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Java 8 on Android Alex Florescu YPlan @flor3scu

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Outline • Backward compatible features • Lambdas, method references etc. • Android N+ only features • Streams, default methods etc. • Setup: how to get this working • Other useful tidbits and analysis

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Notes • Slides are online • All code is online

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Backward-compatible features • Lambdas • Method references • Type annotations

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API support • Official website
 “Also available on API level 23 and lower” • Google I/O 2016
 “Backward compatible with Gingerbread”

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Backward-compatible features • Lambdas • Method references • Type annotations

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Lambdas & Method references

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Functional interface • Has exactly one abstract method • Can annotate with @FunctionalInterface • Not necessary for using lambdas with it

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OnClickListener public interface OnClickListener {
 void onClick(View v);
 }

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Functional interface • A lot of standard ones added in java.util.function • Consumer — an operation that accepts a single input argument and returns no result • Function — a function that accepts one argument and produces a result • Predicate — a predicate (boolean-valued function) of one argument • Supplier — a supplier of results

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Lambdas & Method references

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Lambdas Language construct that simplifies (or flattens) anonymous inner classes

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Setting listener button1.setOnClickListener(new View.OnClickListener() {
 @Override public void onClick(View v) {
 Snackbar.make(mainPanel, "Click listener
 triggered", Snackbar.LENGTH_SHORT).show();
 }
 });

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Setting listener button1.setOnClickListener(new View.OnClickListener() {
 @Override public void onClick(View v) {
 Snackbar.make(mainPanel, "Click listener
 triggered", Snackbar.LENGTH_SHORT).show();
 }
 });

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Setting listener button1.setOnClickListener(v -> 
 Snackbar.make(mainPanel, "Click listener triggered", 
 Snackbar.LENGTH_SHORT).show());

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Setting listener button1.setOnClickListener(v -> 
 Snackbar.make(mainPanel, "Click listener triggered", 
 Snackbar.LENGTH_SHORT).show());

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Lambda examples // Lambda with no parameters
 Runnable r2 = () -> Timber.d("Lambda test");

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Example with params participantRedList.setAdapter(
 new ParticipantAdapter(
 
 filterParticipants(participants,
 new FilterCondition() {
 @Override
 public boolean accept(Participant candidate) {
 return candidate.getTeamColour() == TeamColour.RED;
 }
 })));

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Example with params participantRedList.setAdapter(
 new ParticipantAdapter(
 
 filterParticipants(participants,
 new FilterCondition() {
 @Override
 public boolean accept(Participant candidate) {
 return candidate.getTeamColour() == TeamColour.RED;
 }
 })));

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Example with params participantRedList.setAdapter(
 new ParticipantAdapter(
 
 filterParticipants(participants,
 new FilterCondition() {
 @Override
 public boolean accept(Participant candidate) {
 return candidate.getTeamColour() == TeamColour.RED;
 }
 })));

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Example with params participantRedList.setAdapter(
 new ParticipantAdapter(
 filterParticipants(participants,
 (candidate) -> candidate.getTeamColour() == TeamColour.RED)));

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Example with params participantRedList.setAdapter(
 new ParticipantAdapter(
 filterParticipants(participants,
 (candidate) -> candidate.getTeamColour() == TeamColour.RED)));

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Syntax p -> p.getAge() >= 18 (a,b) -> a.getAge() < b.getAge() () -> Timber.d(“Ping”) (p) -> {
 Timber.d(“Participant age : %d”, p.getAge());
 return p.getAge() < 18;
 }

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Syntax p -> p.getAge() >= 18 (a,b) -> a.getAge() < b.getAge() () -> Timber.d(“Ping”) (p) -> {
 Timber.d(“Participant age : %d”, p.getAge());
 return p.getAge() < 18;
 }

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Syntax p -> p.getAge() >= 18 (a,b) -> a.getAge() < b.getAge() () -> Timber.d(“Ping”) (p) -> {
 Timber.d(“Participant age : %d”, p.getAge());
 return p.getAge() < 18;
 }

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Syntax p -> p.getAge() >= 18 (a,b) -> a.getAge() < b.getAge() () -> Timber.d(“Ping”) (p) -> {
 Timber.d(“Participant age : %d”, p.getAge());
 return p.getAge() < 18;
 }

