I am trying to pass arrays that can store value of Double, Int, Long or etc.
val input = arrayOf(1.3, 4.5)
val output = arrayOf(3) // Error Kotlin: Type mismatch: inferred type is Array<Int> but Array<Any> was expected
magic(input, output)
fun magic(input: Array<Any>, output: Array<Any>) {
// Do the magic
}
What type of parameters I have to use to do that?
You are probably looking for Number
fun magic(input: Array<Number>, output: Array<Number>) {
// Do the magic
}
val input = arrayOf<Number>(1.3, 4.5)
val output = arrayOf<Number>(3)
magic(input, output)
Based on your requirement, you could utilize the Number class. The documentation on the Kotlin website states that - Number is a "Superclass for all platform classes representing numeric values"
You could modify the magic function as follows -
fun magic(input: Array<Number>, output: Array<Number>) {
// Do the magic
}
Then, invoke the function by constructing the required parameters to the function -
val input = arrayOf<Number>(1.3, 4.5)
val output = arrayOf<Number>(3)
magic(input, output)
Related
I'm trying my hands on Kotlin. Being from a Python background is really giving me a tough time to get the knack of the Kotlin syntax. I'm trying to do a simple dictionary (Mutable Map) operation. However, its giving me exceptions.
This is what I tried. Kotlin compiler
Adding the code snippet for reference.
fun main() {
val openActivityMap = mutableMapOf<String, MutableMap<String, Any>>()
val packageName = "amazon"
val currentTime = 23454321234
if(openActivityMap.containsKey(packageName)){
if(openActivityMap[packageName]?.get("isAlreadyApplied")){
if((openActivityMap[packageName]?.get("lastAppliedAt") - currentTime) > 3600){
openActivityMap[packageName]?.put("isAlreadyApplied", false)
}
}
else{
openActivityMap[packageName]?.put("isAlreadyApplied", false)
}
}
}
I'm a bit late to the party, but I'd like to point out another solution here.
As I commented on the OP, heterogeneous maps with fixed string keys like this are usually better expressed with classes in Kotlin. For instance, in your case, the class for your main map's values could be the following:
data class PackageInfo(
var isAlreadyApplied: Boolean,
var lastAppliedAt: Long,
)
(you could obviously add more properties if need be)
This would save you all the casts on the final values.
Another point I'd like to make is that if you access the value for a key anyway, you don't need to check up front the existence of the key with containsKey. Maps return null for keys that are not associated with any value (this is why you need to check for null after getting the value).
The compiler cannot see the correlation between containsKey and the subsequent get or [] access. However, it's smart enough to understand a null check if you simply get the value first and then check for null.
This always applies unless you want to tell the difference between keys that aren't in the map and keys that are in the map but associated null values (which is quite rare).
All in all, I would write something like that:
fun main() {
val openActivityMap = mutableMapOf<String, PackageInfo>()
val packageName = "amazon"
val currentTime = 23454321234
val packageInfo = openActivityMap[packageName]
if (packageInfo != null) { // the key was found and the value is smart cast to non-null in the next block
if (packageInfo.isAlreadyApplied) {
if ((packageInfo.lastAppliedAt - currentTime) > 3600) {
packageInfo.isAlreadyApplied = false
}
} else {
packageInfo.isAlreadyApplied = false
}
}
}
data class PackageInfo(
var isAlreadyApplied: Boolean,
var lastAppliedAt: Long,
)
I would recommend writing tests first and working in small increments, but this should fix your compilation issues:
fun main() {
val openActivityMap = mutableMapOf<String, MutableMap<String, Any>>()
val packageName = "amazon"
val currentTime = 23454321234
if(openActivityMap.containsKey(packageName)){
if(openActivityMap[packageName]?.get("isAlreadyApplied") as Boolean){
if((openActivityMap[packageName]?.get("lastAppliedAt") as Long - currentTime) > 3600){
openActivityMap[packageName]?.put("isAlreadyApplied", false)
}
}
else {
openActivityMap[packageName]?.put("isAlreadyApplied", false)
}
}
}
EDIT: Also I prefer to avoid nullable variables and mutable objects in general, but I suppose there's an exception to every rule.
Couldn't you just declare your Map<String, Any> to return a Boolean instead of Any? So,
val openActivityMap = mutableMapOf<String, MutableMap<String, Boolean>>()
It looks like you're trying to use your second Map to store both Booleans and Ints, which is complicating the logic. You'll need to typecast if you decide to approach it without Typing.
