Lets say I have the following structure:
src
- main
- scala
- model
- One.scala
- Two.scala
- main
- Test.scala
where Test.scala extends App and takes in a parameter:
object Test extends App {
val param: String = args.head
// Based on param, I want to use either One or Two?
}
// sbt run Two
How do i use definitions in either One.scala or Two.scala depending on the runtime value of param.
Appreciate any/all inputs..
Make sure that One and Two share some common interface, choose the instance of this interface at runtime, then import the members of the instance:
trait CommonInterface {
def foo(): Unit
}
object One extends CommonInterface { def foo() = println("1") }
object Two extends CommonInterface { def foo() = println("2") }
object Main extends App {
// check args etc...
val ci = if (args(0) == "one") One else Two
import ci._
// do something with `foo` here
foo()
}
Related
I have a class. It has a companion object A with a factory method.
class A private[somepackage](x: Int) {
}
object A {
def createA(y: Int): A = {
new A(y)
}
}
Now I need to create the mock object of A in a scalatest file which is in a different package.
When I give
private val a = mock[A] --> I get compilation error.
constructor A in class A cannot be accessed in <<somewhere>>.
Is there a better way to mock the object ??
In your test sources, create a test double in the same package:
package somepackage
class MockableA extends A(0)
then just create a mock[MockableA] in your tests and continue as usual.
But the answer with a proxy/facade should work too if you are willing to change production sources to facilitate tests.
Consider using a proxy to access class A, and stub/mock that proxy class instead.
E.g., if A.doStuff is what you want to mock/stub, and A.accessStuff is what you need in your code, create a class
class ADecorated(underlying: A) {
def doStuff() {
underlying.doStuff()
// whatever I want to do
}
def accessStuff() {
x = underlying.accessStuff()
// do something else and return
}
// Any other method you want to use
}
Replace usage of A.createA with new ADecorated(A.createA()). ADecorated is what you work with now
I am relatively new to scala so please bear me if I asked silly questions.
I have a requirement where I need to invoke a method run time.
I have a trait which is being extended by two classes
trait Animal {
def walk():DataFrame
}
This is extended by two classes.
class Dog(sparkSession: SparkSession) extends Animal {
def walk():DataFrame = {
.............
}
}
class Cat(sparkSession: SparkSession) extends Animal {
def walk():DataFrame = {
.............
}
}
Now from a config file I will get a list of these class names
Lets say like this
val animals = ["com.xy.Dog","com.xy.Cat"]
I need to invoke these classes and execute walk methods.
Can I do something like this?
animals.forEach{ animalString =>
val animalObject = Class.forName(animalString ).newInstance().asInstanceOf(Animal)
animalObject.walk
}
There are a few issues, let's take them one by one:
To build a list in scala you need to do:
val animals = List("com.xy.Dog", "com.xy.Cat")
The forEach method is actually foreach, so there's a small typo. Finally, when you call the newInstance you should get the appropriate constructor before that, otherwise it will use the default one.
animals.foreach { animalString =>
val animalObject = Class.forName(animalString)
.getConstructor(classOf[DataFrame]) // Get the constructor for a DataFrame argument
.newInstance(dataframe) // Pass the dataframe instance
.asInstanceOf[Animal]
animalObject.walk
}
I've made a couple of small changes to the code for you to see it working. You can run the app to see the output:
class Dog extends Animal {
def walk(): Unit = { println("I'm a dog.") }
}
class Cat extends Animal {
def walk(): Unit = { println("I'm a cat.") }
}
object AnimalTest extends App {
val animals = List("com.xy.Dog", "com.xy.Cat")
animals.foreach { animalString =>
val animalObject = Class.forName(animalString)
.newInstance()
.asInstanceOf[Animal]
animalObject.walk
}
}
Note that I've removed the constructor arguments here to easily build instances. The rest is about the same. I hope this helps you.
I have a Scala application, where pretty much every object extends a specific trait, which holds all the main functions and variables used by pretty much the entire system.
I want to add a --testing flag to my app's command line variables, which will shift the the results of some of the functions in the trait.
Putting it simply, I'd like the variable accepted in the main to have an affect that alters something in the trait before it is extended by the objects - without sending it explicitly to all objects.
Any ideas how that can be performed?
I doubt you really want to dynamically modify a trait, and I am not sure if it possible that all your classes inheriting that trait would be affected. I don't know enough about the compiler and byte code.
A way to accomplish something similar would be to have your trait take a parameter, and make your trait act conditionally on the parameter.
trait Foo {
val testing: Boolean
def fn1(): Unit = {
if (testing) {
println("testing")
} else {
println("production")
}
}
}
class Bar(val testing: Boolean) extends Foo {
def fn2(): Unit = {
fn1()
}
}
new Bar(true).fn2()
new Bar(false).fn2()
Your question is broad and this is just my 5 cents.
Update
trait Foo {
def fn1(): Unit = {
if (Foo.testing) {
println("testing")
} else {
println("production")
}
}
}
object Foo {
var testing: Boolean = false
}
class Bar extends Foo {
def fn2(): Unit = {
fn1()
}
}
object SOApp extends App {
new Bar().fn2()
Foo.testing = true
new Bar().fn2()
}
Consider passing the 'testing' flag to the trait's initializer block like this:
trait MyTrait {
var testMode: Boolean = _
def doSomething(): Unit = {
if (testMode)
println("In Test Mode")
else
println("In Standard Mode")
}
}
// IMPORTANT: Your best bet would be to create some Config object
// that is loaded and initialized in a main method.
