Im trying to create a GsonRequest class that extends a normal volley request, and also implements the error listener. The code I'm using to do this is
public class GsonRequest<T>(url: String, val clazz: Class<T>) :
Request<T>(Request.Method.GET, url, this), ErrorListener
However, I'm getting the following error: 'this' is not defined in this context. Is what I'm trying to do possible?
You can not use this on the JVM before the super-constructor finished working, so you can not pass it to the super-constructor.
What you can do is use an object expression or a lambda for an error listener:
public class GsonRequest<T>(url: String, val clazz: Class<T>) :
Request<T>(Request.Method.GET, url, { <handling code here> } )
Alternatively, consider taking the listener as a parameter for constructor of class GsonRequest
Related
I'm trying to use inline classes in Kotlin to create a class inlining the String class, such that if I have an instance of my class that it will always be true for the contained string that s == s.trim().
I was initially expecting there to be a straightforward way to do this, like perhaps:
#JvmInline
value class Trimmed private constructor(val str: String) : {
constructor(s : String) : super(s.trim())
}
but that doesn't work, and neither do the other direct approaches I considered ("this(s.trim())", etc.).
This problem has turned out to be surprisingly tricky:
Kotlin seems to provide no easy way to have the primary constructor filter or modify the data that is passed to the constructor of the contained String object.
Even if I make the primary constructor private, I can't declare another constructor with the same signature (taking a single String as a parameter).
If this were a normal (non-inlined) class, I could just set the value after superclass class construction (e.g. "init { str = str.trim() }", but since it's an inline class, I can't do that. ("this=this.trim()" doesn't work either, and String objects themselves are immutable so I can't change the contents of 'str'.)
I tried making the class constructor private and creating a factory function in the same file with the same name as the class, but then I couldn't call the class constructor from within the factory function due to access restrictions.
I then tried making the factory function within the class's companion object, but then Kotlin tried to make that function call itself recursively instead of calling the class's constructor. I wasn't able to find a way to syntactially disambiguate this. I managed to work around this by creating a file-private typealias to give another name for the class so I could call the constructor from within the factory function. (Annoyingly, I couldn't declare the typealias in the companion object next to the factory function: I had to declare it outside.)
This worked, but seemed ugly:
typealias Trimmed2 = Trimmed
#JvmInline
value class Trimmed private constructor(val str: String) {
init { assert(str == str.trim()) }
companion object {
// Kotlin won't let me put the typealias here. :-(
fun Trimmed(s: String): Trimmed = Trimmed2(s.trim()) // Don't want recursion here!
}
}
Another working solution is here, using a private constructor with a dummy argument. Of course Kotlin complained that the dummy argument was unused and so I had to put in a big (why is it so big?) annotation suppressing the warning, which is, again, ugly:
#JvmInline
value class Trimmed private constructor(val str: String) {
private constructor (untrimmed: String, #Suppress("UNUSED_PARAMETER") dummy: Unit) : this(untrimmed.trim())
init { assert(str == str.trim()) }
companion object {
fun Trimmed(s: String): Trimmed = Trimmed(s, Unit)
}
}
Is there a simpler, cleaner way to do this? For instance, a syntactic way to clarify to Kotlin that the companion function is trying to call the class constructor and not itself and so avoid the need for a dummy parameter?
Goals:
Code to construct instances of the class from outside this file should look like constructing an instance of a normal class: 'Trimmed("abc")', not using some factory function with a different name (e.g. "of" or "trimmedOf") or other alternate syntax.
It should be impossible to construct the object containing an untrimmed string. Outside code, and the Trimmed class itself, should be able to trust that if a Trimmed instance exists, that its contained str will be a trimmed string.
The class has a function:
fun theFunc(uri: Uri, theMap: Map<String, String>?, callback: ICallback) {
......
}
and would like to verify it is called with proper params type
io.mockk.verify { mock.theFunc(ofType(Uri::class), ofType(Map<String, String>::class), ofType(ICallbak::class)) }
the ofType(Uri::class) is ok,
the ofType(Map<String, String>::class got error:
the ofType(ICallbak::class) got error:
ICallback does not have a companion object, thus must be initialized
here.
How to use the ofType() for Map and interface?
The problem is that generic parameters are lost at runtime due to type erasure, and for this reason the syntax doesn't allow generic parameters to be specified in that context. You can write Map::class but not Map<String, String>::class because a Map<String, String> is just a Map at runtime.
