I have a lot of enum class in my Kotlin application. They all represent bit flags that I need to convert between EnumSet<> and UInt for serial communication with an FPGA board.
How can I provide some extension methods that apply to all those Enums?
Enums don't allow class inheritance. EnumSet can't reference an interface. I'm not sure what else to try.
Thanks!
enum class ReadFlag(val bits: UInt) {
DATA_READY(1u)
BUSY(2u)
}
typealias ReadFlags = EnumSet<ReadFlag>
enum class WriteFlag(val bits: UInt) {
UART_RESET(1u)
BUSY(2u)
PAGE_DATA(4u)
}
typealias WriteFlags = EnumSet<WriteFlag>
fun ReadFlags.asUInt(): UInt =
this.fold(0u) { acc, next -> acc or next.bits }
fun WriteFlags.asUInt(): UInt =
this.fold(0u) { acc, next -> acc or next.bits }
This results in this error:
error: platform declaration clash: The following declarations have the same JVM signature (asUInt(Ljava/util/EnumSet;)I):
Write an interface for the common members:
interface Flags {
val bits: UInt
}
Implement the interface:
enum class ReadFlag(override val bits: UInt): Flags {
DATA_READY(1u),
BUSY(2u)
}
enum class WriteFlag(override val bits: UInt): Flags {
UART_RESET(1u),
BUSY(2u),
PAGE_DATA(4u)
}
Then you can make your asUInt generic:
fun <E> EnumSet<E>.asUInt(): UInt where E : Enum<E>, E: Flags =
this.fold(0u) { acc, next -> acc or next.bits }
Related
I have an enum in the main repo:
enum class PilotType {
REMOVABLE,
FIXED
}
And I have another enum in another repo that is imported:
enum class PilotTypeDto {
REMOVABLE,
FIXED
}
In a class in my main repo I need to build this object:
(pilotType is of type PilotType)
(pilotTypeDto is of type PilotTypeDto)
return Pilot(
... = ...
pilotType = pilotTypeDto
... = ...
)
I need to convert pilotTypeDto to a pilotType.
I started building an extension function but it does not seem to let me create an enum:
fun pilotType(pilotTypeDto: PilotTypeDto): PilotType {
return PilotType(
... // this does not work
)
}
You can write this:
fun pilotType(pilotTypeDto: PilotTypeDto): PilotType =
when (pilotTypeDto) {
PilotTypeDto.REMOVABLE -> PilotType.REMOVABLE
PilotTypeDto.FIXED -> PilotType.FIXED
}
But as extension you could write this:
fun PilotTypeDto.toPilotType() = when (this) {
PilotTypeDto.REMOVABLE -> PilotType.REMOVABLE
PilotTypeDto.FIXED -> PilotType.FIXED
}
or make it part of the enum by writing this
enum class PilotTypeDto {
REMOVABLE,
FIXED;
fun toPilotType() = when (this) {
REMOVABLE -> PilotType.REMOVABLE
FIXED -> PilotType.FIXED
}
}
I'm trying to write serialization functions to be able to serialize any vector (=ArrayList) in Kotlin, as well as primitive types and classes extending a Serialize class having a toBinary() function.
I also have a custom WriteDataStream class (code below) to serialize fields with the right format, endianness, etc.
I'm new to Kotlin but have experience in C++. In C++, I used templates and template specialization to solve that problem easily, but with Kotlin I've been struggling for a few days, without success.
I have a custom vector class MyVector which extends ArrayList and adds a maximum size. I want to serialize it with any generic type T, including inner vectors like a MyVector<MyVector<MyClass>>.