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Lambdas & Method references

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Method references Compact lambda expressions for methods that already have a name

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Example 
 Collections.sort(participants, new Comparator() {
 @Override
 public int compare(Participant p1, Participant p2) {
 return Integer.compare(p1.getAge(), 
 p2.getAge());
 }
 });

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Example 
 Collections.sort(participants, (p1, p2) -> Integer.compare(p1.getAge(), 
 p2.getAge()));

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Example public final class Participant {
 ….
 public static int compareByAge(Participant a, 
 Participant b) {
 return Integer.compare(a.getAge(), b.getAge());
 }
 ….
 }

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Example public final class Participant {
 ….
 public static int compareByAge(Participant a, 
 Participant b) {
 return Integer.compare(a.getAge(), b.getAge());
 }
 ….
 }

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Example 
 Collections.sort(participants, new Comparator() {
 @Override
 public int compare(Participant p1, Participant p2) {
 return Participant.compareByAge(p1,p2);
 }
 });

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Example 
 Collections.sort(participants, (p1, p2) -> Participant.compareByAge(p1, p2));

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Example 
 Collections.sort(participants,
 Participant::compareByAge);

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From 
 Collections.sort(participants, new Comparator() {
 @Override
 public int compare(Participant p1, Participant p2) {
 return Participant.compareByAge(p1,p2);
 }
 });

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To 
 Collections.sort(participants,
 Participant::compareByAge);

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Android N 
 Collections.sort(participants,
 Comparator.comparingInt(Participant::getAge));

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Syntax

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Syntax • Reference to a static method (s) -> String.valueOf(s)

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Syntax • Reference to a static method (s) -> String.valueOf(s) String::valueOf

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Syntax • Reference to an instance method of a particular object (o) -> o.toString()

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Syntax • Reference to an instance method of a particular object (o) -> o.toString() o::toString

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Syntax • Reference to an instance method of an arbitrary object of a particular type Arrays.sort((a,b) -> a.compareToIgnoreCase(b))

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Syntax • Reference to an instance method of an arbitrary object of a particular type Arrays.sort((a,b) -> a.compareToIgnoreCase(b)) Arrays.sort(String::compareToIgnoreCase)

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Syntax • Reference to an instance method of an arbitrary object of a particular type Arrays.sort((a,b) -> a.compareToIgnoreCase(b)) Arrays.sort(String::compareToIgnoreCase)

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Syntax • Reference to a constructor () -> new String()

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Syntax • Reference to a constructor () -> new String() String::new

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Method count

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Method count • Exploring Java’s Hidden Costs (Jake Wharton) • 23:30 — Lambdas

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Method count From: https://speakerdeck.com/jakewharton/exploring-hidden-java-costs-360-andev-july-2016?slide=126

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Comparison with Kotlin • Claims no increase in method count, bytecode inserted directly (see post)

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Backward-compatible features • Lambdas • Method references • Type annotations

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Type annotations Annotations can now be used anywhere you use a type

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Annotations before Java 8 • Method: @Override void onCreate()… • Class: 
 @RunWith(JUnit4Runner.class) 
 public class TestDoggieTreats {… • Parameters, return values
 @NonNull String prepareText(@Nullable input);

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Type annotations • Anywhere you use a type • Class instantiation: new @Interned MyObject(); • Type cast: myString = (@NonNull String) str; • Implements clause: class UnmodifiableList implements @Readonly List<@Readonly T> { ... } • Throws: void monitorTemperature() throws @Critical TemperatureException { ... }

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Type annotations • Anywhere you use a type • Class instantiation: new @Interned MyObject(); • Type cast: myString = (@NonNull String) str; • Implements clause: class UnmodifiableList implements @Readonly List<@Readonly T> { ... } • Throws: void monitorTemperature() throws @Critical TemperatureException { ... }

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Requirements • Using the Jack compiler • Setting language compatibility to 1.8 • (more on that later)

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Nougat only

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Nougat only features • Streams • Default and static interface methods • Repeatable annotations

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Streams A new abstraction that lets you process data in a declarative way

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Streams Data manipulation like you always dreamed of. Can filter, map & reduce while being traversed, sequentially or in parallel.

