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How do I... (Builder edition)

This page answers common how-to questions that may come up when using AutoValue with the builder option. You should read and understand AutoValue with builders first.

If you are not using a builder, see Introduction and How do I... instead.

How do I...

... use (or not use) set prefixes?
... use different names besides builder()/Builder/build()?
... specify a default value for a property?
... initialize a builder to the same property values as an existing value instance
... include with- methods on my value class for creating slightly altered instances?
... validate property values?
... normalize (modify) a property value at build time?
... expose both a builder and a factory method?
... handle Optional properties?
... use a collection-valued property?
... access nested builders while building?

... use (or not use) `set` prefixes?

Just as you can choose whether to use JavaBeans-style names for property getters (getFoo() or just foo()) in your value class, you have the same choice for setters in builders too (setFoo(value) or just foo(value)). As with getters, you must use these prefixes consistently or not at all.

Using get/is prefixes for getters and using the set prefix for setters are independent choices. For example, it is fine to use the set prefixes on all your builder methods, but omit the get/is prefixes from all your accessors.

Here is the Animal example using get prefixes but not set prefixes:

@AutoValue
abstract class Animal {
  abstract String getName();
  abstract int getNumberOfLegs();

  static Builder builder() {
    return new AutoValue_Animal.Builder();
  }

  @AutoValue.Builder
  abstract static class Builder {
    abstract Builder name(String value);
    abstract Builder numberOfLegs(int value);
    abstract Animal build();
  }
}

... use different names besides `builder()`/`Builder`/`build()`?

Use whichever names you like; AutoValue doesn't actually care.

(We would gently recommend these names as conventional.)

... specify a default value for a property?

What should happen when a caller does not supply a value for a property before calling build()? If the property in question is nullable, it will simply default to null as you would expect. And if it is Optional it will default to an empty Optional as you might also expect. But if it isn't either of those things (including if it is a primitive-valued property, which can't be null), then build() will throw an unchecked exception.

But this presents a problem, since one of the main advantages of a builder in the first place is that callers can specify only the properties they care about!

The solution is to provide a default value for such properties. Fortunately this is easy: just set it on the newly-constructed builder instance before returning it from the builder() method.

Here is the Animal example with the default number of legs being 4:

@AutoValue
abstract class Animal {
  abstract String name();
  abstract int numberOfLegs();

  static Builder builder() {
    return new AutoValue_Animal.Builder()
        .setNumberOfLegs(4);
  }

  @AutoValue.Builder
  abstract static class Builder {
    abstract Builder setName(String value);
    abstract Builder setNumberOfLegs(int value);
    abstract Animal build();
  }
}

Occasionally you may want to supply a default value, but only if the property is not set explicitly. This is covered in the section on normalization.

... initialize a builder to the same property values as an existing value instance

Suppose your caller has an existing instance of your value class, and wants to change only one or two of its properties. Of course, it's immutable, but it would be convenient if they could easily get a Builder instance representing the same property values, which they could then modify and use to create a new value instance.

To give them this ability, just add an abstract toBuilder method, returning your abstract builder type, to your value class. AutoValue will implement it.

  public abstract Builder toBuilder();

... include `with-` methods on my value class for creating slightly altered instances?

This is a somewhat common pattern among immutable classes. You can't have setters, but you can have methods that act similarly to setters by returning a new immutable instance that has one property changed.

If you're already using the builder option, you can add these methods by hand:

@AutoValue
public abstract class Animal {
  public abstract String name();
  public abstract int numberOfLegs();

  public static Builder builder() {
    return new AutoValue_Animal.Builder();
  }

  abstract Builder toBuilder();

  public Animal withName(String name) {
    return toBuilder().setName(name).build();
  }

  @AutoValue.Builder
  public abstract static class Builder {
    public abstract Builder setName(String value);
    public abstract Builder setNumberOfLegs(int value);
    public abstract Animal build();
  }
}

Note that it's your free choice what to make public (toBuilder, withName, neither, or both).

... validate property values?

Validating properties is a little less straightforward than it is in the non-builder case.

What you need to do is split your "build" method into two methods:

the non-visible, abstract method that AutoValue implements
and the visible, concrete method you provide, which calls the generated method and performs validation.

We recommend naming these methods autoBuild and build, but any names will work. It ends up looking like this:

@AutoValue
public abstract class Animal {
  public abstract String name();
  public abstract int numberOfLegs();

  public static Builder builder() {
    return new AutoValue_Animal.Builder();
  }

  @AutoValue.Builder
  public abstract static class Builder {
    public abstract Builder setName(String value);
    public abstract Builder setNumberOfLegs(int value);

    abstract Animal autoBuild();  // not public

    public Animal build() {
      Animal animal = autoBuild();
      Preconditions.checkState(animal.numberOfLegs() >= 0, "Negative legs");
      return animal;
    }
  }
}

... normalize (modify) a property value at `build` time?

Suppose you want to convert the animal's name to lower case.