My WriteDataStream contains the following:
inline fun <reified T> write(vector: MyVector<T>) {
this.writeSize(vector.size.toULong(), vector.MAX_SIZE)
for (element in vector) {
write<T>(element)
}
}
inline fun <reified T: Serialize> write(value: T) {
writeSerialize(value as Serialize)
}
inline fun <reified T> write(value: T) {
when (T::class) {
UByte::class -> {
writeUInt8(value as UByte)
}
UShort::class -> {
writeUInt16(value as UShort)
}
UInt::class -> {
writeUInt32(value as UInt)
}
ULong::class -> {
writeUInt64(value as ULong)
}
Byte::class -> {
writeInt8(value as Byte)
}
Short::class -> {
writeInt16(value as Short)
}
Int::class -> {
writeInt32(value as Int)
}
Long::class -> {
writeInt64(value as Long)
}
Boolean::class -> {
writeBoolean(value as Boolean)
}
Float::class -> {
writeFloat(value as Float)
}
Double::class -> {
writeDouble(value as Double)
}
else -> {
error("Default serialization:" + T::class.qualifiedName)
}
}
}
All the underlying functions writeXXX() are tested and work fine. However, when tying to serialize a MyVector with a class extending Serialize, I fall in the "Default serialization" case:
#Test
fun writeVectorOfStructure() {
class TestStructure: Serialize() {
override fun toBinary(stream: WriteDataStream) {
stream.writeUInt32(17U)
stream.writeUInt8(3U)
stream.writeDouble(555.555)
}
}
val value = MyVector<TestStructure>(MAX_SIZE, arrayListOf(TestStructure(), TestStructure()))
writeStream.write(value)
val bytes: UByteArray = writeStream.byteArray()
Assert.assertEquals(bytes.size, 28) // = 2 (for size) + 2*(4+1+8) = 28 bytes
}
So my question is: Why does Kotlin not use the function
inline fun <reified T: Serialize> write(value: T)
when it serializes an element of the vector (write<T>(element)) with generic T = Serialize, but instead uses the more generic one?
inline fun <reified T> write(value: T)
In C++, the compiler always uses the most fitted function.
Is there a way to overcome this limitation in Kotlin?
I have tried with and without reified types, I have tried a non-generic function as well: inline fun write(value: Serialize), but without success. The only thing that seems to work was to add a case for classes "instance of" Serialize in the fully-generic inline fun <reified T> write(value: T), but this is not really a nice solution.
Thanks you !
JVM and its bad implementation of generics
You are a victim of Java's implementation of generics, more specifically the erasure. C++ uses what is called type expansion to implement generics, meaning if you declare MyType<A> and MyType<B>, at runtime you will have two different types, language runtime will create them for you.
On the other hand what Java does is called the erasure implementation. so in java world when you say List<String> and List<Integer>, at runtime they are both identical types, that is system doesn't have any information to make a distinction between both of these, they are List type (Note that there is no type parameter, it got removed during the compilation).
Lets decompile your code and see for yourself
I wrote following code in kotlin, it matches yours
class SomeType {
inline fun <reified T: String> write(value: T) {}
inline fun <reified T> write(value: T) {}
inline fun <reified T: Any> write(vector: List<T>) {
for (element in vector) {
write(element)
}
}
}
And when I decompile the code it gives me following. (Only relevant code included)
public final class SomeType {
public final void write(#NotNull String value) {}
public final void write(Object value) {}
public final void write(#NotNull List vector) {
boolean var6;
for(Iterator var4 = vector.iterator(); var4.hasNext(); var6 = false) {
Object element = var4.next();
}
}
}
Look at the write(vector: List<T>) method's decompilation. parameter type got changed to List which is a Raw Type and its components are objects.
And for an Object best method match is public final void write(Object value) and not the one with String or in your case Serialize.
Class Aim: read review point and comment, ensure that point is within 0-5
class LimitedReview(val point:Int, val comment:String):Review {
if (point<0){
point=0
}
if (point>5){
point = 5
}
override fun stars(): Int =point
override fun info(): String =comment
}
interface Review{
fun stars():Int
fun info():String
}
Error:(2, 5) Kotlin: Expecting member declaration
Error:(2, 17) Kotlin: Conflicting overloads: public final fun (): Unit defined in LimitedReview, public final fun (): Unit defined in LimitedReview
Error:(2, 17) Kotlin: Function declaration must have a name
may i know how to change my code?
which topic should i learn to avoid the same error again?
Thanks!
The property you have in the primary constructor of the class will assign the value that you call the constructor with to the point property directly, when your class is created, and you can't modify it any more.
Basically, this code:
class LimitedReview(val point: Int)
Is the same as this:
class LimitedReview(point: Int) {
val point: Int = point // ctor param to property
}
If you want to perform logic before assigning the value to the property, you have to move the property outside the constructor, and initialize it manually.
This can be done in an initializer block, if you have complex logic for it:
class LimitedReview(point: Int) {
val point: Int
init {
if (point < 0) {
this.point = 0
} else if (point > 5) {
this.point = 5
} else {
this.point = point
}
}
}
Or if you can fit it into a single expression (coerceIn comes in handy here), then inline with the property declaration:
class LimitedReview(point: Int) {
val point: Int = point.coerceIn(0..5)
}
How to implement an abstract class in Go? As Go doesn't allow us to have fields in interfaces, that would be a stateless object. So, in other words, is it possible to have some kind of default implementation for a method in Go?
