I want to get the class name of the class pass-in into a generic class
class GernericClass<T> {
fun printOut() {
println(T::class.java.name) // I want something like this
}
}
This is my main function
fun main() {
val className = GernericClass<SomeObjectClass>()
className.printOut() // wanted output: SomeObjectClass
}
Is it possible to get the class name only by calling GernericClass<SomeObjectClass>()
Need a subclass to specify the type of T, because of generic erasure，
fun main() {
val className = object:GernericClass<SomeObjectClass>(){}
className.printOut()
}
fun printOut() {
val clazz =
(this.javaClass.genericSuperclass as ParameterizedType).actualTypeArguments[0] as Class<*>
println(clazz) // I want something like this
}
Related
Is it possible to have a trait be a singleton?
What I am trying to achieve is to have a clean and lightweight API I can extend throughout my application like the following:
trait SingletonTrait {
// element I wish to be unique throughout my application
val singletonElement = ///
...
}
// uses *singletonElement*
object MainApplication extends SingletonTrait {
...
}
// uses *singletonElement*
class SomeClass(...) extends SingletonTrait {
...
}
In the same idea implied by a getOrCreate() function that would retrieve an existing instance of an element if one already exists or creates it otherwise.
Maybe just create value in companion object and reference it in trait?
trait SingletonTrait {
final lazy val singletonElement = SingletonTrait.SingletonElement
}
object SingletonTrait {
lazy val SingletonElement = {
println("Creating singleton element!")
"singleton element"
}
}
// uses *singletonElement*
class SomeClass() extends SingletonTrait {
println(s"Using ${singletonElement} in class.")
}
new SomeClass()
new SomeClass()
new SomeClass()
It prints:
Creating singleton element!
Using singleton element in class.
Using singleton element in class.
Using singleton element in class.
Technically you could do this like so
object SingletonElement {
var count = 0
}
trait SingletonTrait {
final val singletonElement = SingletonElement
}
object MainApplication extends SingletonTrait {
singletonElement.count = singletonElement.count + 1
}
class SomeClass extends SingletonTrait {
singletonElement.count = singletonElement.count + 1
}
We can test that the same object is used like so
new SomeClass
MainApplication
SingletonElement.count
which should output
res2: Int = 2
which shows the same SingletonElement was used.
I would like to write a Scala test which checks the exact class of an object created with factory method.
class Base { }
class Derived extends Base { }
class TestSpec {
test("test instance class") {
val result = new Derived()
// I want to check that result is exactly Derived type
result should be a[Derived]
result should not be a[Base]
}
}
I'm looking for something to test if object returned from my factory method for specific parameters is always base class. So it would be something like this is C#:
public class Base { }
public class Derived : Base { }
public class Program
{
public static void Main(string[] args)
{
var baseObject = new Base();
var derivedObject = new Derived();
var baseResult = baseObject.GetType().IsSubclassOf(typeof(Base));
var derivedResult = derivedObject.GetType().IsSubclassOf(typeof(Base));
Console.WriteLine(string.Format("Base is subclass of Base: {0}\nDerived is subclass of Base: {1}", baseResult, derivedResult));
}
}
you can do this :
class A{}
class B extends A{}
val result = new B()
result.getClass.getName shouldEqual classOf[B].getName
result.getClass.getName.equals(classOf[A].getName) shouldBe false
new Derived() will always be an instance of Base as it inherits from Base, which means:
new Derived().isInstanceOf[Base]
will return true.
But the opposite is false: Base won't be an instance of Derived.
Thus, to check if it's the Base class (and not the Derived class), you can use these two combined conditions:
new Base() should not be a[Derived]
new Base() shouldBe a[Base]
and to check if it's the Derived class, the following is enough:
new Derived() shouldBe a[Derived]
I am writing Play 2.5 application using Scala. I have following piece of code:
#ImplementedBy(classOf[BarRepositoryImpl])
trait BarRepository {
def bar = //some actions
}
class BarRepositoryImpl extends BarRepository
case class Foo( /*some fields*/) {
#Inject private var barRepository: BarRepository = null
def foo1 = {
val a = barRepository.bar //here barRepository is always null
// some actions with 'a' and returning some result which depends on 'a'
}
}
I also have a controller where I inject BarRepository as well, but through constructor and there everything works well while in the class Foo on the line val a = barRepository.bar I get a NullPointerException. Could someone help to figure out what's the problem? Is it forbidden to use injection in case class?
