code> on constructors and types, and the name of the return type for <code>@Builder</code>
on static methods. For example, if <code>@Builder</code> is applied to a class named <code>com.yoyodyne.FancyList<T></code>, then the builder name will be
<code>FancyListBuilder<T></code>. If <code>@Builder</code> is applied to a static method that returns <code>void</code>, the builder will be named
<code>VoidBuilder</code>.
</p><p>
The configurable aspects of builder are:<ul>
<li>The <em>builder's class name</em> (default: return type + 'Builder')</li>
<li>The <em>build()</em> method's name (default: <code>"build"</code>)</li>
<li>The <em>builder()</em> method's name (default: <code>"builder"</code>)</li>
<li>The 'fluent' nature of the generated 'setter'-like methods. A fluent 'setter' is named just <code>fieldName()</code>, a non-fluent one is named <code>setFieldName()</code>. (default: <code>true</code>)</li>
<li>The 'chain' nature of the generated 'setter'-like methods. A chainable 'setter' returns the builder object itself, letting you chain 'set' calls. A non-chainable setter returns <code>void</code>. (default: <code>true</code>)</li>
</ul>
Example usage where all options are changed from their defaults:<br />
<code>@Builder(builderClassName = "HelloWorldBuilder", buildMethodName = "execute", builderMethodName = "helloWorld", fluent = false, chain = false)</code><br />
</p>
</div>
<div class="snippets">
<div class="pre">
<h3>With Lombok</h3>
<div class="snippet">@HTML_PRE@</div>
</div>
<div class="sep"></div>
<div class="post">
<h3>Vanilla Java</h3>
<div class="snippet">@HTML_POST@</div>
</div>
</div>
<div style="clear: left;"></div>
<div class="overview confKeys">
<h3>Supported configuration keys:</h3>
<dl>
<dt><code>lombok.builder.flagUsage</code> = [<code>warning</code> | <code>error</code>] (default: not set)</dt>
<dd>Lombok will flag any usage of <code>@Builder</code> as a warning or error if configured.</dd>
</dl>
</div>
<div class="overview">
<h3>Small print</h3><div class="smallprint">
<p>
Another strategy for fluent APIs is that the programmer using your library statically imports your 'builder' method. In this case, you might want to name your builder
method equal to your type's name. So, the builder method for a class called <code>Person</code> would become <code>person()</code>. This is nicer if the builder method
is statically imported.
</p><p>
If the return type of your target static method is a type parameter (such as <code>T</code>), lombok will enforce an explicit builder class name.
</p><p>
You don't HAVE to use <code>@Builder</code> to build anything; you can for example put it on a static method that has lots of parameter to improve the API of it.
In this case, we suggest you use <code>buildMethodName = </code> to rename the build method to <code>execute()</code> instead.
</p><p>
The builder class will NOT get an auto-generated implementation of <code>hashCode</code> or <code>equals</code> methods! These would suggest that it is sensible to use
instances of a builder as keys in a set or map. However, that's not a sensible thing to do. Hence, no <code>hashCode</code> or <code>equals</code>.
</p><p>
Generics are sorted out for you.
</p><p>
If an explicit constructor is present, but <code>@Builder</code> is placed on the class, then the builder will be generated as if an explicit constructor is present with the
same arguments list as what <code>@AllArgsConstructor</code> would produce. If this constructor does not exist, a compile time error will result. Usually you should just either let
lombok make this constructor (delete your constructor from the source), or, move the <code>@Builder</code> annotation to the constructor.
</p>
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