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Example public interface FilterCondition {
 boolean accept(T candidate);
 }
 List filter(List 
 participants, 
 FilterCondition 
 filterCondition)

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Example List output = new ArrayList<>();
 
 for (Participant participant : participants) {
 if (filterCondition.accept(participant)) {
 output.add(participant);
 }
 }
 
 return output;


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Example return participants.stream()
 .filter(filterCondition::accept)
 .collect(Collectors.toList());

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Example return participants.stream()
 .filter(filterCondition::accept)
 .collect(Collectors.toList());

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Example return participants.stream()
 .filter((participant) -> 
 filterCondition.accept(participant))
 .collect(Collectors.toList());

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Example return participants.stream()
 .filter(filterCondition::accept)
 .collect(Collectors.toList());

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Example return participants.stream()
 .filter(filterCondition::accept)
 .collect(Collectors.toList());

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Example List filter(
 List participants, Predicate predicate)
 
 return participants.stream()
 .filter(predicate)
 .collect(Collectors.toList());

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Example List filter(
 List participants, Predicate predicate)
 
 return participants.stream()
 .filter(predicate)
 .collect(Collectors.toList());

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Find the youngest participants
 .stream()
 .min(Comparator.comparingInt(
 Participant::getAge))
 .ifPresent(
 participant -> 
 Timber.d("Youngest participant %s", 
 participant.getName()));


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Find the youngest participants
 .stream()
 .min(Comparator.comparingInt(
 Participant::getAge))
 .ifPresent(
 participant -> 
 Timber.d("Youngest participant %s", 
 participant.getName()));


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Find the youngest participants
 .stream()
 .min(Comparator.comparingInt(
 Participant::getAge))
 .ifPresent(
 participant -> 
 Timber.d("Youngest participant %s", 
 participant.getName()));


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Find the youngest participants
 .stream()
 .min(Comparator.comparingInt(
 Participant::getAge))
 .ifPresent(
 participant -> 
 Timber.d("Youngest participant %s", 
 participant.getName()));


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Find the youngest participants
 .parallelStream()
 .min(Comparator.comparingInt(
 Participant::getAge))
 .ifPresent(
 participant -> 
 Timber.d("Youngest participant %s", 
 participant.getName()));


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Map Set names =
 participants.stream()
 .map(Participant::getName)
 .sorted()
 .collect(Collectors.toSet());

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Map Set names =
 participants.stream()
 .map(Participant::getName)
 .sorted()
 .collect(Collectors.toSet());

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Map Set names =
 participants.stream()
 .map(Participant::getName)
 .sorted()
 .collect(Collectors.toSet());

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Map SortedSet names =
 participants.stream()
 .map(Participant::getName)
 .sorted()
 .collect(Collectors.toCollection(TreeSet::new));

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Map SortedSet names =
 participants.stream()
 .map(Participant::getName)
 .sorted()
 .collect(Collectors.toCollection(TreeSet::new));

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Streams are lazy • The intermediate operations are always lazy • The “terminal” operations produce values or side- effects

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Map SortedSet names =
 participants
 .stream()
 .map(Participant::getName)
 .sorted()
 .collect(Collectors.toCollection(TreeSet::new));

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Map SortedSet names =
 participants
 .stream()
 .map(Participant::getName)
 .sorted()
 .collect(Collectors.toCollection(TreeSet::new));

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Map SortedSet names =
 participants
 .stream()
 .map(Participant::getName)
 .sorted()
 .collect(Collectors.toCollection(TreeSet::new));

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Map SortedSet names =
 participants
 .stream()
 .map(Participant::getName)
 .sorted()
 .collect(Collectors.toCollection(TreeSet::new));

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Map SortedSet names =
 participants
 .stream()
 .map(Participant::getName)
 .sorted()
 .limit(2)
 .collect(Collectors.toCollection(TreeSet::new));

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Nougat only features • Streams • Default and static interface methods • Repeatable annotations

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Default methods

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Default methods How do streams work?