There's a problem with the 2 statement below
if(openActivityMap[packageName]?.get("isAlreadyApplied"))
if((openActivityMap[packageName]?.get("lastAppliedAt") - currentTime) > 3600)
As we all know, an IF statement requires a boolean value for it's param. The types of both statement are unknown at compilation time as they are of a Generic type, Any. As such,
openActivityMap[packageName]?.get("isAlreadyApplied") could be a null or of type Any (Not Boolean).
openActivityMap[packageName]?.get("lastAppliedAt") could be a null or of type Any (an Int was expected here for computation).
This would throw compilation errors as the compiler does not know the types to go with. What could be done is to cast to it's proper types.
Solution
openActivityMap[packageName]?.get("isAlreadyApplied") as Boolean ?: false
((openActivityMap[packageName]?.get("lastAppliedAt") as Int ?: 0) - currentTime)
Giving a default value if it's null.
maybe you can try something like this
if (openActivityMap.containsKey(packageName)) {
val packageMap = openActivityMap[packageName]!!
val applyRequired = (packageMap["lastAppliedAt"] as Long - currentTime) > 3600
packageMap["isAlreadyApplied"] = packageMap.containsKey("isAlreadyApplied") && !applyRequired
}
btw. do you really want to have lastAppliedAt to be in te future? otherewise it will never be > 3600
This is the function declaration for rememberCoilPainter:
#Composable
fun rememberCoilPainter(
request: Any?,
imageLoader: ImageLoader = CoilPainterDefaults.defaultImageLoader(),
shouldRefetchOnSizeChange: ShouldRefetchOnSizeChange = ShouldRefetchOnSizeChange { _, _ -> false },
requestBuilder: (ImageRequest.Builder.(size: IntSize) -> ImageRequest.Builder)? = null,
fadeIn: Boolean = false,
fadeInDurationMs: Int = LoadPainterDefaults.FadeInTransitionDuration,
#DrawableRes previewPlaceholder: Int = 0,
): LoadPainter<Any> {
}
The line of code I am having difficulty understanding is:
requestBuilder: (ImageRequest.Builder.(size: IntSize) -> ImageRequest.Builder)? = null
A dot appears after Builder followed by (size: IntSize)
This is the first time I've seen this construct in Kotlin and am not sure how to interpret it. This is a lambda. Normally the dot after an object refers to a sub component of a class or a package. But the ( ) after the dot isn't clear.
How do I implement the requestBuilder parameter?
This is a function with receiver type as described here: https://kotlinlang.org/docs/lambdas.html#function-types
Function types can optionally have an additional receiver type, which is specified before a dot in the notation: the type A.(B) -> C represents functions that can be called on a receiver object of A with a parameter of B and return a value of C. Function literals with receiver are often used along with these types.
It could be tricky to understand at first, but this is like you are providing a function/lambda that is a method of ImageRequest.Builder. Or in other words: your lambda receives one additional parameter of type ImageRequest.Builder and it is available in the lambda as this.
You can provide requestBuilder as any other lambda, but note that inside it you will have access to properties and methods of ImageRequest.Builder object that was provided to you.
What you are looking at is a "function literal with receiver". Speaking generically, a type A.(B) -> C represents a function that can be called on a receiver object of A with a parameter of B and return a value of C. Or in your example:
requestBuilder: (ImageRequest.Builder.(size: IntSize) -> ImageRequest.Builder)?
We have a function requestBuilder which can be called on a ImageRequest.Builder with a parameter size: IntSize and returns another ImageRequest.Builder.
Calling this function is just like calling any other function with a lambda as a parameter. The difference: You have access to ImageRequest.Builder as this inside your lambda block.
Hope the following example helps understand lambdas with receiver type:
data class Person(val name: String)
fun getPrefixSafely(
prefixLength: Int,
person: Person?,
getPrefix: Person.(Int) -> String): String
{
if (person?.name?.length ?: 0 < prefixLength) return ""
return person?.getPrefix(prefixLength).orEmpty()
}
// Here is how getPrefixSafely can be called
getPrefixSafely(
prefixLength = 2,
person = Person("name"),
getPrefix = { x -> this.name.take(x) }
)
How do I implement the requestBuilder parameter?
Hope this part of the code snippet answers the above:
getPrefix = { x -> this.name.take(x) }
PS: These lambdas with receiver types are similar to extension functions IMO.
This is the main body of my function
val client = ConnectionFactory.createClient() # <- Return lettice.io RedisClusterClient
val conn = client.connect()
val command = conn.sync()
var index: String? = null
index = readDataStructure(command, key)
This is my first try to define my readDataStructure function:
fun readDataStructure(command: RedisCommand, key: String): String {
...
kotlin complaints error: 3 type arguments expected for interface RedisCommand<K : Any!, V : Any!, T : Any!>
I want to be able to NOT specifying K, V and T because I am just writing a throwaway script.
