Usage from Java

Scala’s Functions

Due to Scala’s Function0 and Function1 being traits, these are difficult to instantiate directly from Java.

Use com.twitter.util.Function instead of scala.Function1s and scala.PartialFunctions. While com.twitter.util.Function0 is a replacement for scala.Function0.

While the scala.runtime.AbstractFunctions look tempting, they were not intended to be used for Java interop and we got mixed messages from the scalac team as to whether it was a good idea to use them. The com.twitter.util versions of these work perfectly well and should be used instead of those.

There are also construction methods on the Function object such as func which improves interop with Java 8’s lambdas as well as ofCallable and ofRunnable.

import com.twitter.util.Future;
import com.twitter.util.Function;
import com.twitter.util.Function0;
import static com.twitter.util.Function.func;
import static com.twitter.util.Function.func0;
import static com.twitter.util.Function.cons;
import scala.runtime.BoxedUnit;

// use a static import of func0 and lambdas for interop with
// Scala's Function0 and call-by-name method arguments.
Function0<String> fn0 = func0(() -> "example");
// and used inline:
Future<String> fs = Future.apply(func0(() -> "example"));

// use a static import of func and lambdas for interop with
// Scala's Function1.
Function<String, Integer> fn = func(s -> s.length());
// and used inline, with method references:
Future<Integer> fi =;

// use a static import of cons and lambdas for interop with
// Scala Function1's that return Unit.
Function<String, BoxedUnit> consumer = cons(s -> System.out.println(s));
// and used inline, with method references:
Future<String> f2 = fs.onSuccess(cons(System.out::println));

Compatibility APIs

For many common classes and methods, there are static compatibility classes that provide Java developers with more idiomatic APIs and less Foo$.MODULE$s. Note that Scala will sometimes create the static forwarding methods on Foo itself so it is worth looking there as well.

The class or object’s scaladocs should point you towards where to look. These are typically located at $module/src/main/java. There are also Java compatibility tests which exist under $module/src/test/java/ which can be helpful for seeing how to use the APIs from Java. Often, the Java analog will follow the convention of being in the same package, but with a plural name. For example, has Java’s

As an example, this uses com.twitter.util.FuturePools in order to access the FuturePool companion object methods.

import com.twitter.util.FuturePool;
import com.twitter.util.FuturePools;
import java.util.concurrent.Executors;

FuturePool unboundedPool = FuturePools.unboundedPool();
FuturePool execServicePool =