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Roel Spilker
99 lines
7.2 KiB
HTML
99 lines
7.2 KiB
HTML
<!DOCTYPE html>
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<meta name="description" content="Spice up your java" />
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<title>@XArgsConstructor</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>@NoArgsConstructor, @RequiredArgsConstructor, @AllArgsConstructor</h1>
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<div class="byline">Constructors made to order: Generates constructors that take no arguments, one argument per final / non-null field, or one argument for every field.</div>
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<div class="overview">
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<h3>Overview</h3>
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<p>
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This set of 3 annotations generate a constructor that will accept 1 parameter for certain fields, and simply assigns this parameter to the field.
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</p><p>
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<code>@NoArgsConstructor</code> will generate a constructor with no parameters. If this is not possible (because of final fields), a compiler error will result instead.
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For fields with constraints, such as <code>@NonNull</code> fields, <em>no</em> check or assignment is generated, so be aware that these constraints may then not be
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fulfilled until those fields are properly initialized later. Certain java constructs, such as hibernate and the Service Provider Interface require a no-args constructor.
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This annotation is useful primarily in combination with either <code>@Data</code> or one of the other constructor generating annotations.
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</p><p>
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<code>@RequiredArgsConstructor</code> generates a constructor with 1 parameter for each field that requires special handling. All <code>final</code> fields get a parameter,
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as well as any fields that are marked as <code>@NonNull</code> that aren't initialized where they are declared. For those fields marked with <code>@NonNull</code>, an explicit
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null check is also generated. The constructor will throw a <code>NullPointerException</code> if any of the parameters intended for the fields marked with <code>@NonNull</code>
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contain <code>null</code>. The order of the parameters match the order in which the fields appear in your class.
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</p><p>
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<code>@AllArgsConstructor</code> generates a constructor with 1 parameter for each field in your class. Fields marked with <code>@NonNull</code> result in null checks on
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those parameters.
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</p><p>
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Each of these annotations allows an alternate form, where the generated constructor is always private, and an additional static factory method that wraps around the
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private constructor is generated. This mode is enabled by supplying the <code>staticName</code> value for the annotation, like so: <code>@RequiredArgsConstructor(staticName="of")</code>.
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Such a static factory method will infer generics, unlike a normal constructor. This means your API users get write <code>MapEntry.of("foo", 5)</code> instead of the much longer
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<code>new MapEntry<String, Integer>("foo", 5)</code>.
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</p><p>
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Static fields are skipped by these annotations. Also, a <code>@java.beans.ConstructorProperties</code> annotation is added for all constructors with at least 1 argument,
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which allows bean editor tools to call the generated constructors. <code>@ConstructorProperties</code> is now in Java 1.6, which means that if your code is intended for
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compilation on Java 1.5, a compiler error will occur. <em>Running</em> on a JVM 1.5 should be no problem (the annotation will be ignored). To suppress the generation of
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the <code>@ConstructorProperties</code> annotation, add a parameter to your annotation: <code>@AllArgsConstructor(suppressConstructorProperties=true)</code>. However,
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as java 1.5, which has already been end-of-lifed, fades into obscurity, this parameter will eventually be removed. It has also been marked deprecated for this reason.
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</p><p>
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Unlike most other lombok annotations, the existence of an explicit constructor does not stop these annotations from generating their own constructor. This means you can write
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your own specialized constructor, and let lombok generate the boilerplate ones as well. If a conflict arises (one of your constructors ends up with the same
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signature as one that lombok generates), a compiler error will occur.
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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">
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<h3>Small print</h3><div class="smallprint">
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<p>
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Even if a field is explicitly initialized with <code>null</code>, lombok will consider the requirement to avoid null as fulfilled, and will <em>NOT</em> consider
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the field as a 'required' argument. The assumption is that if you explicitly assign <code>null</code> to a field that you've also marked as <code>@NonNull</code>
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signals you must know what you're doing.
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</p><p>
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The <code>@java.beans.ConstructorProperties</code> annotation is never generated for a constructor with no arguments. This also explains why <code>@NoArgsConstructor</code>
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lacks the <code>suppressConstructorProperties</code> annotation method. The <code>@ConstructorProperties</code> annotation is also omitted for private constructors. The
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generated static factory methods also do not get <code>@ConstructorProperties</code>, as this annotation can only be added to real constructors.
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</p><p>
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<code>@XArgsConstructor</code> can also be used on an enum definition. The generated constructor will always be
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private, because non-private constructors aren't legal in enums. You don't have to specify <code>AccessLevel.PRIVATE</code>.
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</p><p>
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While <code>suppressConstructorProperties</code> has been marked deprecated in anticipation of a world where all java environments have the
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<code>@ConstructorProperties</code> annotation available, first GWT 2.2 and Android 2.3.3, which do not (yet) have this annotation, will have
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to be ancient history before this annotation parameter will be removed.
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</p>
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</div>
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</div>
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<a href="index.html">Back to features</a> | <a href="EqualsAndHashCode.html">Previous feature (@EqualsAndHashCode)</a> | <a href="Data.html">Next feature (@Data)</a><br />
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<a href="../credits.html" class="creditsLink">credits</a> | <span class="copyright">Copyright © 2010-2011 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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