Is there any Kotlin lang syntax and can allow me to just pass the command variable as is?
I suppose you are after:
fun readDataStructure(command: RedisCommand<*,*,*>, key: String): String {
?
From Kotlin docs https://kotlinlang.org/docs/tutorials/kotlin-for-py/generics.html:
If you don't have any idea (or don't care) what the generic type might be, you can use a star-projection:
fun printSize(items: List<*>) = println(items.size)
When using a generic type where you have star-projected one or more of its type parameters, you can:
Use any members that don't mention the star-projected type parameter(s) at all
Use any members that return the star-projected type parameter(s), but the return type will appear to be Any? (unless the type parameter is constrained, in which case you'll get the type mentioned in the constraint)
Not use any members that take a star-projected type as a parameter
I'm using jackson-module-kotlin:2.11.2 to parse some YAML. I'm trying to produce objects which contain a map, whose values are objects of a class that I have declared. This map instead contains values which are HashMaps.
Here are my declarations:
import com.fasterxml.jackson.module.kotlin.readValue
object Parser {
// ObjectMapper is thread safe as long as we don't mess with the config after this declaration.
val mapper: ObjectMapper = ObjectMapper(YAMLFactory()).registerKotlinModule()
.registerModule(JavaTimeModule())
.registerModule(nullMapDeserialiserModule)
.registerModule(SimpleModule().setDeserializerModifier(ValidatingDeserializerModifier()))
// When parsing timestamps, we don't want to lose the offset information
.disable(DeserializationFeature.ADJUST_DATES_TO_CONTEXT_TIME_ZONE)
// We would prefer an error if we're trying to store a float in an int
.disable(DeserializationFeature.ACCEPT_FLOAT_AS_INT)
// If a primitive field (like Int) is non-nullable (as in the Kotlin meaning), then we don't want nulls being converted to 0
.enable(DeserializationFeature.FAIL_ON_NULL_FOR_PRIMITIVES)
// Because enums could change order in future versions (if we keep them in lexicographic order, for example),
// we don't want the client to expect that they can give us the ordinal value of the enum.
.enable(DeserializationFeature.FAIL_ON_NUMBERS_FOR_ENUMS)
// When serialising schedule expressions, don't include null values
.setSerializationInclusion(JsonInclude.Include.NON_NULL)
#Throws(JsonProcessingException::class)
inline fun <reified T: Any> deserialise(yaml: String): T = mapper.readValue(yaml)
}
data class ListValue (
val listValueKey: String,
val someOtherValue: Int
)
data class ExpectedValue (
val expectedValueKey: String,
val list: List<ListValue>
)
data class TestClass (
val testClassKey: String,
#param:JsonDeserialize(contentAs = ExpectedValue::class)
val testMap: Map<String, ExpectedValue>
)
Here is my test case:
#Test
fun `map parse test`() {
val testObj: TestClass? = RuleParser.deserialise(
//language=YAML
"""
testClassKey: the-key
testMap:
key1:
expectedValueKey: subKey1
list:
- listValueKey: someKey1
someOtherValue: 5
- listValueKey: anotherKey1
someOtherValue: 6
key2:
expectedValueKey: subKey2
list:
- listValueKey: someKey2
someOtherValue: 7
- listValueKey: anotherKey2
someOtherValue: 8
"""
)
assertTrue(testObj is TestClass)
assert(testObj.testMap is HashMap)
assertNotNull(testObj.testMap["key1"])
assert(testObj.testMap["key1"] is ExpectedValue)
assertEquals(
ExpectedValue(
expectedValueKey = "subKey1",
list = listOf(ListValue("someKey1", 5), ListValue("anotherKey1", 6))
),
testObj.testMap["key1"]
)
}
Currently, this test is failing on the final assertion, with the following error
Expected :ExpectedValue(expectedValueKey=subKey1, list=[ListValue(listValueKey=someKey1, someOtherValue=5), ListValue(listValueKey=anotherKey1, someOtherValue=6)])
Actual :{expectedValueKey=subKey1, list=[{listValueKey=someKey1, someOtherValue=5}, {listValueKey=anotherKey1, someOtherValue=6}]}
This is clearly not what I expected. If I instead parse a list of a declared class, this works correctly (example test follows).
#Test
fun `list parse test`() {
val testObj: ExpectedValue? = RuleParser.deserialise(
//language=YAML
"""
expectedValueKey: subKey1
list:
- listValueKey: someKey1
someOtherValue: 5
- listValueKey: anotherKey1
someOtherValue: 6
"""
)
assertTrue(testObj is ExpectedValue)
assertTrue(testObj.list[0] is ListValue)
assertEquals(
ListValue("someKey1", 5),
testObj.list[0]
)
}
So I'm a bit surprised that it is possible to parse a generic list in this way, but not a map. How do I get Jackson to create the map values that I expect, rather than HashMaps?