If you don't want to pollute your case class signature with Guice injected annotation and fields then simply add an implicit dependency on the method that needs it instead:
case class Foo( /*some fields*/) {
def bar1(someField: Int)(implicit barRepository: BarRepository) = {
// some code that interacts with barRepository
}
}
The calling class will have to have the BarRepository as an implicitly injected parameter. E.g. a Play controller like:
#Singleton
class HomeController #Inject()(cc: ControllerComponents)
(implicit barRepository: BarRepository)
extends AbstractController(cc) {
def index() = Action { implicit request =>
val foo = Foo("field")
val bar = foo.bar1
// ...
}
}
I would have assumed that you inject the object in your class signature?
case class Foo #Inject()(barRepository:BarRepository, /* your fields */){
/** some stuff **/
}
I have the following class setup:
class MyClass {
class MyInnerClass(memberVar: String)
def getAInner: MyInnerClass = {
new MyInnerClass("hello")
}
}
Then I have the following code outside of the class:
def myFunction = {
val a = new MyClass
val b = a.getAInner.memberVar // value memberVar is not a member of a.MyInnerClass
}
Why is this?
You need to add the keyword val to make memberVar public otherwise it's a private value:
class MyClass {
class MyInnerClass(val memberVar: String)
def getAInner: MyInnerClass = {
new MyInnerClass("hello")
}
}
#Noah's answer is totally correct, but I would also throw out the option of using case class. See here for some of the sugar it provides. I use it almost reflexively. In your example, it would be:
object MyClass {
case class MyInnerClass(memberVar: String)
def getAInner: MyInnerClass = {
new MyInnerClass("hello")
}
}
def myFunction = {
val b = MyClass.getAInner.memberVar
}
I tend to do it this way because invariably, I want to take advantage of the sane defaults case class provides.
I also chose to use object for the outer type, because it doesn't have any parameters, although you may have just done that for simplicity's sake.
I have the following class in my mind:
abstract class MyClass (data: MyData) {
def update(): MyClass = {
new MyClass(process())
}
def process(): MyData = {
...
}
}
However, abstract classes cannot be instantiated so the line new MyClass(process()) is an error. My question is - is there any way to tell the compiler that in case of each of the child classes of MyClass I want to create an object of exactly that child class? It seems an overkill to write this method awhole in all child classes. Playing with type parameters of the class or method I could not acheive that myself.
How about something like this? MyClass is parametrized with the concrete type. Of course, all concrete classes have to implement a method that actually returns a new instance of Self.
trait MyClass[+Self <: MyClass[Self]] {
def update(): Self = {
makeNew(process())
}
def process(): MyData = {
// ...
}
protected def makeNew(data: MyData): Self
}
class Concrete0 extends MyClass[Concrete0] {
protected def makeNew(data: MyData) = new Concrete0
}
class RefinedConcrete0 extends Concrete0 with MyClass[RefinedConcrete0] {
override protected def makeNew(data: MyData) = new RefinedConcrete0
}
Credit: IttayD’s second update to his answer to this question.
To completly avoid implementing almost identical method in all subclasses you would need to use reflection. I guess that would be your last resort if you have chosen Scala.
So here is how to minimize the repetitive code:
// additional parameter: a factory function
abstract class MyClass(data: MyData, makeNew: MyData => MyClass) {
def update(): MyClass = {
makeNew(process())
}
def process(): MyData = {
...
}
}
class Concrete(data: MyData) extends MyClass(data, new Concrete(_))
This way you repeat only the shortest fragment required to instantiate the subclass.