I have written this methods in Kotlin and analysed the bytecode:
Situation 1
class A {
object b {
fun doSomething() {}
}
}
Situation 2
class A {
companion object b {
fun doSomething() {}
}
}
Situation 3
fun doSomething() {}
Bytecode Result
Situation 1: class Test$asb, public final doSomething()I
Situation 2: class Test$Companion, public final doSomething()I
Situation 3: class TestKt, public final static doSomething()I
My questions are:
I have an enum class, and I want to return an enum instace given an enum variable, for instance, findById (enum(id, color)). How would I do it? Companion Object? object?
It seems the only way to have a real static method is in package level, without class declaration. But that becomes a little bit too global. Is there any way to access it via: ClassName.staticMethod, staticMethod being really static.
Provide meaningfull examples of package declaration methods, companion object and object.
Context. I have been coding in Kotlin and I find it amazing. But sometimes I need to make a decision: for example, a heavy immutable property which in java I would declare as static final, but in Kotlin I find it hard to "find an equivalent".
If you have a function which performs some action closely related to a class but doesn't require a class instance, such as your findById example, you should put it in the companion object of the class.
If you want to expose a method as a static method to Java code, you can annotate it with the #JvmStatic annotation.
If a function does not require an instance of a class, then it is your design decision where to put it. Use package level if it is package-specific, use a class companion if it closely relets to the class (for example other classes in the package have similar functions).
Note that enum has several in-build properties and patterns:
enum class Colour(val value: Int) {
black(100), red(200), green(300)
}
fun colourById(id: Int) = Colour.values[id]
fun colourByValue(value: Int) = Colour.values.first {it.value == value}
fun colourByName(name: String) = Colour.valueOf(name)
I would suggest to develop voddan answer:
enum class Color {
RED,
BLUE,
GREEN;
companion object Utils {
fun findById(color: Color): Color {
return color;
}
}
}
And to test
#Test
fun testColor() {
println(Color.Utils.findById(Color.valueOf("RED")));
}
Related
Both Java and Javascript allow for a different way of executing static code. Java allows you to have static code in the body of a class while JS allows you to execute static code outside class definitions. Examples:
Java:
public class MyClass {
private static Map<String,String> someMap = new HashMap<String,String();
static {
someMap.put("key1","value");
someMap.put("key2","value");
SomeOtherClass.someOtherStaticMethod();
System.out.println(someMap);
}
}
JS (basically any JS code outside a class):
var myint = 5;
callSomeMethod();
$(document).ready(function () {
$("#hiddenelement").hide();
});
However, it seems like Dart supports either of both ways. Declaring global variables and methods is supported, but calling methods and executing code like in JS is not. This can only be done in a main() method. Also, static code inside a class is not allowed either.
I know Dart has other ways to statically fill a Map like my first example, but there is another case that I can think of for which this is required.
Let's consider the following CarRegistry implementation that allows you to map strings of the car model to an instance of the corresponding class. F.e. when you get the car models from JSON data:
class CarRegistry {
static Map<String, Function> _factoryMethods = new HashMap<String, Function>();
static void registerFactory(String key, Car factory()) {
_factoryMethods[key] = factory;
}
static Car createInstance(String key) {
Function factory = _factoryMethods[key];
if(factory != null) {
return factory();
}
throw new Exception("Key not found: $key");
}
}
class TeslaModelS extends Car {
}
class TeslaModelX extends Car {
}
In order to be able to call CarRegistry.createInstance("teslamodelx");, the class must first be registered. In Java this could be done by adding the following line to each Car class: static { CarRegistry.registerFactory("teslamodelx" , () => new TeslaModelX()); }. What you don't want is to hard-code all cars into the registry, because it will lose it's function as a registry, and it increases coupling. You want to be able to add a new car by only adding one new file. In JS you could call the CarRegistry.registerFactory("teslamodelx" , () => new TeslaModelX()); line outside the class construct.
How could a similar thing be done in Dart?
Even if you would allow to edit multiple files to add a new car, it would not be possible if you are writing a library without a main() method. The only option then is to fill the map on the first call of the Registry.createInstance() method, but it's no longer a registry then, just a class containing a hard-coded list of cars.
EDIT: A small addition to the last statement I made here: filling this kind of registry in the createInstance() method is only an option if the registry resided in my own library. If, f.e. I want to register my own classes to a registry provided by a different library that I imported, that's no longer an option.
Why all the fuss about static?
You can create a getter that checks if the initialization was already done (_factoryMethods != null) if not do it and return the map.
As far a I understand it, this is all about at what time this code should be executed.
The approach I showed above is lazy initialization.
