Java 8: Under the Hood

Vlastimil Menčík

λ - syntax

x => x + 1

_ + 1

Integer.parseInt Integer::parseInt

x -> x + 1

MyClass MyClass::new

if MyClass is a case class

λ - types

• functional interfaces aka SAM interfaces

• target typing

• applicable to legacy interfaces

• FunctionN

• String => Int

• Currying

• PartialFunction

Functional Interface

Interoperability with Scala

λ - implementation in Scala

• instances of FunctionX traits

• compiled into anonymous inner classesMyClass$$anonfun$1.class

• a lot of extra bytecode

λ - implementation in Java 8

• implementation in synthetic methods

• invokedynamic

• LambdaMetafactory

λ in bytecode

invokedynamic

• since Java 7

• java.lang.invoke

• what really gets invoked is resolved at runtime by delegating to a bootstrap method

LambdaMetafactory

• contains bootstrap methods for converting λ-expressions to functional interface objects

invokedynamic + LambdaMetafactory

invokedynamic #2

x -> x + 5

When will this be in Scala?

• after Scala moves away from Java 6

• 2.11 will have experimental Java 7 backend

• 2.12 will target Java 8

Java 8: Stream API

Parallel streams

• very similar to parallel collections in Scala

stream.parallel()

stream.sequential()

col.par

col.seq

More lambdas in Java API

• java.util.function– “standard” functional interfaces– Function, BiFunction, Predicate, Supplier,

…

• java.util.Optional<T>

public T orElseGet(Supplier<? extends T> other)

Default methods

public interface Iterable<T> {default void forEach(…)

}

• necessary for backwards compatibility• useful in many other cases

Does Java now have traits?

• sort of, but not really – no state, no self types, no linearization (possible

diamond inheritance issues), …

• default methods are means of painless API evolution

• traits are means of modularization

Future benefits for Scala?

• better bytecode representation for method-only traits

• implemented methods no longer copied to subtypes

• could help with binary compatibility problems

java.time

• (finally) an immutable date representation

• inspired by Joda Time

Changes in JVM

• PermGen replaced with Metaspace

• hopefully future optimizations for functional programming style

And the release date?

• umm…

• when it’s done

• hopefully next spring

Where to go next?

• https://jdk8.java.net/lambda/

• http://www.lambdafaq.org/

• http://lambdadoc.net/api/

• http://youtu.be/-7OB2PxB_zg– my more Java-focused talk (in Czech)

Q&A