So, you can call it like this:
verify { mock.theFunc(ofType(Uri::class), ofType(Map::class), ofType(ICallback::class)) }
that will work. However, there is also a version of function ofType which takes generic parameters, so you can use this:
verify { mock.theFunc(ofType<Uri>(), ofType<Map<String, String>>(), ofType<ICallback>()) }
You need to use mapOf<String,String>::class
io.mockk.verify { mock.theFunc(ofType(Uri::class), ofType(mapOf<String,String>()::class), ofType(ICallbak)) }
For interface, you can create mocck object. And put it into ofType.
val callbackMock: ICallback = mockk()
io.mockk.verify { mock.theFunc(ofType(Uri::class), ofType(mapOf<String,String>()::class), ofType(callbackMock::class)) }
I am trying to parse a yaml in Kotlin using SnakeYAML following this tutorial:
data class Case(val args: List<String>, val expected: String, val score: Int)
data class TestCases(val target: String, val cases: List<Case>)
val yaml = Yaml(Constructor(TestCases::class.java))
but the code returns these errors in IntelliJ
Cannot access '<init>': it is public/*package*/ in 'Constructor'
for using Constructor as in java.lang.reflect.Constructor, and
None of the following functions can be called with the arguments supplied:
public constructor Yaml(dumperOptions: DumperOptions!) defined in org.yaml.snakeyaml.Yaml
public constructor Yaml(loadingConfig: LoaderOptions!) defined in org.yaml.snakeyaml.Yaml
public constructor Yaml(constructor: BaseConstructor!) defined in org.yaml.snakeyaml.Yaml
public constructor Yaml(representer: Representer!) defined in org.yaml.snakeyaml.Yaml
for using Yaml as in org.yaml.snakeyaml.Yaml, which I think is expecting a BaseConstructor from Constructor.
How can I fix this problem? Thank you.
I guess you are importing wrong class. It should be org.yaml.snakeyaml.constructor.Constructor instead of java.lang.reflect.Constructor.
I'm not sure if this is a limitation, a bug or just bad use of GSON. I need to have a hierarchy of Kotlin objects (parent with various subtypes) and I need to deserialize them with GSON. The deserialized object has correct subtype but its field enumField is actually null.
First I thought this is because the field is passed to the "super" constructor but then I found out that "super" works well for string, just enum is broken.
See this example:
import com.google.gson.Gson
import com.google.gson.GsonBuilder
import com.google.gson.typeadapters.RuntimeTypeAdapterFactory
open class Parent(val stringField: String,
val enumField: EnumField) {
enum class EnumField {
SUBTYPE1,
SUBTYPE2,
SUBTYPE3
}
}
class Subtype1() : Parent("s1", EnumField.SUBTYPE1)
class Subtype2(stringField: String) : Parent(stringField, EnumField.SUBTYPE2)
class Subtype3(stringField: String, type: EnumField) : Parent(stringField, type)
val subtypeRAF = RuntimeTypeAdapterFactory.of(Parent::class.java, "enumField")
.registerSubtype(Subtype1::class.java, Parent.EnumField.SUBTYPE1.name)
.registerSubtype(Subtype2::class.java, Parent.EnumField.SUBTYPE2.name)
.registerSubtype(Subtype3::class.java, Parent.EnumField.SUBTYPE3.name)
fun main() {
val gson = GsonBuilder()
.registerTypeAdapterFactory(subtypeRAF)
.create()
serializeAndDeserialize(gson, Subtype1()) // this works (but not suitable)
serializeAndDeserialize(gson, Subtype2("s2")) // broken
serializeAndDeserialize(gson, Subtype3("s3", Parent.EnumField.SUBTYPE3)) // broken
}
private fun serializeAndDeserialize(gson: Gson, obj: Parent) {
println("-----------------------------------------")
val json = gson.toJson(obj)
println(json)
val obj = gson.fromJson(json, Parent::class.java)
println("stringField=${obj.stringField}, enumField=${obj.enumField}")
}
Any ideas how to achieve to deserialization of enumField?
(deps: com.google.code.gson:gson:2.8.5, org.danilopianini:gson-extras:0.2.1)
P.S.: Note that I have to use RuntimeAdapterFactory because I have subtypes with different set of fields (I did not do it in the example so it is easier to understand).
Gson requires constructors without arguments to work properly (see deep-dive into Gson code below). Gson constructs raw objects and then use reflection to populate fields with values.
So if you just add some argument-less dummy constructors to your classes that miss them, like this:
class Subtype1() : Parent("s1", EnumField.SUBTYPE1)
class Subtype2(stringField: String) : Parent(stringField, EnumField.SUBTYPE2) {
constructor() : this("")
}
class Subtype3(stringField: String, type: EnumField) : Parent(stringField, type) {
constructor() : this("", EnumField.SUBTYPE3)
}
you will get the expected output:
-----------------------------------------
{"stringField":"s1","enumField":"SUBTYPE1"}
stringField=s1, enumField=SUBTYPE1
-----------------------------------------
{"stringField":"s2","enumField":"SUBTYPE2"}
stringField=s2, enumField=SUBTYPE2
-----------------------------------------
{"stringField":"s3","enumField":"SUBTYPE3"}
stringField=s3, enumField=SUBTYPE3
Gson deep-dive
If you want to investigate the internals of Gson, a tip is to add an init { } block to Subtype1 since it works and then set a breakpoint there. After it is hit you can move up the call stack, step through code, set more breakpoints etc, to reveal the details of how Gson constructs objects.