// Define test-specific Config class:
case class Config(testMode: Boolean) {
def isTestMode: Boolean = this.testMode
}
// Instantiate in main method:
val config = new Config(true)
// Later, extend the trait:
class MyObj extends MyTrait { testMode = config.isTestMode() }
// Then just invoke
new MyObject().doSomething()
say I have the following code:
package my
class Foo
class Bar extends Foo
object Chooser {
val isFoo = true
}
I import Foo as:
import my.{Foo => MyClass}
I want to be able to do something like:
If Chooser.isFoo, then:
import my.{Foo => MyClass}
else:
import my.{Bar => MyClass}
I have used Foo in my code as something like this:
object MyObj {
val a = new MyClass
// ...
}
Are there any hidden features of Scala that let me use Bar in place of Foo without modifying the code of MyObj at all. Also what would be the best way to design such code in future so that such extensions are easy?
There isn't but I'm guessing you are more interested in different implementations at runtime than in conditional type import. Your Chooser.isFoo sounds like something that happens during runtime and not in the type system.
One example of how you could do it, since the common type for Foo and Bar is Foo:
val a: Foo =
if (Chooser.isFoo) new my.Foo
else new my.Bar
Edit based on your edit: you could delay the choice by having an abstract type or a type parameter, like so:
class MyObj[T :> Foo] {
val a: T
}
val anInstance = new MyObj[Foo]
val anotherInstance = new MyObj[Bar]
Note the type bound that says that T must be a subclass of Foo, or else you wouldn't know anything about what you can do with a T.
Happen to see this old post & a little curious why it can't do conditional imports in Scala? Maybe old version limit, not sure? But see this, we can import in any place of scala code.
For your scenario, it could be:
try.scala:
package my
class Foo {
}
class Bar extends Foo
object Chooser {
val isFoo = true
}
object MyObj extends App {
if (Chooser.isFoo) {
import my.{Foo => MyClass}
val a = new MyClass
println(a)
} else {
import my.{Bar => MyClass}
val a = new MyClass
println(a)
}
}
run output:
C:\abc\abc>scalac try.scala
C:\abc\abc>scala my.MyObj
my.Foo#6f4a47c7
I need a smart mechanism for component composition which allows mixed in traits to initialize after the composed component. The following throws a NullPointerException:
class Component {
def addListener(pf: PartialFunction[Any, Unit]) {}
}
trait DynamicComponent {
protected def component: Component
component.addListener {
case x =>
}
}
class Foo extends DynamicComponent {
protected val component = new Component
}
new Foo // -> NullPointerException
The following things are not options for me:
Using protected lazy val component; that would produce an avalange of dozens of vals needing to become lazy, something I do not want.
Putting addListener in a method, e.g. initDynamic(); because I will be mixing in many traits, and I don't want to remember to call half a dozen initFoo() methods.
Using DelayedInit. This doesn't work with traits, at least according to the scaladocs.
I could live with a single init() call, but only under the following conditions:
all mixed in traits can easily declare to be invoked in this one single call
it is a compile error to forget the init() statement.
You can delay the initialization of a trait by by using early definitions. (See section 5.1.6 of the scala language specification)
class Foo extends {
protected val component = new Component
} with DynamicComponent
It's even clunkier than your solution, but you can always require the creation of a val that must be set with the init() method. You could choose to not do it last and get an error at runtime, but at least you won't forget it entirely:
class Component {
def addListener(pf: PartialFunction[Any, Unit]) {
println("Added")
}
}
trait Dyn {
protected def component: Component
protected val initialized: Init
class Init private () {}
private object Init { def apply() = new Init() }
def init() = { component.addListener{ case x => }; Init() }
}
class Foo extends Dyn {
protected val component = new Component
protected val initialized = init()
}
No cheating!:
> class Bar extends Dyn { protected val component = new Component }
<console>:12: error: class Bar needs to be abstract, since value
initialized in trait Dyn of type Bar.this.Init is not defined
class Bar extends Dyn { protected val component = new Component }
The advantage this has is if you need multiple things to be in place before you initialize all of them cooperatively, or if your Component class is final so you can't mix in anything else.
AN idea could be to use the trick described here:
Cake pattern: how to get all objects of type UserService provided by components
All your components that should be initialized could be registered in some Seq[InitializableComponent]. And then you could initialize all registered components with a foreach.
No component will be forgotten in that Seq because they are registered automatically, but you can still forget to call the foreach anyway...
Here is one idea (I am happy to read about other suggestions):
class Component {
def addListener(pf: PartialFunction[Any, Unit]) {
println("Added")
}
}
trait DynamicComponentHost {
protected def component: Component with DynamicPeer
protected trait DynamicPeer {
_: Component =>
addListener {
case x =>
}
}
}
class Foo extends DynamicComponentHost {
protected val component = new Component with DynamicPeer
}
new Foo
So basically I am forcing the component to mix in a type that can only be provided by the mixed in trait. Reasonable? Looks a bit too complicated in my eyes.