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Kevin Most Idiomatic Interop

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Doesn't Kotlin already have 100% interop? • Yes, but the interop can either be pleasant or clumsy • And some features from Kotlin won't work in Java

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Who should be thinking about this? • Java library developers • Kotlin library developers • Anyone working in a mixed codebase

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Your interop story should be • Safe • Performant • Readable • Discoverable

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@JvmHappiness

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@Jvm Annotations • Annotations that tell the Kotlin compiler how to expose code to Java

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@JvmOverloads fun Date.format( formatString: String, locale: Locale = defaultLocale() ): String format(date, "yyyyMMdd");

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@JvmOverloads fun Date.format( formatString: String, locale: Locale = defaultLocale() ): String format( date, "yyyyMMdd", defaultLocale()) );;

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@JvmOverloads @JvmOverloads fun Date.format( formatString: String, locale: Locale = defaultLocale() ): String format( date, "yyyyMMdd", defaultLocale()) );;

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@JvmOverloads @JvmOverloads fun Date.format( formatString: String, locale: Locale = defaultLocale() ): String format(date, "yyyyMMdd"); defaultLocale()

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Don't get too carried away data class User @JvmOverloads constructor( val id: String? = null, val name: String? = null, val username: String? = null, val gender: String? = null, val points: Int = 0 )

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"Consider a builder when faced with many constructor parameters" - Joshua Bloch

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@JvmStatic • Only for use in companion and named objects • On fun and val: • Generates a static method in the bytecode that delegates through to that function/property • On var: • Also generates a static setter that delegates through

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@JvmStatic User.Companion.create(); class User {A companion object {B fun create(): User }C }D

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@JvmStatic class User {A companion object {B @JvmStatic fun create(): User }C }D User.Companion.create();

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@JvmStatic User.create(); class User {A companion object {B @JvmStatic fun create(): User }C }D

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A less nested way? User.create(); class User {A companion object {B @JvmStatic fun create(): User }C }D

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A less nested way? User.create(); fun create(): User class User {A }D

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A less nested way? User.create(); fun create(): User class User {A }D

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A less nested way? User.create(); fun create(): User class User {A }D

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A less nested way? User.create(); @file:JvmName("User") fun create(): User class User {A }D

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@Throws

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• For Kotlin functions, property getters/setters, constructors • Adds throws FooException to generated method header @Throws

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@Throws interface Repository { /** * @throws IOException if can't save */ fun save(obj: T) }D

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@Throws interface Repository { /** * @throws IOException if can't save */ fun save(obj: T) }D

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@Throws interface Repository { /** * @throws IOException if can't save */ fun save(obj: T) }D class UserRepository implements Repository { @Override public void save(User obj) { throw new IOException(""); } }

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@Throws interface Repository { /** * @throws IOException if can't save */ fun save(obj: T) }D class UserRepository implements Repository {A @OverrideB public void save(User obj) {C throw new IOException("");D }E }F

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@Throws interface Repository { /** * @throws IOException if can't save */ fun save(obj: T) }D class UserRepository implements Repository {A @OverrideB public void save(User obj) throws IOException {C throw new IOException("");D }E }F

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@Throws interface Repository { /** * @throws IOException if can't save */ fun save(obj: T) }D class UserRepository implements Repository {A @OverrideB public void save(User obj) throws IOException {C throw new IOException("");D }E }F

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@Throws interface Repository { /** * @throws IOException if can't save */ fun save(obj: T) }D class UserRepository implements Repository {A @OverrideB public void save(User obj) throws IOException {C throw new IOException("");D }E }F

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@Throws interface Repository { /** * @throws IOException if can't save */ @Throws(IOException::class) fun save(obj: T) }D class UserRepository implements Repository {A @OverrideB public void save(User obj) throws IOException {C throw new IOException("");D }E }F

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Extension functions

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Extension functions • You have a huge Java codebase with many Utils classes • You add Kotlin • You still have all these Utils • Awesome new Kotlin code has to call into old Java utils

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Extension functions • Can we convert our Utils classes to Kotlin extensions • While preserving their signatures in Java so existing call-sites can stay as they are?