Consider an example:
type Daemon interface {
start(time.Duration)
doWork()
}
func (daemon *Daemon) start(duration time.Duration) {
ticker := time.NewTicker(duration)
// this will call daemon.doWork() periodically
go func() {
for {
<- ticker.C
daemon.doWork()
}
}()
}
type ConcreteDaemonA struct { foo int }
type ConcreteDaemonB struct { bar int }
func (daemon *ConcreteDaemonA) doWork() {
daemon.foo++
fmt.Println("A: ", daemon.foo)
}
func (daemon *ConcreteDaemonB) doWork() {
daemon.bar--
fmt.Println("B: ", daemon.bar)
}
func main() {
dA := new(ConcreteDaemonA)
dB := new(ConcreteDaemonB)
start(dA, 1 * time.Second)
start(dB, 5 * time.Second)
time.Sleep(100 * time.Second)
}
This won't compile as it's not possible to use interface as a receiver.
In fact, I have already answered my question (see the answer below). However, is it an idiomatic way to implement such logic? Are there any reasons not to have a default implementation besides language's simplicity?
The other answers provide an alternative to your problem, however they proposed solution without using abstract classes/struct, and I guess if you were interested in using abstract class like solution, here is very precise solution to your problem:
Go plaground
package main
import (
"fmt"
"time"
)
type Daemon interface {
start(time.Duration)
doWork()
}
type AbstractDaemon struct {
Daemon
}
func (a *AbstractDaemon) start(duration time.Duration) {
ticker := time.NewTicker(duration)
// this will call daemon.doWork() periodically
go func() {
for {
<- ticker.C
a.doWork()
}
}()
}
type ConcreteDaemonA struct {
*AbstractDaemon
foo int
}
func newConcreteDaemonA() *ConcreteDaemonA {
a:=&AbstractDaemon{}
r:=&ConcreteDaemonA{a, 0}
a.Daemon = r
return r
}
type ConcreteDaemonB struct {
*AbstractDaemon
bar int
}
func newConcreteDaemonB() *ConcreteDaemonB {
a:=&AbstractDaemon{}
r:=&ConcreteDaemonB{a, 0}
a.Daemon = r
return r
}
func (a *ConcreteDaemonA) doWork() {
a.foo++
fmt.Println("A: ", a.foo)
}
func (b *ConcreteDaemonB) doWork() {
b.bar--
fmt.Println("B: ", b.bar)
}
func main() {
var dA Daemon = newConcreteDaemonA()
var dB Daemon = newConcreteDaemonB()
dA.start(1 * time.Second)
dB.start(5 * time.Second)
time.Sleep(100 * time.Second)
}
If this is still not obvious how to use abstract classes/multi-inheritance in go-lang here is the post with comprehensive details. Abstract Classes In Go
If you want to provide a "default" implementation (for Daemon.start()), that is not the characteristic of an interface (at least not in Go). That is a characteristic of a concrete (non-interface) type.
So Daemon in your case should be a concrete type, conveniently a struct since you want it to have fields. And the task to be done can be either a value of an interface type, or in a simple case just a function value (a simple case means it would only have one method).
With interface type
Try the complete app on the Go Playground.
type Task interface {
doWork()
}
type Daemon struct {
task Task
}
func (d *Daemon) start(t time.Duration) {
ticker := time.NewTicker(t)
// this will call task.doWork() periodically
go func() {
for {
<-ticker.C
d.task.doWork()
}
}()
}
type MyTask struct{}
func (m MyTask) doWork() {
fmt.Println("Doing my work")
}
func main() {
d := Daemon{task: MyTask{}}
d.start(time.Millisecond*300)
time.Sleep(time.Second * 2)
}
With a function value
In this simple case this one is shorter. Try it on the Go Playground.
type Daemon struct {
task func()
}
func (d *Daemon) start(t time.Duration) {
ticker := time.NewTicker(t)
// this will call task() periodically
go func() {
for {
<-ticker.C
d.task()
}
}()
}
func main() {
d := Daemon{task: func() {
fmt.Println("Doing my work")
}}
d.start(time.Millisecond * 300)
time.Sleep(time.Second * 2)
}
An easy solution is to move daemon *Daemon to the argument list (thus removing start(...) from the interface):
type Daemon interface {
// start(time.Duration)
doWork()
}
func start(daemon Daemon, duration time.Duration) { ... }
func main() {
...
start(dA, 1 * time.Second)
start(dB, 5 * time.Second)
...
}
You can implement abstract class in go.
The definition:
type abstractObject interface{
print()
}
type object struct{
a int
abstractObject
}
Now object is an abstract class, like java's.
