Kotlin Extension Functions

Transcript

1. Extension Functions
   Christoph Leiter
   Kotlin Vienna, April 18th 2017
   

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2. Agenda
   1 Basics
   2 General Extension Functions
   3 Handy Extension Functions in stdlib
   4 Defining your own Extension Functions
   5 Summary
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3. Basics
   

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4. What are EF?
   EFs allow to add new functionality to a class without
   modifying it
   Better alternative to creating *Util classes
   They are statically resolved, there’s no patching of the original
   classes
   Works with final classes, e.g. String
   Kotlin also has extension properties which are similar
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5. Declaring an EF
   Extension function:
   fun SomeType. extensionFunction
   (x: String ): Int { ... }
   Inside the fun you can reference the instance with this
   Even works with nullable types (this will be null)
   Extension property:
   val SomeType.empty: Boolean
   get() = size == 0
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6. Interoperability
   You can call EF from Java as well but you need to call the
   static method directly
   If your EFs are in a file called foo.kt Kotlin will generate the
   class FooKt with static methods
   First argument is the receiver object (this)
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7. Interoperability
   You can call EF from Java as well but you need to call the
   static method directly
   If your EFs are in a file called foo.kt Kotlin will generate the
   class FooKt with static methods
   First argument is the receiver object (this)
   Example extensions.kt:
   fun Int.toBigInteger () =
   BigInteger.valueOf(this.toLong ())
   Java:
   ExtensionsKt.toBigInteger (10);
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8. General Extension Functions
   

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9. General Extension Functions
   There are several handy extension functions defined in stdlib
   (Standard.kt) which work on Any:
   run
   let
   apply
   also
   takeIf
   takeUnless
   to
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10. let
    Calls the specified function block with this value as its argument
    and returns its result.
    public inline fun T.let
    (block: (T) -> R): R = block(this)
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11. let
    Calls the specified function block with this value as its argument
    and returns its result.
    public inline fun T.let
    (block: (T) -> R): R = block(this)
    Example:
    class C {
    var d: D? = null
    fun f() {
    d?.let { it.g() }
    }
    }
    Use to capture variable if smart cast not possible due to mutability.
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12. run
    Calls the specified function block with this value as its receiver
    and returns its result.
    public inline fun T.run
    (block: T.() -> R): R = block ()
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13. run
    Calls the specified function block with this value as its receiver
    and returns its result.
    public inline fun T.run
    (block: T.() -> R): R = block ()
    Example:
    val x = y.run() {
    a = 1 // refers to y.a
    b = 2 // refers to y.b
    compute () // refers to y.compute ()
    }
    Use if you want to access multiple members of an instance without
    repeating yourself but you want to return something different.
    Might also be useful to execute multiple methods of an instance
    with a null check by using ?.run(..).
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14. apply
    Calls the specified function block with this value as its receiver
    and returns this value.
    public inline fun T.apply
    (block: T.() -> Unit ): T
    { block (); return this }
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15. apply
    Calls the specified function block with this value as its receiver
    and returns this value.
    public inline fun T.apply
    (block: T.() -> Unit ): T
    { block (); return this }
    Example:
    val bean = MyBean (). apply {
    a = 1
    b = 2
    c = true
    }
    Use to instantiate instances and set some values.
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16. also
    Calls the specified function block with this value as its argument
    and returns this value.
    public inline fun T.also
    (block: (T) -> Unit ): T
    { block(this ); return this }
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17. also
    Calls the specified function block with this value as its argument
    and returns this value.
    public inline fun T.also
    (block: (T) -> Unit ): T
    { block(this ); return this }
    Example:
    fun MyBean.copy () =
    MyBean (). also {
    it.a = a // it refers to new instance
    it.b = b // implicit this refers to outer
    }
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18. to
    Creates a tuple of type Pair from this and that
    public infix fun A.to(that: B):
    Pair = Pair(this , that)
    Useful for creating map entries with less verbosity:
    val m = mapOf("foo" to 23, "bar" to 42)
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19. takeIf
    Returns this value if it satisfies the given predicate or null, if it
    doesn’t.
    public inline fun T.takeIf
    (predicate: (T) -> Boolean ): T? =
    if (predicate(this )) this else null
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20. takeIf
    Returns this value if it satisfies the given predicate or null, if it
    doesn’t.
    public inline fun T.takeIf
    (predicate: (T) -> Boolean ): T? =
    if (predicate(this )) this else null
    Example:
    val x = foo.takeIf { it.p() } ?: return true
    Like filter for a single value.
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21. takeUnless
    Returns this value if it does not satisfy the given predicate or
    null, if it does.
    public inline fun T.takeUnless
    (predicate: (T) -> Boolean ): T? =
    if (! predicate(this )) this else null
    Same as a negated takeIf.
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22. Handy Extension Functions in
    stdlib
    