By using this method, you can find the Gson internal class com.google.gson.internal.ConstructorConstructor and its method newDefaultConstructor(Class<? super T>) that has code like this (I have simplified for brevity):
final Constructor<? super T> constructor = rawType.getDeclaredConstructor(); // rawType is e.g. 'class Subtype3'
Object[] args = null;
return (T) constructor.newInstance(args);
i.e. it tries to construct an object via a constructor without arguments. In your case for Subtype2 and Subtype3, the code will result in a caught exception:
} catch (NoSuchMethodException e) { // java.lang.NoSuchMethodException: Subtype3.<init>()
return null; // set breakpoint here to see
}
i.e. your original code fails since Gson can't find constructors without arguments for Subtype2 and Subtype3.
In simple cases, the problem with missing argument-less constructors is worked around with the newUnsafeAllocator(Type, final Class<? super T>)-method in ConstructorConstructor, but with RuntimeTypeAdapterFactory that does not work correctly.
I may be missing something in what you're trying to achieve, but is it necessary to use the RuntimeTypeAdapterFactory? If we take out the line where we register that in the Gson builder, so that it reads
val gson = GsonBuilder()
.create()
Then the output returns the enum we would expect, which looks to be serialising / deserialising correctly. I.e. the output is:
-----------------------------------------
{"stringField":"s1","enumField":"SUBTYPE1"}
stringField=s1, enumField=SUBTYPE1
-----------------------------------------
{"stringField":"s2","enumField":"SUBTYPE2"}
stringField=s2, enumField=SUBTYPE2
-----------------------------------------
{"stringField":"s3","enumField":"SUBTYPE3"}
stringField=s3, enumField=SUBTYPE3
It also may be an idea to implement Serializable in Parent. i.e.
open class Parent(val stringField: String, val enumField: EnumField) : Serializable {
enum class EnumField {
SUBTYPE1,
SUBTYPE2,
SUBTYPE3
}
}
Try adding #SerializedName annotation to each enum.
enum class EnumField {
#SerializedName("subtype1")
SUBTYPE1,
#SerializedName("subtype2")
SUBTYPE2,
#SerializedName("subtype3")
SUBTYPE3
}
I'm creating a builder (for Java compat), where context is both a private property and public method.
private lateinit var context: Context
fun context(appContext: Context) = apply {
context = appContext
}
fun build(): MySdk {
// this::context fails to compile because it cannot differentiate between the
// method `context()` vs property `context`
require(this::context.isInitialized) {
"context == null"
}
But I get a compilation issue for ::context.isInitialized, because it cannot differentiate between the method context() vs property context
Does Kotlin have a workaround for this? or am I forced to use unique property/method names?
This is a case of overload resolution ambiguity and the kotlin compiler is unable to identify whether you are using the property or the method.
This is because of callable references (::) . Internally when you are using the callable references it calls a method.
Callable references : References to functions, properties, and
constructors, apart from introspecting the program structure, can also
be called or used as instances of function types.
The common supertype for all callable references is KCallable, where R is the return value type, which is the property type for properties, and the constructed type for constructors.
KCallable<out R> // supertype for all callable references
So, for function the type is KFunction and for properties the type is KProperty
interface KFunction<out R> : KCallable<R>, Function<R> (source)
interface KProperty<out R> : KCallable<R> (source)
When you use a function like :
fun context(appContext: Context) = apply {
context = appContext
}
It can be used as a Function reference
::context // This is a Function reference i.e. KFunction
When you use a property reference, like
private lateinit var context: Context
fun something(){
::context // this is a property reference, KProperty
}
A property reference can be used where a function with one parameter is expected:
val strs = listOf("a", "bc", "def")
println(strs.map(String::length))
So, its not that Kotlin forces you to use different property and function names("although it is not recommended"). Its just that its unable to differentiate in this case as
Both are KCallable and have the same name
A property reference can be used where a function with one parameter is expected
You can resolve the ambiguity between the property and the method by giving the expected type:
val prop: kotlin.reflect.KProperty0<*> = this::context
Alas, prop.isInitialized then gives a compilation error:
This declaration can only be called on a property literal (e.g. 'Foo::bar')
So this doesn't appear to be possible currently. OTOH, since the error shows isInitialized is already handled specially by the compiler, it's likely possible to fix; I suggest reporting it on http://youtrack.jetbrains.com/ (after searching for duplicates).