@Builder

Overview

The @Builder annotation produces complex builder APIs for your classes.

@Builder lets you automatically produce the code required to have your class be instantiable with code such as:
Person.builder().name("Adam Savage").city("San Francisco").job("Mythbusters").job("Unchained Reaction").build();

@Builder can be placed on a class, or on a constructor, or on a method. While the "on a class" and "on a constructor" mode are the most common use-case, @Builder is most easily explained with the "method" use-case.

A method annotated with @Builder (from now on called the target) causes the following 7 things to be generated:

• An inner class named FooBuilder, with the same type arguments as the method (called the builder).
• In the builder: One private non-static non-final field for each parameter of the target.
• In the builder: A package private no-args empty constructor.
• In the builder: A 'setter'-like method for each parameter of the target: 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.
• In the builder: A build() method which calls the method, passing in each field. It returns the same type that the target returns.
• In the builder: A sensible toString() implementation.
• In the class containing the target: A static builder() method, which creates a new instance of the builder.

Each listed generated element will be silently skipped if that element already exists (disregarding parameter counts and looking only at names). This includes the builder 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 @EqualsAndHashCode on the builder class.

@Builder 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: Person.builder().job("Mythbusters").job("Unchained Reaction").build(); would result in the List<String> jobs field to have 2 strings in it. To get this behaviour, the field/parameter needs to be annotated with @Singular. The feature has its own documentation.

Now that the "method" mode is clear, putting a @Builder 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.

Finally, applying @Builder to a class is as if you added @AllArgsConstructor(access = AccessLevel.PACKAGE) to the class and applied the @Builder 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 @Builder annotation on the constructor instead of on the class.

If using @Builder to generate builders to produce instances of your own class (this is always the case unless adding @Builder to a static method that doesn't return your own type), you can use @Builder(toBuilder = true) to also generate an instance method in your class called toBuilder(); it creates a new builder that starts out with all the values of this instance. You can put the @Builder.ObtainVia annotation on the parameters (in case of a constructor or static method) or fields (in case of @Builder on a type) to indicate alternative means by which the value for that field/parameter is obtained from the instance. For example, you can specify a method to be invoked: @Builder.ObtainVia(method = "calculateFoo").

The name of the builder class is FoobarBuilder, where Foobar is the simplified, title-cased form of the return type of the target - that is, the name of your type for @Builder on constructors and types, and the name of the return type for @Builder on methods. For example, if @Builder is applied to a class named com.yoyodyne.FancyList<T>, then the builder name will be FancyListBuilder<T>. If @Builder is applied to a method that returns void, the builder will be named VoidBuilder.

The configurable aspects of builder are:

• The builder's class name (default: return type + 'Builder')
• The build() method's name (default: "build")
• The builder() method's name (default: "builder")
• If you want toBuilder() (default: no)

Example usage where all options are changed from their defaults:
@Builder(builderClassName = "HelloWorldBuilder", buildMethodName = "execute", builderMethodName = "helloWorld", toBuilder = true)

@Singular

By annotating one of the parameters (if annotating a method or constructor with @Builder) or fields (if annotating a class with @Builder) with the @Singular 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:

• When invoking build(), the produced collection will be immutable.
• Calling one of the 'adder' methods after invoking build() does not modify any already generated objects, and, if build() is later called again, another collection with all the elements added since the creation of the builder is generated.
• The produced collection will be compacted to the smallest feasible format while remaining efficient.

@Singular can only be applied to collection types known to lombok. Currently, the supported types are:

• java.util:
  • Iterable, Collection, and List (backed by a compacted unmodifiable ArrayList in the general case).
  • Set, SortedSet, and NavigableSet (backed by a smartly sized unmodifiable HashSet or TreeSet in the general case).
  • Map, SortedMap, and NavigableMap (backed by a smartly sized unmodifiable HashMap or TreeMap in the general case).
• Guava's com.google.common.collect:
  • ImmutableCollection and ImmutableList (backed by the builder feature of ImmutableList).
  • ImmutableSet and ImmutableSortedSet (backed by the builder feature of those types).
  • ImmutableMap, ImmutableBiMap, and ImmutableSortedMap (backed by the builder feature of those types).

If your identifiers are written in common english, lombok assumes that the name of any collection with @Singular on it is an english plural and will attempt to automatically singularize that name. If this is possible, the add-one method will use this name. For example, if your collection is called statuses, then the add-one method will automatically be called status. You can also specify the singular form of your identifier explictly by passing the singular form as argument to the annotation like so: @Singular("axis") List<Line> axes;.
If lombok cannot singularize your identifier, or it is ambiguous, lombok will generate an error and force you to explicitly specify the singular name.

The snippet below does not show what lombok generates for a @Singular field/parameter because it is rather complicated. You can view a snippet here.

Small print