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23. Array
    flatten: Returns a single list of all elements from all arrays in
    the given array.
    fun Array >. flatten ():
    List
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24. Array
    flatten: Returns a single list of all elements from all arrays in
    the given array.
    fun Array >. flatten ():
    List
    unzip: Returns a pair of lists, where first list is built from the first
    values of each pair from this array, second list is built from the
    second values of each pair from this array.
    fun Array >. unzip ():
    Pair , List >
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25. Iterable
    flatten: Returns a single list of all elements from all collections
    in the given collection.
    fun Iterable >. flatten ():
    List
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26. Iterable
    flatten: Returns a single list of all elements from all collections
    in the given collection.
    fun Iterable >. flatten ():
    List
    unzip: Returns a pair of lists, where first list is built from the first
    values of each pair from this collection, second list is built from
    the second values of each pair from this collection.
    fun Iterable >. unzip ():
    Pair , List > {
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27. List
    orEmpty: Returns this List if it’s not null and the empty list
    otherwise.
    inline fun List ?. orEmpty ():
    List = this ?: emptyList ()
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28. List
    orEmpty: Returns this List if it’s not null and the empty list
    otherwise.
    inline fun List ?. orEmpty ():
    List = this ?: emptyList ()
    binarySearch: Searches this list or its range for the provided
    element using the binary search algorithm.
    fun > List . binarySearch (
    element: T?,
    fromIndex: Int = 0,
    toIndex: Int = size
    ): Int
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29. Map
    getOrElse: Returns the value for the given key, or the result of
    the defaultValue function if there was no entry for the given key.
    inline fun Map . getOrElse(
    key: K,
    defaultValue: () -> V
    ): V = get(key) ?: defaultValue ()
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30. Map
    getOrElse: Returns the value for the given key, or the result of
    the defaultValue function if there was no entry for the given key.
    inline fun Map . getOrElse(
    key: K,
    defaultValue: () -> V
    ): V = get(key) ?: defaultValue ()
    Alternative to using ?:
    val v = map.getOrElse(k) { myDefaultValue }
    val w = map[k] ?: myDefaultValue
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31. Map II
    getOrPut: Returns the value for the given key. If the key is not
    found in the map, calls the defaultValue function, puts its result
    into the map under the given key and returns it.
    inline fun
    MutableMap . getOrPut(
    key: K,
    defaultValue: () -> V
    ): V
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32. Map II
    getOrPut: Returns the value for the given key. If the key is not
    found in the map, calls the defaultValue function, puts its result
    into the map under the given key and returns it.
    inline fun
    MutableMap . getOrPut(
    key: K,
    defaultValue: () -> V
    ): V
    Easy way to get a value out of a cache and if it doesn’t exist
    compute it and put it in the cache
    val v = cache.getOrPut(key) {
    computeValue(key)
    }
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33. Map III
    Methods to filter Maps based on Entrys, keys or values:
    inline fun Map . filter(
    predicate: (Map.Entry ) -> Boolean
    ): Map
    inline fun Map . filterKeys(
    predicate: (K) -> Boolean
    ): Map {
    inline fun Map . filterValues (
    predicate: (V) -> Boolean
    ): Map
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34. Comparator
    thenBy: Creates a comparator comparing values after the primary
    comparator defined them equal. It uses the function to transform
    value to a Comparable instance for comparison.
    inline fun Comparator . thenBy(
    crossinline selector: (T) -> Comparable <*>?
    ): Comparator {
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35. Comparator
    thenBy: Creates a comparator comparing values after the primary
    comparator defined them equal. It uses the function to transform
    value to a Comparable instance for comparison.
    inline fun Comparator . thenBy(
    crossinline selector: (T) -> Comparable <*>?
    ): Comparator {
    Allows chaining of multiple comparators
    list.sortedWith(
    compareBy
    { it.shoeSize }. thenBy { it.age }
    )
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36. Lock
    withLock: Executes the given action under this lock and returns
    its value.
    inline fun Lock.withLock(
    action: () -> T
    ): T
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37. Lock
    withLock: Executes the given action under this lock and returns
    its value.
    inline fun Lock.withLock(
    action: () -> T
    ): T
    Allows easier lock usage:
    lock.withLock () {
    doSomething ()
    }
    Also has read and write EFs for ReentrantReadWriteLock
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38. Closeable
    use: Executes the given block function on this resource and then
    closes it down correctly whether an exception is thrown or not.
    inline fun T.use(
    block: (T) -> R
    ): R
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39. Closeable
    use: Executes the given block function on this resource and then
    closes it down correctly whether an exception is thrown or not.
    inline fun T.use(
    block: (T) -> R
    ): R
    Works like try-with-resources in Java but is not a language
    feature
    stream.use {
    readFrom(it)
    }
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40. Defining your own Extension
    Functions
    