I think this is usually the preferred way I guess.
If you want to do initialization when the library is loaded
I don't know another way as calling an init() method of the library from main() and add initialization code this libraries init() method.
Here is a discussion about this topic executing code at library initialization time
I encountered the same issue when trying to drive a similarly themed library.
My initial attempt explored using dart:mirrors to iterate over classes in a library and determine if they were tagged by an annotation like this (using your own code as part of the example):
#car('telsamodelx')
class TelsaModelX extends Car {
}
If so, they got automatically populated into the registry. Performance wasn't great, though, and I wasn't sure if how it was going to scale.
I ended up taking a more cumbersome approach:
// Inside of CarRegistry.dart
class CarRegister {
static bool _registeredAll = false;
static Car create() {
if (!_registeredAll) { _registerAll()); }
/* ... */
}
}
// Inside of the same library, telsa_model_x.dart
class TelsaModelX extends Car {}
// Inside of the same library, global namespace:
// This method registers all "default" vehicles in the vehicle registery.
_registerAll() {
register('telsamodelx', () => new TelsaModelX());
}
// Inside of the same library, global namespace:
register(carName, carFxn) { /* ... */ }
Outside of the library, consumers had to call register(); somewhere to register their vehicle.
It is unnecessary duplication, and unfortunately separates the registration from the class in a way that makes it hard to track, but it's either cumbersome code or a performance hit by using dart:mirrors.
YMMV, but as the number of register-able items grow, I'm starting to look towards the dart:mirrors approach again.
I am trying my best to explain the situation. I hope, what I wrote, is understandable.
We already have class defined like
public ref class TestClass
{
public:
TestClass();
virtual ~TestClass();
protected:
Car* m_car;
}
TestClass is managed C++ and Car is unmanaged C++.
So far so good, but now I need to make static object of TestClass also. So I modify the code like below
public ref class TestClass
{
private:
static TestClass^ s_test = nullptr ;
public:
TestClass();
virtual ~TestClass();
static TestClass^ Instance();
protected:
Car* m_car;
}
When I want to use static instant of the class, I just get it from calling
TestClass staticobj = TestClass::Instance();
Elsewhere, just call
TestClass normalobj = gcnew TestClass();
Instance function is creating s_test static object and returns it.
TestClass ^ TestClass::Instance()
{
if(s_test == nullptr)
{
s_test = gcnew TestClass();
s_test->m_car = new Car();
}
return s_test;
}
Is it a good approach?
Is there any other better approach to accomplish same thing?
Edit :
FYI Above code works.
I combined Krizz and Reed Copsey’s solutions. That solve independent Singleton and memory leak.
Here is my sample code,
Special Singleton class derived from test class,
public ref class SpecialSingletonTestClass: public TestClass
{
private:
static SpecialSingletonTestClass ^ s_ SpecialSingletonTestClass = nullptr;
public:
SpecialSingletonTestClass ();
static SpecialSingletonTestClass ^ Instance();
};
Changed the testclass so it has now one more finalizer function.
public ref class TestClass
{
public:
TestClass ();
virtual ~ TestClass ();
! TestClass ();
protected:
Car* m_car;
}
I tested above pattern , it worked.
Thanks you guys,
L.E.
Is it a good approach?
I would probably not consider this a good approach, as you're making a single class both a singleton and a normal class that you can instance directly.
Typically, if you need a singleton, this would preclude the need or desire to be able to instantiate the class.
If you truly need to have a way to have a "global" instance of this class, I would encapsulate that in a separate class which implements the singleton. This would, at least, make it clear that you are dealing with something that's a single instance in that case. I would not mix both use cases into a single class.
Well, actually there is an issue with memory leaks in your code.
You declare only virtual ~TestClass(); which, for managed classes, are internally turned by C++/CLI compiler into implementation of IDisposable.Dispose().
Therefore, if you put delete car into it, it will be called only if you delete test_class or, e.g. wrap into using (TestClass tst) {} block when using from C#.
It will not be called when object is GCed!
To be sure it is called you need to add finalizer to your class !MyClass(); which is turned by compiler into virtual void Finalize() and thus non-deterministically called when GC is freeing an object.
And it is the only way to free m_car of singleton object.
Therefore, I suggest:
TestClass()
{
m_car = new Car();
}
~TestClass()
{
if (m_car)
delete m_car;
m_car = NULL;
}
!TestClass()
{
if (m_car)
delete m_car;
m_car = NULL;
}
I'm unsure as to what situation you could possibly be in that would require both singleton-style semantics and normal creation semantics for the same class.