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Extension functions class UserUtils { static boolean hasName(User user) {} static String getDisplayableName(User user) {} static boolean isAnonymous(User user) {} static boolean isFriendsWith(User subject, User other) {} } UserUtils.isAnonymous(aUser); UserUtils.hasName(aUser); UserUtils.isAnonymous(aUser) UserUtils.hasName(aUser)

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Extension functions val User.hasName: Boolean get() {} val User.displayableName: String get() {} val User.isAnonymous: Boolean get() {} fun User.isFriendsWith(other: User): Boolean {} UserUtils.isAnonymous(aUser); UserUtils.hasName(aUser); UserUtils.isAnonymous(aUser) UserUtils.hasName(aUser)

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Extension functions val User.hasName: Boolean get() {} val User.displayableName: String get() {} val User.isAnonymous: Boolean get() {} fun User.isFriendsWith(other: User): Boolean {} UserUtils.isAnonymous(aUser); UserUtils.hasName(aUser); UserUtils.isAnonymous(aUser) UserUtils.hasName(aUser)

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Extension functions val User.hasName: Boolean get() {} val User.displayableName: String get() {} val User.isAnonymous: Boolean get() {} fun User.isFriendsWith(other: User): Boolean {} UserUtils.isAnonymous(aUser); UserUtils.hasName(aUser); aUser.isAnonymous aUser.hasName

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Extension functions UserUtils.isAnonymous(aUser); UserUtils.hasName(aUser); aUser.isAnonymous aUser.hasName @file:JvmName("UserUtils") // default is UserUtilsKt val User.hasName: Boolean get() {} val User.displayableName: String get() {} val User.isAnonymous: Boolean get() {} fun User.isFriendsWith(other: User): Boolean {}

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Extension functions UserUtils.isAnonymous(aUser); UserUtils.hasName(aUser); aUser.isAnonymous aUser.hasName @file:JvmName("UserUtils") // default is UserUtilsKt val User.hasName: Boolean get() {} val User.displayableName: String get() {} val User.isAnonymous: Boolean get() {} fun User.isFriendsWith(other: User): Boolean {}

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Extension functions UserUtils.isAnonymous(aUser); UserUtils.hasName(aUser); aUser.isAnonymous aUser.hasName @file:JvmName("UserUtils") // default is UserUtilsKt val User.hasName: Boolean @JvmName("hasName") get() {} val User.displayableName: String get() {} val User.isAnonymous: Boolean get() {} fun User.isFriendsWith(other: User): Boolean {}

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Extension functions UserUtils.isAnonymous(aUser); UserUtils.hasName(aUser); aUser.isAnonymous aUser.hasName @file:JvmName("UserUtils") // default is UserUtilsKt val User.hasName: Boolean @JvmName("hasName") get() {} val User.displayableName: String get() {} val User.isAnonymous: Boolean get() {} fun User.isFriendsWith(other: User): Boolean {}

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Inline functions

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Inline functions • Java compiler doesn't support inlining • Can still use inline functions, but they won't be inlined • Be mindful of performance • Cannot call inline functions with reified generics from Java

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Reified generics workaround inline fun View.firstViewOfType(): T? {}

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Reified generics workaround inline fun View.firstViewOfType(): T? = firstViewOfType(T::class.java) fun View.firstViewOfType(type: Class): T? {}

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Visibility

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Visibility • public, protected, private behave as expected • Java package-local has no equivalent in Kotlin • Kotlin internal has no equivalent in Java

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internal • Kotlin module-level visibility • Module: IDEA, Maven, Gradle, or Ant compilation unit • So external Java shouldn't be able to see it, right? • Unfortunately, internal -> public in bytecode

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Package-local • Java default modifier • Only visible within the same package • Kotlin doesn't (currently?) have a way to restrict members to the same package

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private • Java classes can access an inner class' private member • Kotlin classes cannot

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!

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! • The dreaded platform type • Blows up when dereferenced • Most calls into Java will return a platform type • You should try to eliminate most/all of these in your own code • Solution: Nullability Annotations

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Nullability Annotations! interface Request {A Response execute(); }B val request: Request = getUser(id) request.execute().value.displayableName interface Response {C T getValue(); }D NPE

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Nullability Annotations! interface Request {A Response execute(); }B val request: Request = getUser(id) request.execute().value.displayableName interface Response {C @Nullable T getValue(); }D

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Nullability Annotations! kmost@kmost: ~/work/foursquare-android dev $ rg "@NonNull" --count --no-filename | paste -d+ -s - | bc 759 kmost@kmost: ~/work/foursquare-android dev $ rg "@Nullable" --count --no-filename | paste -d+ -s - | bc 475 • Annotating everything is super-tedious

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Nullability Annotations! static List getUsers(); getUsers() // (Mutable)List!