You can inherit it and use its members:
type concreteObject struct{
*object
}
(o *concreteObject) print() {
fmt.Println(o.a)
}
func newConcreteObject(o *object) {
obj := &concreteObject{object: o}
o.abstractObject = obj // all magics are in this statement.
}
And use the object with concreteObject's methods:
o := &object{}
newConcereteObject(o)
o.print()
And cast abstract object to concrete object:
concObj := o.abstractObject.(*concreteObject)
Just like other OOP languages.
The solution by Max Malysh would work in some cases if you don't need a factory. However the solution given by Adrian Witas could cause cyclic dependencies issues.
This is the way I achieved implementing an abstract class the easy way respecting cyclic dependencies and good factory patterns.
Let us assume we have the following package structure for our component
component
base
types.go
abstract.go
impl1
impl.go
impl2
impl.go
types.go
factory.go
Define the definition of the component, in this example it will be defined here:
component/types.go
package component
type IComponent interface{
B() int
A() int
Sum() int
Average() int
}
Now let's assume we want to create an abstract class that implements Sum and Average only, but in this abstract implementation we would like to have access to use the values returned by the implemented A and B
To achieve this, we should define another interface for the abstract members of the abstract implementation
component/base/types.go
package base
type IAbstractComponentMembers {
A() int
B() int
}
And then we can proceed to implement the abstract "class"
component/base/abstract.go
package base
type AbstractComponent struct {
IAbstractComponentsMember
}
func (a *AbstractComponent) Sum() int {
return a.A() + a.B()
}
func (a *AbstractComponent) Average() int {
return a.Sum() / 2
}
And now we proceed to the implementations
component/impl1/impl.go // Asume something similar for impl2
package impl1
type ComponentImpl1 struct {
base.AbstractComponent
}
func (c *ComponentImpl1) A() int {
return 2
}
func (c *ComponentImpl1) A() int {
return 4
}
// Here is how we would build this component
func New() *ComponentImpl1 {
impl1 := &ComponentImpl1{}
abs:=&base.AbstractComponent{
IAbstractComponentsMember: impl1,
}
impl1.AbstractComponent = abs
return impl1
}
The reason we use a separate interface for this instead of using Adrian Witas example, is because if we use the same interface in this case, if we import the base package in impl* to use the abstract "class" and also we import the impl* packages in the components package, so the factory can register them, we'll find a circular reference.
So we could have a factory implementation like this
component/factory.go
package component
// Default component implementation to use
const defaultName = "impl1"
var instance *Factory
type Factory struct {
// Map of constructors for the components
ctors map[string]func() IComponent
}
func (f *factory) New() IComponent {
ret, _ := f.Create(defaultName)
return ret
}
func (f *factory) Create(name string) (IComponent, error) {
ctor, ok := f.ctors[name]
if !ok {
return nil, errors.New("component not found")
}
return ctor(), nil
}
func (f *factory) Register(name string, constructor func() IComponent) {
f.ctors[name] = constructor
}
func Factory() *Factory {
if instance == nil {
instance = &factory{ctors: map[string]func() IComponent{}}
}
return instance
}
// Here we register the implementations in the factory
func init() {
Factory().Register("impl1", func() IComponent { return impl1.New() })
Factory().Register("impl2", func() IComponent { return impl2.New() })
}
The functionality of abstract class has below requirements
1. It should not be possible to create direct instance of abstract class
2. It should provide default fields and methods.
A combination of interface and struct can be used to fulfill above two requirements. For example we can see below
package main
import "fmt"
//Abstract Interface
type iAlpha interface {
work()
common(iAlpha)
}
//Abstract Concrete Type
type alpha struct {
name string
}
func (a *alpha) common(i iAlpha) {
fmt.Println("common called")
i.work()
}
//Implementing Type
type beta struct {
alpha
}
func (b *beta) work() {
fmt.Println("work called")
fmt.Printf("Name is %s\n", b.name)
}
func main() {
a := alpha{name: "test"}
b := &beta{alpha: a}
b.common(b)
}
Output:
common called
work called
Name is test
One important point to mention here is that all default method should have iAlpha as first argument, and if default method needs to call any unimplemented method they they will call on this interface. This is same as we did in common method above - i.work().
Source: https://golangbyexample.com/go-abstract-class/
Is there a way to specify the return type of a function to be the type of the called object?
e.g.
trait Foo {
fun bar(): <??> /* what to put here? */ {
return this
}
}
class FooClassA : Foo {
fun a() {}
}
class FooClassB : Foo {
fun b() {}
}
// this is the desired effect:
val a = FooClassA().bar() // should be of type FooClassA
a.a() // so this would work
val b = FooClassB().bar() // should be of type FooClassB
b.b() // so this would work
In effect, this would be roughly equivalent to instancetype in Objective-C or Self in Swift.