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Default methods • How to build “streams” without breaking all custom collections? • Collection interface adds: • boolean removeIf(Predicate super E> filter) • Spliterator spliterator() • Stream stream() • Stream parallelStream()

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Default methods • default boolean removeIf(Predicate super E> filter) • default Spliterator spliterator() • default Stream stream() • default Stream parallelStream()

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Default methods Enables you to add new (default) implementations to interfaces so implementing classes don’t need to change

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Collection @Override
 default Spliterator spliterator() {
 return Spliterators.spliterator(this, 0);
 }
 default Stream stream() {
 return StreamSupport.stream(spliterator(), false);
 }

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Implementing classes • When you implement an interface that contains a default method, you can do the following: • Not mention the default method at all, which lets your implementation use the default method • Override the default method • Reuse the default method implementation using super

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Implementing classes • Not mention the default method at all, which lets your implementation use the default method

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Implementing classes • Not mention the default method at all, which lets your implementation use the default method class FancyList implements Collection

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Implementing classes • Override the default method

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Implementing classes • Override the default method class FancyList implements Collection @Override
 public Stream stream() {
 return ….. 
 }

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Implementing classes • Reuse the default method implementation using super

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Implementing classes • Reuse the default method implementation using super class FancyList implements Collection @Override
 Stream stream() {
 // ……
 return super.stream();
 }

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Extending interface • When you extend an interface that contains a default method, you can do the following: • Not mention the default method at all, which lets your extended interface inherit the default method • Redeclare the default method, which makes it abstract • Redefine the default method, which overrides it

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Extending interface • Not mention the default method at all, which lets your extended interface inherit the default method

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Extending interface • Not mention the default method at all, which lets your extended interface inherit the default method interface Set extends Collection {
 ….
 }

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Extending interface • Redeclare the default method, which makes it abstract

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Extending interface • Redeclare the default method, which makes it abstract
 
 interface FancySet extends Collection {
 ….
 Stream stream();
 }

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Extending interface • Redefine the default method, which overrides it

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Extending interface • Redefine the default method, which overrides it interface Set extends Collection {
 ….
 @Override default Spliterator spliterator() {
 return Spliterators.spliterator(this, Spliterator.DISTINCT);
 }
 }

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Multiple inheritance? public interface A {
 default void foo() {
 Timber.d(“Calling A.foo()");
 }
 }
 
 
 
 
 
 
 public class ProblematicClass implements A, B {
 //…..
 } public interface B {
 default void foo() {
 Timber.d(“Calling B.foo()");
 }
 }

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Multiple inheritance? public interface A {
 default void foo() {
 Timber.d(“Calling A.foo()");
 }
 }
 
 
 
 
 
 
 public class ProblematicClass implements A, B {
 //…..
 } public interface B {
 default void foo() {
 Timber.d(“Calling B.foo()");
 }
 }

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Multiple inheritance public class ProblematicClass implements A, B { } // error: class ProblematicClass inherits unrelated defaults for foo() from types A and B

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Multiple inheritance public class ProblematicClass implements A, B { } // error: class ProblematicClass inherits unrelated defaults for foo() from types A and B

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Multiple inheritance public class ProblematicClass implements A, B { @Override public void foo() { } }

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Multiple inheritance public class ProblematicClass implements A, B { @Override public void foo() { A.super.foo(); B.super.foo(); } }

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Instances methods vs default methods public class Horse { public String identifyMyself() { return "I am a horse."; } } public interface Flyer { default public String identifyMyself() { return "I am able to fly."; } } public interface Mythical { default public String identifyMyself() { return "I am a mythical creature."; } } public class Pegasus extends Horse implements Flyer, Mythical { public static void main(String... args) { Pegasus myApp = new Pegasus(); System.out.println(myApp.identifyMyself()); } }

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Instances methods vs default methods public class Horse { public String identifyMyself() { return "I am a horse."; } } public interface Flyer { default public String identifyMyself() { return "I am able to fly."; } } public interface Mythical { default public String identifyMyself() { return "I am a mythical creature."; } } public class Pegasus extends Horse implements Flyer, Mythical { public static void main(String... args) { Pegasus myApp = new Pegasus(); System.out.println(myApp.identifyMyself()); } }