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Nullability Annotations! @NonNull static List getUsers(); getUsers() // (Mutable)List!

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Nullability Annotations! @NonNull static List getUsers(); getUsers() // (Mutable)List

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Nullability Annotations! @NonNull static List<@NonNull String> getUsers(); getUsers() // (Mutable)List

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Nullability Annotations! @NonNull static List<@NonNull String> getUsers(); getUsers() // (Mutable)List

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Nullability Annotations! interface Request {A Response execute(); }B val request: Request = getUser(id) request.execute().value.displayableName interface Response {C @Nullable T getValue(); }D

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Nullability Annotations! interface Request {A Response< A @Nullable T> execute(); }B val request: Request = getUser(id) request.execute().value.displayableName interface Response {C T getValue(); }D

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Nullability Annotations! interface Request {A Response execute(); }B val request: Request = getUser(id) request.execute().value.displayableName interface Response<@Nullable T> {C T getValue(); }D

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Nullability Annotations! interface Request {A Response execute(); }B val request: Request = getUser(id) request.execute().value.displayableName interface Response { @Nullable T getValue(); }D

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Nullability Annotations! interface Request {A Response execute(); }B val request: Request = getUser(id) request.execute().value.displayableName interface Response { @Nullable T getValue(); @Nullable Error getError(); @Nullable HttpResponse getRawResponse(); }D

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Nullability Annotations! interface Request {A Response execute(); }B val request: Request = getUser(id) request.execute().value.displayableName interface Response { @Nullable T getValue(); @Nullable Error<@Nullable T> getError(); @Nullable HttpResponse getRawResponse(); }D

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Nullability Annotations

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Default Nullability Annotations • Added in Kotlin 1.1.50 • Specify default annotations: • Per package • Per class • Per method

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Default Nullability Annotations dependencies { compile "com.google.code.findbugs:jsr305:3.0.2" } compileKotlin.kotlinOptions.freeCompilerArgs = [ "-Xjsr305-annotations=enable" ]

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Default Nullability Annotations • JSR-305 comes with: • @ParametersAreNonnullByDefault • @ParametersAreNullableByDefault • Annotate a package to make all parameters for all functions non-null or nullable by default

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Annotating a package package-info.java @ParametersAreNonnullByDefault package com.example.kotlinconf;

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DIY Default Nullability Annotations • You're not limited to @ParametersAreNonnullByDefault • You can make your own nullability annotations • Let's look at the source for @ParametersAreNonnullByDefault

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ParametersAreNonnullByDefault.java @Nonnull @TypeQualifierDefault(ElementType.PARAMETER) public @interface ParametersAreNonnullByDefault {} DIY Default Nullability Annotations

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ParametersAreNonnullByDefault.java @Nonnull @TypeQualifierDefault(ElementType.PARAMETER) public @interface ParametersAreNonnullByDefault {} DIY Default Nullability Annotations

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FieldsAreNonnullByDefault.java @Nonnull @TypeQualifierDefault(ElementType.FIELD) public @interface FieldsAreNonnullByDefault {} DIY Default Nullability Annotations

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FieldsAreNonnullByDefault.java @Nonnull @TypeQualifierDefault(ElementType.FIELD) public @interface FieldsAreNonnullByDefault {} DIY Default Nullability Annotations

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FieldsAreNullableByDefault.java @Nullable @TypeQualifierDefault(ElementType.FIELD) public @interface FieldsAreNullableByDefault {} DIY Default Nullability Annotations

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FieldsAreNullableByDefault.java @Nullable @TypeQualifierDefault(ElementType.FIELD) public @interface FieldsAreNullableByDefault {} // not quite DIY Default Nullability Annotations

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FieldsAreNullableByDefault.java @CheckForNull @TypeQualifierDefault(ElementType.FIELD) public @interface FieldsAreNullableByDefault {} // not quite DIY Default Nullability Annotations

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Living the dream package-info.java @ParametersAreNonnullByDefault @FieldsAreNullableByDefault @MethodsReturnNullableByDefault package com.example.kotlinconf;

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Nulls in libraries • These solutions only work if you control the code in question • How do you deal with Java libs that have null everywhere? • ex: Android