As far as what you've coded though, it looks completely fine. My only comments would be that your Instance() function shouldn't need to perform construction on Car, the Instance() function should just call the default constructor of TestClass which should do all that.
EDIT
In reference to:
#crush . The class is already define i just need to get static object of it. Singleton means only one object of the class, but in this case, class have multiple normal object. But i want to use only one object of this class for one specific goal only for limited period of time. – L.E. 2 mins ago
A singleton is (usually) a sign of bad design - alot of people call it an anti-pattern actually. Chances are if you just need this one single specific instance of this class for a limited period of time there are some issues:
Singleton design is made for static-style existence - the variable will live for the scope of your program after lazily initialized.
Allowing global access will move your code towards spaghetti logic. You'd be better off dynamically allocating the one you need and passing the pointer to it to where you need it to be. A shared_ptr would be good for this.
You should find a way around the singleton-style implementation in this case even if it's more work for you - it'll almost certainly be better design.
I got a ViewModel which I export with MEF. I'd like this ViewModel to be initialized differently each time it's being imported, according to an enum/specific object parameter that will be provided to it.
I've been reading a little on the subject and I found that maybe this -
http://msdn.microsoft.com/en-us/library/ee155691.aspx#metadata_and_metadata_views
would be able to fit my needs, but I'm not sure that this would be the best way to do it.
Another method I've been thinking about is importing the class normally, and then once I've an instance, to call a special initialization method that would receive my parameter. However this doesn't seem like a classic MEF implementation, and maybe losses some of its "magic".
I'm hoping someone would be able to point out for me what would be the recommended method to achieve this.
Thanks!
A workaround is exporting a factory that creates instances of your type. While this means you cannot directly import thos instances, it does have the benefit that the logic to create them is the responsability of the factory so users of the class do not have to know about it:
public class ServiceWithParameter
{
public ServiceWithParameter( int a )
{
this.a = a;
}
private readonly int a;
}
[Export]
public class ServiceWithParameterFactory
{
public ServiceWithParameterFactory()
{
instance = 0;
}
public ServiceWithParameter Instance()
{
return new ServiceWithParameter( instance++ );
}
private int instance;
}
//now everywhere you need ServiceWithParameter:
[Import]
ServiceWithParameterFactory serviceFactory;
var instanceA = serviceFactory.Instance(); //instanceA.a = 0
var instanceB = serviceFactory.Instance(); //instanceB.a = 1
A more extensible way is telling the container you have a factory and an example is presented here: http://pwlodek.blogspot.com/2010/10/mef-object-factories-using-export.html
In php we can call static member functions using class objects. For example
class Human
{
public static function Speak()
{
echo "I am a human.";
}
}
$human = new Human();
$human->Speak();
What we would expect is that a static member function can only be called using the class name and not the class instance variable (object). But what i have seen while programming is that php allows calling a static member function using the class object also. Is there any practical use or some important reason that this feature has been provided in php ?
This feature exists in java and c++ also. Thanks Oli for pointing this out in your response.
This is the same as in other OO languages, such as C++ and Java. Why would you want the interpreter to prevent this?
UPDATE
My best guess for this (and this is only a guess) is "for convenience". In essence, why should the user of your class necessarily care whether a given member function is static or not? In some circumstances, this will certainly matter; in others, maybe not. I'm not saying this is a great justification, but it's all I can come up with!
it allows you to abstract from the particular definition of the method, so that for example if you had to turn it into a static one at some point, you don't have to rewrite all the method calls!
I can't answer for PHP, (or really for anything) but consider this hypothetical C++:
class base{
public:
static void speak(){cout<<"base\n";}
};
class sub :public base {
public:
static void speak(){cout<<"sub\n"; }
};
int _tmain(int argc, _TCHAR* argv[]){
base *base1 = new base();
base1->speak();
sub *sub1 = new sub();
sub1->speak();
base *sub2 = new sub();
sub2->speak();
((sub*)sub2)->speak();
}
The output would be:
base
sub
base
sub
I'm sure it could be useful... maybe helping you determine which class's static method you should call based on the object currently in hand.
Like java, I want to create a method that accepts an array list of particular object type.
In java:
public void addStudents(List<Student> students) {
...
}
In actionscript
public function addStudents(students:ArrayCollection):void {
.....
}
Here I want to have public function addStudents(students:ArrayCollection).
Thanks
If you have a Student object and publish for FP10 you can use the Vector object.
public function addStudents(students:Vector.<Student>):void {}
For further information: http://help.adobe.com/en_US/AS3LCR/Flash_10.0/Vector.html
As far as i know, AS has no template-like generics. But you can extend ArrayCollection into something like StudentArrayCollection with more rigid type check inside.