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Instances methods vs default methods public class Horse { public String identifyMyself() { return "I am a horse."; } } public interface Flyer { default public String identifyMyself() { return "I am able to fly."; } } public interface Mythical { default public String identifyMyself() { return "I am a mythical creature."; } } public class Pegasus extends Horse implements Flyer, Mythical { public static void main(String... args) { Pegasus myApp = new Pegasus(); System.out.println(myApp.identifyMyself()); } }

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Instances methods vs default methods public class Horse { public String identifyMyself() { return "I am a horse."; } } public interface Flyer { default public String identifyMyself() { return "I am able to fly."; } } public interface Mythical { default public String identifyMyself() { return "I am a mythical creature."; } } public class Pegasus extends Horse implements Flyer, Mythical { public static void main(String... args) { Pegasus myApp = new Pegasus(); System.out.println(myApp.identifyMyself()); } }

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Instances methods vs default methods public class Horse { public String identifyMyself() { return "I am a horse."; } } public interface Flyer { default public String identifyMyself() { return "I am able to fly."; } } public interface Mythical { default public String identifyMyself() { return "I am a mythical creature."; } } public class Pegasus extends Horse implements Flyer, Mythical { public static void main(String... args) { Pegasus myApp = new Pegasus(); System.out.println(myApp.identifyMyself()); } }

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Instances methods vs default methods • Instance methods are preferred over interface default methods • The method Pegasus.identifyMyself returns the string I am a horse.

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Static methods • Comparator gains:
 
 public static > Comparator reverseOrder() {
 return Collections.reverseOrder();
 }

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Nougat only features • Streams • Default and static interface methods • Repeatable annotations

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Repeatable annotations You may now create annotations that can be repeated

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Example @Schedule(dayOfMonth="last")
 @Schedule(dayOfWeek="Fri", hour="23")
 public void doPeriodicCleanup() { ... }

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How-to • Declare the annotation as repeatable • Declare a containing annotation type • See: https://docs.oracle.com/javase/tutorial/java/ annotations/repeating.html

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Pre-N alternatives

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Alternative options • Lambdas -> Retrolambda • Streams -> Lightweight Stream API

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Build setup

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Requirements • This applies for all Java 8 features • Requirements: • Use Jack & Jill • Set language compatibility • Android Studio 2.1+ • Build tools 21.1.1+

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A bit about Jack • Jack is a new Android toolchain • Compiles Java source into Android dex bytecode • Replaces the previous toolchain • javac, ProGuard, jarjar, dx etc.

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A bit about Jack

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What’s Jill? • Jill translates the existing .jar libraries into the new library format

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A bit about Jill

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How to setup • Instructions at : https://developer.android.com/ guide/platform/j8-jack.html

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Annotation processor • If you’re using Dagger, Butterknife etc. apply plugin: 'com.neenbedankt.android-apt'
 apt 'com.google.dagger:dagger-compiler:2.2'

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Annotation processor annotationProcessor ‘com.google.dagger:dagger-compiler:2.2’

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Annotation processor • Guide: https://bitbucket.org/hvisser/android-apt/ wiki/Migration

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Known issues • Jack does not generate intermediate class files when compiling an app, so it breaks some tools • Instant run doesn’t work with Jack • Lint that operates on class files • JaCoCo instrumentation (now fixed) • Possible Kotlin issues

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Known issues • Updated list: https://source.android.com/source/ known-issues.html • The one under Java 8 features doesn’t seem to be updated • File bugs: http://tools.android.com/filing-bugs

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Resources

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Links — Lambdas • Many lambda examples: docs.oracle.com/javase/ tutorial/java/javaOO/lambdaexpressions.html • Lambda tutorial: https://github.com/ AdoptOpenJDK/lambda-tutorial • Official docs on Streams: http://docs.oracle.com/ javase/tutorial/collections/streams/ • Streams tutorial: http://blog.hartveld.com/2013/03/ jdk-8-33-stream-api.html

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Other talks • Exploring Java’s Hidden Costs (@JakeWharton) • Jack and Jill build system (Eric Lafortune) • API Design With Java 8 Lambda and Streams (@stuartmarks & @BrianGoetz)

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Thank you http://bit.do/droidcon-java8
 https://github.com/anothem/ java8-android