There's no language feature supporting this, but you can always use recursive generics (which is the pattern many libraries use):
// Define a recursive generic parameter Me
trait Foo<Me: Foo<Me>> {
fun bar(): Me {
// Here we have to cast, because the compiler does not know that Me is the same as this class
return this as Me
}
}
// In subclasses, pass itself to the superclass as an argument:
class FooClassA : Foo<FooClassA> {
fun a() {}
}
class FooClassB : Foo<FooClassB> {
fun b() {}
}
You can return something's own type with extension functions.
interface ExampleInterface
// Everything that implements ExampleInterface will have this method.
fun <T : ExampleInterface> T.doSomething(): T {
return this
}
class ClassA : ExampleInterface {
fun classASpecificMethod() {}
}
class ClassB : ExampleInterface {
fun classBSpecificMethod() {}
}
fun example() {
// doSomething() returns ClassA!
ClassA().doSomething().classASpecificMethod()
// doSomething() returns ClassB!
ClassB().doSomething().classBSpecificMethod()
}
You can use an extension method to achieve the "returns same type" effect. Here's a quick example that shows a base type with multiple type parameters and an extension method that takes a function which operates on an instance of said type:
public abstract class BuilderBase<A, B> {}
public fun <B : BuilderBase<*, *>> B.doIt(): B {
// Do something
return this
}
public class MyBuilder : BuilderBase<Int,String>() {}
public fun demo() {
val b : MyBuilder = MyBuilder().doIt()
}
Since extension methods are resolved statically (at least as of M12), you may need to have the extension delegate the actual implementation to its this should you need type-specific behaviors.
Recursive Type Bound
The pattern you have shown in the question is known as recursive type bound in the JVM world. A recursive type is one that includes a function that uses that type itself as a type for its parameter or its return value. In your example, you are using the same type for the return value by saying return this.
Example
Let's understand this with a simple and real example. We'll replace trait from your example with interface because trait is now deprecated in Kotlin. In this example, the interface VitaminSource returns different implementations of the sources of different vitamins.
In the following interface, you can see that its type parameter has itself as an upper bound. This is why it's known as recursive type bound:
VitaminSource.kt
interface VitaminSource<T: VitaminSource<T>> {
fun getSource(): T {
#Suppress("UNCHECKED_CAST")
return this as T
}
}
We suppress the UNCHECKED_CAST warning because the compiler can't possibly know whether we passed the same class name as a type argument.
Then we extend the interface with concrete implementations:
Carrot.kt
class Carrot : VitaminSource<Carrot> {
fun getVitaminA() = println("Vitamin A")
}
Banana.kt
class Banana : VitaminSource<Banana> {
fun getVitaminB() = println("Vitamin B")
}
While extending the classes, you must make sure to pass the same class to the interface otherwise you'll get ClassCastException at runtime:
class Banana : VitaminSource<Banana> // OK
class Banana : VitaminSource<Carrot> // No compiler error but exception at runtime
Test.kt
fun main() {
val carrot = Carrot().getSource()
carrot.getVitaminA()
val banana = Banana().getSource()
banana.getVitaminB()
}
That's it! Hope that helps.
Depending on the exact use case, scope functions can be a good alternative. For the builder pattern apply seems to be most useful because the context object is this and the result of the scope function is this as well.
Consider this example for a builder of List with a specialized builder subclass:
open class ListBuilder<E> {
// Return type does not matter, could also use Unit and not return anything
// But might be good to avoid that to not force users to use scope functions
fun add(element: E): ListBuilder<E> {
...
return this
}
fun buildList(): List<E> {
...
}
}
class EnhancedListBuilder<E>: ListBuilder<E>() {
fun addTwice(element: E): EnhancedListBuilder<E> {
addNTimes(element, 2)
return this
}
fun addNTimes(element: E, times: Int): EnhancedListBuilder<E> {
repeat(times) {
add(element)
}
return this
}
}
// Usage of builder:
val list = EnhancedListBuilder<String>().apply {
add("a") // Note: This would return only ListBuilder
addTwice("b")
addNTimes("c", 3)
}.buildList()
However, this only works if all methods have this as result. If one of the methods actually creates a new instance, then that instance would be discarded.
This is based on this answer to a similar question.
You can do it also via extension functions.
class Foo
fun <T: Foo>T.someFun(): T {
return this
}
Foo().someFun().someFun()