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<title>@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>
<code>@Builder</code> was introduced as experimental feature in lombok v0.12.0.
</p><p>
<code>@Builder</code> gained <code>@Singular</code> support and was promoted to the main <code>lombok</code> package since lombok v1.16.0.
</p>
</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").job("Mythbusters").job("Unchained Reaction").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>
<code>@Builder</code> can generate so-called 'singular' methods for collection parameters/fields. These take 1 element instead of an entire list, and add the
element to the list. For example: <code>Person.builder().job("Mythbusters").job("Unchained Reaction").build();</code> would result in the <code>List<String> jobs</code>
field to have 2 strings in it. To get this behaviour, the field/parameter needs to be annotated with <code>@Singular</code>. The feature has <a href="#singular">its own documentation</a>.
</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>
</ul>
Example usage where all options are changed from their defaults:<br />
<code>@Builder(builderClassName = "HelloWorldBuilder", buildMethodName = "execute", builderMethodName = "helloWorld")</code><br />
</p>
</div>
<div class="overview">
<h3><a name="singular">@Singular</a></h3>
<p>
By annotating one of the parameters (if annotating a static method or constructor with <code>@Builder</code>) or fields (if annotating a class with <code>@Builder</code>) with the
<code>@Singular</code> annotation, lombok will treat that builder node as a collection, and it generates 2 'adder' methods instead of a 'setter' method. One which adds a single element to the collection, and one
which adds all elements of another collection to the collection. No setter to just set the collection (replacing whatever was already added) will be generated. These 'singular' builders
are very complicated in order to guarantee the following properties:
<ul>
<li>When invoking <code>build()</code>, the produced collection will be immutable.</li>
<li>Repeatedly invoking <code>build()</code> works fine and does not corrupt any of the collections already generated.</li>
<li>Calling one of the 'adder' methods after invoking <code>build()</code> does not modify any already generated objects, and, if <code>build()</code> is later called again,
<li>The produced collection will be compacted to the smallest feasible format while remaining efficient.</li>
</ul>
</p><p>
<code>@Singular</code> can only be applied to collection types for which lombok has a recipe to produce the singular methods. Currently, the supported types are:
<ul>
<li><a href="http://docs.oracle.com/javase/8/docs/api/java/util/package-summary.html"><code>java.util</code></a>:<ul>
<li><code>Collection</code> and <code>List</code> (backed by a compacted unmodifiable <code>ArrayList</code> in the general case).</li>
<li><code>Set</code>, <code>SortedSet</code>, and <code>NavigableSet</code> (backed by a smartly sized unmodifiable <code>HashSet</code> or <code>TreeSet</code> in the general case).</li>
<li><code>Map</code>, <code>SortedMap</code>, and <code>NavigableMap</code> (backed by a smartly sized unmodifiable <code>HashMap</code> or <code>TreeMap</code> in the general case).</li>
</ul></li>
<li><a href="https://github.com/google/guava">Guava</a>'s <code>com.google.common.collect</code>:<ul>
<li><code>ImmutableCollection</code> and <code>ImmutableList</code> (backed by the builder feature of <code>ImmutableList</code>).</li>
<li><code>ImmutableSet</code> and <code>ImmutableSortedSet</code> (backed by the builder feature of those types).</li>
<li><code>ImmutableMap</code>, <code>ImmutableBiMap</code>, and ImmutableSortedMap</code> (backed by the builder feature of those types).</li>
</ul></li>
</ul>
</p><p>
If your identifiers are written in common english, lombok assumes that any collection with <code>@Singular</code> on it is an english plural and will attempt to automatically
singularize it. If this is possible, the add-one method will use this name. For example, if your collection is called <code>statuses</code>, then the add-one method will automatically
be called <code>status</code>. If lombok cannot singularize your identifier, or it is ambiguous, lombok will generate an error and force you to explicitly specify the singular name.
To do this, just pass the singular name as string, like so: <code>@Singular("axis") List<Line> axes;</code>.
</p><p>
When using the <code>java.util</code> interfaces, lombok always uses <code>ArrayList</code> to store items added to the builder, because this is more efficient than adding them to a map or
set immediately, as lombok needs to compact and potentially duplicate the result.
</p><p>
The snippet below does not show what lombok generates for a <code>@Singular</code> field/parameter because it is rather complicated.
You can view a snippet <a href="Singular-snippet.html">here</a>.
</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>
<dt><code>lombok.singular.useGuava</code> = [<code>true</code> | <code>false</code>] (default: false)</dt>
<dd>If <code>true</code>, lombok will use guava's <code>ImmutableX</code> builders and types to implement <code>java.util</code> collection interfaces, instead of creating
implementations based on <code>Collections.unmodifiableX</code>. You must ensure that guava is actually available on the classpath and buildpath if you use this setting.
Guava is used automatically if your field/parameter has one of the guava <code>ImmutableX</code> types.
<dt><code>lombok.singular.auto</code> = [<code>true</code> | <code>false</code>] (default: true)</dt>
<dd>If <code>true</code> (which is the default), lombok automatically tries to singularize your identifier name by assuming that it is a common english plural.
If <code>false</code>, you must always explicitly specify the singular name, and lombok will generate an error if you don't (useful if you write your code in a language other than english).
</dl>
</div>
<div class="overview">
<h3>Small print</h3><div class="smallprint">
<p>
@Singular support for <code>java.util.NavigableMap/Set</code> only works if you are compiling with JDK1.8 or higher.
</p><p>
You cannot manually provide some or all parts of a <code>@Singular</code> node; the code lombok generates is too complex for this. If you want to
manually control (part of) the builder nodes associated with some field or parameter, don't use @Singular and add everything you need manually.
</p><p>
The sorted collections (java.util: <code>SortedSet</code>, <code>NavigableSet</code>, <code>SortedMap</code>, <code>NavigableMap</code> and guava: <code>ImmutableSortedSet</code>, <code>ImmutableSortedMap</code>) require that the type argument of the collection has natural order (implements <code>java.util.Comparable</code>). There is no way to pass an explicit <code>Comparator</code> to use in the builder.
</p>
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