You'll need to add a getter to your builder, as shown:

@AutoValue
public abstract class Animal {
  public abstract String name();
  public abstract int numberOfLegs();

  public static Builder builder() {
    return new AutoValue_Animal.Builder();
  }

  @AutoValue.Builder
  public abstract static class Builder {
    public abstract Builder setName(String value);
    public abstract Builder setNumberOfLegs(int value);

    abstract String name(); // must match method name in Animal

    abstract Animal autoBuild(); // not public

    public Animal build() {
      setName(name().toLowerCase());
      return autoBuild();
    }
  }
}

The getter in your builder must have the same signature as the abstract property accessor method in the value class. It will return the value that has been set on the Builder. If no value has been set for a non-nullable property, IllegalStateException is thrown.

Getters should generally only be used within the Builder as shown, so they are not public.

As an alternative to returning the same type as the property accessor method, the builder getter can return an Optional wrapping of that type. This can be used if you want to supply a default, but only if the property has not been set. (The usual way of supplying defaults means that the property always appears to have been set.) For example, suppose you wanted the default name of your Animal to be something like "4-legged creature", where 4 is the numberOfLegs() property. You might write this:

@AutoValue
public abstract class Animal {
  public abstract String name();
  public abstract int numberOfLegs();

  public static Builder builder() {
    return new AutoValue_Animal.Builder();
  }

  @AutoValue.Builder
  public abstract static class Builder {
    public abstract Builder setName(String value);
    public abstract Builder setNumberOfLegs(int value);

    abstract Optional<String> name();
    abstract int numberOfLegs();

    abstract Animal autoBuild(); // not public

    public Animal build() {
      if (!name().isPresent()) {
        setName(numberOfLegs() + "-legged creature");
      }
      return autoBuild();
    }
  }
}

Notice that this will throw IllegalStateException if the numberOfLegs property hasn't been set either.

The Optional wrapping can be any of the Optional types mentioned in the section on Optional properties. If your property has type int it can be wrapped as either Optional<Integer> or OptionalInt, and likewise for long and double.

... expose both a builder and a factory method?

If you use the builder option, AutoValue will not generate a visible constructor for the generated concrete value class. If it's important to offer your caller the choice of a factory method as well as the builder, then your factory method implementation will have to invoke the builder itself.

... handle `Optional` properties?

Properties of type Optional benefit from special treatment. If you have a property of type Optional<String>, say, then it will default to an empty Optional without needing to specify a default explicitly. And, instead of or as well as the normal setFoo(Optional<String>) method, you can have setFoo(String). Then setFoo(s) is equivalent to setFoo(Optional.of(s)).

Here, Optional means either java.util.Optional from Java (Java 8 or later), or com.google.common.base.Optional from Guava. Java 8 also introduced related classes in java.util called OptionalInt, OptionalLong, and OptionalDouble. You can use those in the same way. For example a property of type OptionalInt will default to OptionalInt.empty() and you can set it with either setFoo(OptionalInt) or setFoo(int).

@AutoValue
public abstract class Animal {
  public abstract Optional<String> name();

  public static Builder builder() {
    return new AutoValue_Animal.Builder();
  }

  @AutoValue.Builder
  public abstract static class Builder {
    // You can have either or both of these two methods:
    public abstract Builder setName(Optional<String> value);
    public abstract Builder setName(String value);
    public abstract Animal build();
  }
}

... use a collection-valued property?

Value objects should be immutable, so if a property of one is a collection then that collection should be immutable too. We recommend using Guava's immutable collections to make that explicit. AutoValue's builder support includes a few special arrangements to make this more convenient.

In the examples here we use ImmutableSet, but the same principles apply to all of Guava's immutable collection types, like ImmutableList, ImmutableMultimap, and so on.

We recommend using the immutable type (like ImmutableSet<String>) as your actual property type. However, it can be a pain for callers to always have to construct ImmutableSet instances to pass into your builder. So AutoValue allows your builder method to accept an argument of any type that ImmutableSet.copyOf accepts.

If our Animal acquires an ImmutableSet<String> that is the countries it lives in, that can be set from a Set<String> or a Collection<String> or an Iterable<String> or a String[] or any other compatible type. You can even offer multiple choices, as in this example:

@AutoValue
public abstract class Animal {
  public abstract String name();
  public abstract int numberOfLegs();
  public abstract ImmutableSet<String> countries();

  public static Builder builder() {
    return new AutoValue_Animal.Builder();
  }

  @AutoValue.Builder
  public abstract static class Builder {
    public abstract Builder setName(String value);
    public abstract Builder setNumberOfLegs(int value);
    public abstract Builder setCountries(Set<String> value);
    public abstract Builder setCountries(String... value);
    public abstract Animal build();
  }
}

... let my builder accumulate values for a collection-valued property (not require them all at once)?