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Nulls in libraries • Ask your library maintainers

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Nulls in libraries • Submit your own PR • Annotations are easy and low-risk • Even if you "know" the nullability of members, letting the compiler enforce it for you is better

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Lambdas and SAMs

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Lambdas and SAMs • Kotlin lambdas compile to a functional interface in Java • () -> R becomes Function0 • (T) -> R becomes Function1 • Java SAMs compile to a special syntax in Kotlin • SAMName { ... }

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SAMs • Unfortunately, Kotlin SAMs currently don't offer SAM syntax in Kotlin • Right now, it's best to keep your SAM types in Java

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SAMs public interface JavaSAM {
 void onClick(View view);
 } val sam = JavaSAM { view -> ... }

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SAMs interface KotlinSAM {
 fun onClick(view: View)
 }B val sam = JavaSAM { view -> ... }

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SAMs interface KotlinSAM {
 fun onClick(view: View)
 }B val sam = KotlinSAM { view -> ... }

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SAMs interface KotlinSAM {
 fun onClick(view: View)
 }B val sam = KotlinSAM { view -> ... }

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SAMs interface KotlinSAM {
 fun onClick(view: View)
 }B val sam = object : KotlinSAM {
 override fun onClick(view: View) { ...
 }
 }

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"The other two [collection literals and SAM conversions] seem tractable in the foreseeable future"

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Lambda signatures • Lambdas with receivers exported with receiver as 1st param • Nullability of types is lost in Java! • (Foo) -> Unit is equivalent to Foo?.() -> Unit from Java's perspective

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Special Types

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Special Types • Most types are mapped between Java and Kotlin • There are exceptions: • Unit • Nothing

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Unit • Unit can be mapped to void in most cases in Java • It cannot, however, if Unit is the selected type for a generic • Ex: Lambdas. Java signature FunctionN • Java has to: return Unit.INSTANCE;

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Nothing • Nothing is the subtype of all other types • No instances exist, not even null • So a Nothing function can never return; must throw/loop • No type exists like this in Java

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Nothing • Generics of Nothing become raw types • List in Kotlin becomes List in Java • Actual Nothings become Void • Consumers just have to know the method will never return

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Wildcards

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Wildcards • In Java, all use-sites of a generic need to say if they are: • Covariant: ? extends Foo • Contravariant: ? super Foo • In Kotlin, you just put that declaration on the type param itself: • Covariant: out T • Contravariant: in T

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Wildcards class Box { T foo; }B Box box(T unboxed) { return new Box<>(unboxed); }D T unbox(Box box) { return box.foo; }F Box boxedInt = box(3); List> boxes = new ArrayList<>(); boxes.add(boxedInt);

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Wildcards class Box { T foo; }B Box box(T unboxed) { return new Box<>(unboxed); }D T unbox(Box box) { return box.foo; }F Box boxedInt = box(3); List> boxes = new ArrayList<>(); boxes.add(boxedInt);

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Wildcards class Box { T foo; }B Box box(T unboxed) { return new Box<>(unboxed); }D T unbox(Box box) { return box.foo; }F Box boxedInt = box(3); List> boxes = new ArrayList<>(); boxes.add(boxedInt);

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Wildcards class Box { T foo; }B Box box(T unboxed) { return new Box<>(unboxed); }D T unbox(Box box) { return box.foo; }F Box boxedInt = box(3); List> boxes = new ArrayList<>(); boxes.add(boxedInt);

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Wildcards class Box { T foo; }B Box box(T unboxed) { return new Box<>(unboxed); }D T unbox(Box box) { return box.foo; }F Box boxedInt = box(3); List> boxes; boxes.add(boxedInt);

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Wildcards class Box { T foo; }B Box box(T unboxed) { return new Box<>(unboxed); }D T unbox(Box box) { return box.foo; }F Box boxedInt = box(3); List> boxes; boxes.add(boxedInt); class Box(val foo: T) fun box(unboxed: T) = Box(unboxed) fun unbox(box: Box) = box.foo val boxedInt: Box = box(3) val boxed = mutableListOf>() boxed.add(boxedInt)