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41. Use cases
    There are many good use cases to use EFs in your code:
    Add methods to classes which you don’t control
    Add methods to classes depending on the context
    toDTO in REST layer
    toJPA in storage layer
    Make 3rd party libraries more Kotlin like
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42. DTO Converter
    class UsersController (
    private val userRepository : UserRepository
    ) {
    fun getUser(id: Long) =
    userRepository .find(id)?. toDTO ()
    }
    private fun User.toDTO (): UserDTO =
    UserDTO (). also {
    it.name = "$firstname $lastname"
    it.age = calculateAge (birthday)
    }
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43. SQL like IN
    fun Any?. isIn(vararg objs: Any ?): Boolean =
    objs.any { it == this }
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44. SQL like IN
    fun Any?. isIn(vararg objs: Any ?): Boolean =
    objs.any { it == this }
    Allows compact syntax for comparing a value to multiple
    possibilities. Avoid repetition of the value if it’s a complex
    expression.
    if (foo.bar (). buz (). isIn (23, 42, 404)) {
    doSomething ()
    }
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45. MonetaryAmount
    JSR-354 defines a new MonetaryAmount. We can extend it to feel
    more like a native type with operator EFs:
    operator fun MonetaryAmount .plus(
    other: MonetaryAmount
    ): MonetaryAmount = this.add(other)
    operator fun MonetaryAmount .times(
    other: Number
    ): MonetaryAmount = this.multiply(other)
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46. MonetaryAmount
    JSR-354 defines a new MonetaryAmount. We can extend it to feel
    more like a native type with operator EFs:
    operator fun MonetaryAmount .plus(
    other: MonetaryAmount
    ): MonetaryAmount = this.add(other)
    operator fun MonetaryAmount .times(
    other: Number
    ): MonetaryAmount = this.multiply(other)
    Allows use of arithmetic symbols:
    val a = Money.of(30, "EUR")
    val b = Money.of(50, "EUR")
    val c = a + b * 2
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47. MonetaryAmount II
    You could even define something like
    val Number.EUR: MonetaryAmount
    get() = Money.of(this , "EUR")
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48. MonetaryAmount II
    You could even define something like
    val Number.EUR: MonetaryAmount
    get() = Money.of(this , "EUR")
    Then you could write
    assertThat(calculation ()). isEqualTo (30. EUR)
    I wouldn’t recommend this for production code but for unit tests it
    might be handy.
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49. Formatting for Remote Systems
    When communicating with remote systems it’s handy to have
    functions for data types to convert from/to remote types.
    fun LocalDate.format () =
    format( DateTimeFormatter .ISO_DATE)
    fun MonetaryAmount .convert (): BigDecimal =
    number.numberValue(BigDecimal :: class.java)
    .multiply(BigDecimal.valueOf (100))
    fun XMLGregorianCalendar .toLocalDate () =
    toGregorianCalendar ()
    . toZonedDateTime ()
    .toLocalDate ()
    Make sure to mark them private or internal to avoid polluting
    the namespace.
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50. Redisson Lock
    inline fun RedissonClient .lock(
    lockName: String ,
    waitTime: Duration ,
    leaseTime: Duration ,
    action: (RLock) -> T
    ): LockResult {
    val lock = getLock(lockName)
    val success = lock.tryLock(
    waitTime.toMillis (),
    leaseTime.toMillis (),
    TimeUnit.MILLISECONDS)
    // check - try action - finally unlock
    }
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51. Redisson Lock II
    Easy and consistent usage of locks, no need to repeat the
    try-finally block all the time
    Looks more like a language feature
    Allows usage of Duration instead of milliseconds as Long
    You can set default values for waitTime and leaseTime
    redisson.lock("my -lock") {
    doSomethingInLock ()
    }
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52. EFs for Lambdas
    You can even define EFs on lambdas:
    fun (() -> Any). time (): Long {
    val start = System. currentTimeMillis ()
    this ()
    return System. currentTimeMillis () - start
    }
    val t = { computeSomething () }. time ()
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53. EFs for Lambdas
    You can even define EFs on lambdas:
    fun (() -> Any). time (): Long {
    val start = System. currentTimeMillis ()
    this ()
    return System. currentTimeMillis () - start
    }
    val t = { computeSomething () }. time ()
    Haven’t found a good use case for that but it’s good to know
    nonetheless. :)
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54. Summary
    

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55. Summary
    EFs are syntactic sugar but really helpful
    Help to make your code more concise
    Can be used to “extend” the language (e.g. use is a language
    feature in Java)
    Even though you can define the same EFs multiple times it’s
    best not to pollute the namespace for common types
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56. End
    Thanks for your attention
    Questions?
    Kotlin Vienna Meetup
    https://www.meetup.com/Kotlin-Vienna/
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