Your deserializer function is wrong. You must use the reified generic type in readValue method:
inline fun <reified T: Any> deserialise(yaml: String): T = mapper.readValue(yaml, T::class.java)
While it is odd that the annotation is needed at all (since it is not needed for lists of a declared type), it does work if the annotation is used as follows:
#field:JsonDeserialize(`as` = HashMap::class, contentAs = ExpectedValue::class)
This was not clear at first, because that javadoc for contentAs does not mention that as is also required.
I'm trying to understand why let is needed. In the example below I have a class Test with a function giveMeFive:
public class Test() {
fun giveMeFive(): Int {
return 5
}
}
Given the following code:
var test: Test? = Test()
var x: Int? = test?.giveMeFive()
test = null
x = test?.giveMeFive()
x = test?.let {it.giveMeFive()}
x gets set to 5, then after test is set to null, calling either of the following statements return null for x. Given that calling a method on a null reference skips the call and sets x to null, why would I ever need to use let? Are some cases where just ?. won't work and let is required?
Further, if the function being called doesn't return anything, then ?. will skip the call and I don't need ?.let there either.
let()
fun <T, R> T.let(f: (T) -> R): R = f(this)
let() is a scoping function: use it whenever you want to define a variable for a specific scope of your code but not beyond. It’s extremely useful to keep your code nicely self-contained so that you don’t have variables “leaking out”: being accessible past the point where they should be.
DbConnection.getConnection().let { connection ->
}
// connection is no longer visible here
let() can also be used as an alternative to testing against null:
val map : Map<String, Config> = ...
val config = map[key]
// config is a "Config?"
config?.let {
// This whole block will not be executed if "config" is null.
// Additionally, "it" has now been cast to a "Config" (no
question mark)
}
You need to use let if you want to chain function calls that aren't defined on the type you are chaining from.
Let's say the definition of your function was this instead:
// Not defined inside the Test class
fun giveMeFive(t: Test) {
return 5
}
Now, if you have a nullable Test and want to call this function in a chain, you have to use let:
val x = test?.let { giveMeFive(it) }
The .let{} extension function in Kotlin:
Takes the object reference as the parameter on which .let{} is called.
Returns value of any non-primitive data-type which has been returned from with let{} function. By default, it returns undefined value of kotlin.Any class.
Declaration in package kotlin:
public inline fun <T, R> T.let(block: (T) -> R): R {
return block(this)
}
Simple practical demonstration to see how .let{} extension function works in Kotlin.
Sample Code 1:-
val builder = StringBuilder("Hello ")
println("Print 0: $builder")
val returnVal = builder.let { arg ->
arg.append("World")
println("Print 1: $arg")
"Done" // Returnning some string
}
println("Print 2: $builder")
println("Print 3: $returnVal")
Sample Code 1 Output:-
Print 0: Hello
Print 1: Hello World
Print 2: Hello World
Print 3: Done
In Sample Code 1:
We created the final object of type StringBuilder with initialization value "Hello ".
In builder.let{}, the builder object reference will be passed to arg.
Here, the output Print 2: Hello World and Print 3: Hello World means that the builder and arg, both are pointing to the same StringBuilder object-reference. That's why they both print the same String value.
In the last line of .let{} function block, we are returning "Done" String value which will be assigned to returnVal.
*Hence, we get Print 3: Done output from returnVal.
Sample Code 2:-
val builder = StringBuilder("Hello ")
println("Print 0: $builder")
val returnVal = builder.let { arg ->
arg.append("World")
println("Print 1: $arg")
arg.length
}
println("Print 2: $builder")
println("Print 3: $returnVal") // Now return val is int = length of string.
Sample Code 2 Output:-
Print 0: Hello
Print 1: Hello World
Print 2: Hello World
Print 3: 11
In Sample Code 2:
What's difference:
In the last line of .let{} function block, we are returning the Int value equals to the length of StringBuilder object arg whose value at this line of code is "Hello World". So length = 11 of type Int will be assigned to returnVal.
*Hence, we get Print 3: 11 as output from returnVal.
Also try these:-
.run() {}
.apply() {}
with(obj) {}
.also() {}
Happy Coding...
fun <T, R> T.let(f: (T) -> R): R = f(this)
.let block does not equal to in multiThreading
val x? = null
if(x == null) {
}
and
x?.let{
}
.let block is thread-safe