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Wildcards class Box { T foo; }B Box box(T unboxed) { return new Box<>(unboxed); }D T unbox(Box box) { return box.foo; }F Box boxedInt = box(3); List> boxes; boxes.add(boxedInt); class Box(val foo: T) fun box(unboxed: T) = Box(unboxed) fun unbox(box: Box) = box.foo val boxedInt: Box = box(3) val boxed = mutableListOf>() boxed.add(boxedInt)

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Wildcards class Box { T foo; }B Box box(T unboxed) { return new Box<>(unboxed); }D T unbox(Box box) { return box.foo; }F Box boxedInt = box(3); List> boxes; boxes.add(boxedInt); class Box(val foo: T) fun box(unboxed: T) = Box(unboxed) fun unbox(box: Box) = box.foo val boxedInt: Box = box(3) val boxed = mutableListOf>() boxed.add(boxedInt)

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Wildcards • So out is roughly equivalent to extends • And in is roughly equivalent to super • Kotlin "fakes" declaration-site variance for Java by generating wildcards for all variant generics in parameters • Return types remain invariant • Final covariant types remain invariant

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Wildcards • To override the default generic behavior: • @JvmWildcard if you want variance where there is none • @JvmSuppressWildcards if you don't want variance

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Data classes

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Data classes • Tuple-like classes; properties declared in constructor • Auto-generation of hashCode(), equals(), toString() • These work perfectly in Java • Auto-generation of copy(...) • This works okay in Java • Lack of default + named params makes it clunky

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Data classes data class User( val id: String? = null, val name: String? = null, val username: String? = null, val gender: String? = null, val points: Int = 0 ) u.copy(u.getId(), u.getName(), u.getUsername(), u.getGender(), u.getPoints() + 1);

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Data classes data class User( val id: String? = null, val name: String? = null, val username: String? = null, val gender: String? = null, val points: Int = 0 )

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data class User( val id: String? = null, val name: String? = null, val username: String? = null, val gender: String? = null, val points: Int = 0 ) { fun toBuilder() = Builder(this) class Builder(private var user: User = User()) { fun id(id: String?) = apply { user = user.copy(id = id) } fun name(name: String?) = apply { user = user.copy(name = name) // ... fun build() = user } } Data classes

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"Consider a builder when faced with many constructor parameters" - Joshua Bloch

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data class User( val id: String? = null, val name: String? = null, val username: String? = null, val gender: String? = null, val points: Int = 0 ) { fun toBuilder() = Builder(this) class Builder(private var user: User = User()) { fun id(id: String?) = apply { user = user.copy(id = id) } fun name(name: String?) = apply { user = user.copy(name = name) // ... fun build() = user } } Data classes

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data class User( val id: String? = null, val name: String? = null, val username: String? = null, val gender: String? = null, val points: Int = 0 ) { fun toBuilder() = Builder(this) class Builder(private var user: User = User()) { fun id(id: String?) = apply { user = user.copy(id = id) } fun name(name: String?) = apply { user = user.copy(name = name) // ... fun build() = user } } Data classes

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data class User( val id: String? = null, val name: String? = null, val username: String? = null, val gender: String? = null, val points: Int = 0 ) { fun toBuilder() = Builder(this) class Builder(private var user: User = User()) { fun id(id: String?) = apply { user = user.copy(id = id) } fun name(name: String?) = apply { user = user.copy(name = name) // ... fun build() = user } } Data classes

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data class User( val id: String? = null, val name: String? = null, val username: String? = null, val gender: String? = null, val points: Int = 0 ) { fun toBuilder() = Builder(this) class Builder(private var user: User = User()) { fun id(id: String?) = apply { user = user.copy(id = id) } fun name(name: String?) = apply { user = user.copy(name = name) // ... fun build() = user } } Data classes

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Overall Interop Thoughts

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Overall Interop Thoughts • kt ❤ java • Most of the time, interop Just Works™ • But when writing non-private members, say to yourself: • When writing Kotlin: "How will this look in Java?" • When writing Java: "How will this look in Kotlin?"

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Quick Tip

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Quick Tip • Write (at least some) tests in the other language • If you use Java, write some Kotlin tests • If you write Kotlin, write some Java tests • Gives you insight into the ergonomics of your public API

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Resources • Calling Kotlin from Java: https://kotlinlang.org/docs/ reference/java-to-kotlin-interop.html • Calling Java from Kotlin: https://kotlinlang.org/docs/ reference/java-interop.html

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#kotlinconf17 Kevin Most Thank you!