Instead of defining a setter for an immutable collection foos, you can define a method foosBuilder() that returns the associated builder type for that collection. In this example, we have an ImmutableSet<String> which can be built using the countriesBuilder() method:

@AutoValue
public abstract class Animal {
  public abstract String name();
  public abstract int numberOfLegs();
  public abstract ImmutableSet<String> countries();

  public static Builder builder() {
    return new AutoValue_Animal.Builder();
  }

  @AutoValue.Builder
  public abstract static class Builder {
    public abstract Builder setName(String value);
    public abstract Builder setNumberOfLegs(int value);
    public abstract ImmutableSet.Builder<String> countriesBuilder();
    public abstract Animal build();
  }
}

The name of this method must be exactly the property name (countries here) followed by the string Builder. Even if the properties follow the getCountries() convention, the builder method must be countriesBuilder() and not getCountriesBuilder().

You may notice a small problem with this example: the caller can no longer create their instance in a single chained statement:

  // This DOES NOT work!
  Animal dog = Animal.builder()
      .setName("dog")
      .setNumberOfLegs(4)
      .countriesBuilder()
          .add("Guam")
          .add("Laos")
      .build();

Instead they are forced to hold the builder itself in a temporary variable:

  // This DOES work... but we have to "break the chain"!
  Animal.Builder builder = Animal.builder()
      .setName("dog")
      .setNumberOfLegs(4);
  builder.countriesBuilder()
      .add("Guam")
      .add("Laos");
  Animal dog = builder.build();

One solution for this problem is just below.

... accumulate values for a collection-valued property, without "breaking the chain"?

Another option is to keep countriesBuilder() itself non-public, and only use it to implement a public addCountry method:

@AutoValue
public abstract class Animal {
  public abstract String name();
  public abstract int numberOfLegs();
  public abstract ImmutableSet<String> countries();

  public static Builder builder() {
    return new AutoValue_Animal.Builder();
  }

  @AutoValue.Builder
  public abstract static class Builder {
    public abstract Builder setName(String value);
    public abstract Builder setNumberOfLegs(int value);

    abstract ImmutableSet.Builder<String> countriesBuilder();
    public Builder addCountry(String value) {
      countriesBuilder().add(value);
      return this;
    }

    public abstract Animal build();
  }
}

Now the caller can do this:

  // This DOES work!
  Animal dog = Animal.builder()
      .setName("dog")
      .setNumberOfLegs(4)
      .addCountry("Guam")
      .addCountry("Laos") // however many times needed
      .build();

... offer both accumulation and set-at-once methods for the same collection-valued property?

Yes, you can provide both methods, letting your caller choose the style they prefer.

The same caller can mix the two styles only in limited ways; once foosBuilder has been called, any subsequent call to setFoos will throw an unchecked exception. On the other hand, calling setFoos first is okay; a later call to foosBuilder will return a builder already populated with the previously-supplied elements.

... access nested builders while building?

Often a property of an @AutoValue class is itself an immutable class, perhaps another @AutoValue. In such cases your builder can expose a builder for that nested class. This is very similar to exposing a builder for a collection property, as described earlier.

Suppose the Animal class has a property of type Species:

@AutoValue
public abstract class Animal {
  public abstract String name();
  public abstract Species species();

  public static Builder builder() {
    return new AutoValue_Animal.Builder();
  }

  @AutoValue.Builder
  public abstract static class Builder {
    public abstract Builder setName(String name);
    public abstract Species.Builder speciesBuilder();
    public abstract Animal build();
  }
}

@AutoValue
public abstract class Species {
  public abstract String genus();
  public abstract String epithet();

  public static Builder builder() {
    return new AutoValue_Species.Builder();
  }

  @AutoValue.Builder
  public abstract static class Builder {
    public abstract Builder setGenus(String genus);
    public abstract Builder setEpithet(String epithet);
    public abstract Species build();
  }
}

Now you can access the builder of the nested Species while you are building the Animal:

  Animal.Builder catBuilder = Animal.builder()
      .setName("cat");
  catBuilder.speciesBuilder()
      .setGenus("Felis")
      .setEpithet("catus");
  Animal cat = catBuilder.build();

Although the nested class in the example (Species) is also an @AutoValue class, it does not have to be. For example, it could be a protobuf. The requirements are:

The nested class must have a way to make a new builder. This can be new Species.Builder(), or Species.builder(), or Species.newBuilder().
There must be a way to build an instance from the builder: Species.Builder must have a method Species build().
If there is a need to convert Species back into its builder, then Species must have a method Species.Builder toBuilder().

In the example, if Animal has an abstract toBuilder() method then Species must also have a toBuilder() method. That also applies if there is an abstract setSpecies method in addition to the speciesBuilder method.

As an alternative to having a method Species.Builder toBuilder() in Species, Species.Builder can have a method called addAll or putAll that accepts an argument of type Species. This is how AutoValue handles ImmutableSet for example. ImmutableSet does not have a toBuilder() method, but ImmutableSet.Builder does have an addAll method that accepts an ImmutableSet. So given ImmutableSet<String> strings, we can achieve the effect of strings.toBuilder() by doing:
```
ImmutableSet.Builder<String> builder = ImmutableSet.builder();
builder.addAll(strings);
```

There are no requirements on the name of the builder class. Instead of Species.Builder, it could be Species.Factory or SpeciesBuilder.

If speciesBuilder() is never called then the final species() property will be set as if by speciesBuilder().build(). In the example, that would result in an exception because the required properties of Species have not been set.

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