I have a class that needs to write to a file to interface with some legacy C++ application.
Since it will be instantiated several times in a concurrent manner,
it is a good idea to give the file an unique name.
I could use System.currentTimemili or hashcode, but there exists the possibility of collisions.
Another solution is to put a var field inside a companion object.
As an example, the code below shows one such class with the last solution, but I am not sure it is the best way to do it (at least it seems thread-safe):
case class Test(id:Int, data: Seq[Double]) {
//several methods writing files...
}
object Test {
var counter = 0
def new_Test(data: Seq[Double]) = {
counter += 1
new Test(counter, data)
}
}
Did you try this :
def uuid = java.util.UUID.randomUUID.toString
See UUID javadoc, and also How unique is UUID? for a discussion of uniqueness guarantee.
it is a good idea to give the file an unique name
Since all you want is a file, not id, the best solution is to create a file with unique name, not a class with unique id.
You could use File.createTempFile:
val uniqFile = File.createTempFile("myFile", ".txt", "/home/user/my_dir")
Vladimir Matveev mentioned that there is a better solution in Java 7 and later - Paths.createTempFile:
val uniqPath = Paths.createTempFile(Paths.get("/home/user/my_dir"), "myFile", ".txt"),
Related
I'm looking for help with a resurrection of a 6502 emulator I wrote in Java many moons ago and now converting to Kotlin. Yes, there's lot out there, but this is my implementation so I could learn how to create emulators and now, how to use Kotlin.
I require a Word class for addresses, i.e.:
class Word(initValue: Int = 0x0000) {
var value = initValue
get() = field
set(newValue) {
field = newValue and 0xFFFF
}
}
I can't extend Int, thus I assume I have an internal copy inside my class (if there's a better way, I'd love to hear it).
Using this:
val address = Word()
Is trivial and I can use it with lots of address.value += 123 to move to another location. Further to this, I can add functions to perform Add, Inc, Dec etc.
However, is there a way I can modify the class so I can:
address += 123
Directly?
I'm not sure how or what the approach is for this? I'd prefer NOT to have a lot of:
address.add(123) or address.value += 123
in my emulator.
Any advice would be really appreciated.
Unlike Java, Kotlin allows for operator overloading.
Find the documentation here
From the documentation you can use operator keyword to create overloaded function
data class Counter(val dayIndex: Int) {
operator fun plus(increment: Int): Counter {
return Counter(dayIndex + increment)
}
}
Perhaps I am simply reading old material, but I can't see a way to do something seemingly very simple.
I have a class called Robot, each instance of which needs a unique id. The id is simply an Int that should be 1,2,3... The normal solution would be to have a class var MaxId that you increment in the init() and then assign that to the instance's id.
Swift (4?) does not have class vars, but does have computed properties at the class level. However, I am a bit mystified about how one might use this to do a MaxId. Am I missing something blindingly obvious here?
a unique id. The id is simply an Int that should be 1,2,3.
You can certainly use a static property and increment it, but note that
those are two different requirements. If all you really want is a unique id, there is no need for the “least available integer” approach. Just use the built-in UUID struct and move on.
Static variables are essentially class variables. Try this in a playground:
class Numbered {
static var serial: Int = 1
let myID: Int
init() {
myID = Numbered.serial
Numbered.serial = Numbered.serial + 1
}
}
print(Numbered().myID)
print(Numbered().myID)
I'm having some issues with a Wicket (8.0.0-M4) NumberTextField in Kotlin (1.1.0).
My stripped-down form looks like this:
class Test : AbstractWebPage() {
val housenumberModel: Model<Int> = Model<Int>()
val housenumber = NumberTextField<Int>("housenumberModel", housenumberModel)
val form: Form<Unit> = object : Form<Unit>("adressForm") {}
override fun onInitialize() {
super.onInitialize()
form.add(housenumber.setRequired(false))
form.add(object : SubmitLink("submit") {
override fun onSubmit() {
super.onSubmit()
println(housenumberModel.`object`) // this is line 28
}
})
add(form)
}
}
After submitting the form I get the following stacktrace:
java.lang.ClassCastException: java.lang.String cannot be cast to
java.lang.Number
at com.mycompany.test.pages.Test$onInitialize$1.onSubmit(Test.kt:28)
at org.apache.wicket.markup.html.form.Form.delegateSubmit(Form.java:1312)
at org.apache.wicket.markup.html.form.Form.process(Form.java:979)
at org.apache.wicket.markup.html.form.Form.onFormSubmitted(Form.java:802)
at org.apache.wicket.markup.html.form.Form.onRequest(Form.java:715)
at org.apache.wicket.core.request.handler.ListenerRequestHandler.internalInvoke(ListenerRequestHandler.java:301)
at org.apache.wicket.core.request.handler.ListenerRequestHandler.invoke(ListenerRequestHandler.java:250)
at org.apache.wicket.core.request.handler.ListenerRequestHandler.invokeListener(ListenerRequestHandler.java:210)
at org.apache.wicket.core.request.handler.ListenerRequestHandler.respond(ListenerRequestHandler.java:203)
at org.apache.wicket.request.cycle.RequestCycle$HandlerExecutor.respond(RequestCycle.java:912)
at org.apache.wicket.request.RequestHandlerExecutor.execute(RequestHandlerExecutor.java:65)
at org.apache.wicket.request.cycle.RequestCycle.execute(RequestCycle.java:283)
at org.apache.wicket.request.cycle.RequestCycle.processRequest(RequestCycle.java:253)
at org.apache.wicket.request.cycle.RequestCycle.processRequestAndDetach(RequestCycle.java:221)
at org.apache.wicket.protocol.http.WicketFilter.processRequestCycle(WicketFilter.java:262)
at org.apache.wicket.protocol.http.WicketFilter.processRequest(WicketFilter.java:204)
at org.apache.wicket.protocol.http.WicketFilter.doFilter(WicketFilter.java:286)
[...]
If I use
val housenumberModel: Model<Int> = Model.of(0)
instead of
val housenumberModel: Model<Int> = Model<Int>()
everything works fine. But since my NumberTextField is optional I don't want to have it pre-initialized with 0.
Me and my colleagues were trying to change the type signature of the Model in every way we could imagine but came to no solution. A co-worker suggested to write a custom Wicket converter since Kotlins Int is represendeted as a primitive type (From the docs: "On the JVM, non-nullable values of this type are represented as values of the primitive type int.") Even though I don't know yet if this would work it seems like an overkill for me.
Another hack I could think of: writing some JavaScript to delete the zero from the input field. Also not really something I would want to do.
Question: Is there a simple solution to my problem?
(And as a bonus-question: has already anyone written a larger Wicket application in Kotlin and could tell me if this combination is ready for prime time to develop a critical project with this stack or is my problem just the tip of the iceberg?)
[edit]
Solution as pointed out by svenmeier:
Using
val housenumber = NumberTextField<Int>("housenumberModel", housenumberModel, Int::class.java)
works.
Or as an alternative:
val housenumbervalue: Int? = null
val housenumberModel: IModel<Int> = PropertyModel<Int>(this, "housenumbervalue")
val housenumber = NumberTextField<Int>("housenumberModel", housenumberModel)
Because of type erasure your NumberTextField cannot detect the generic type parameter of your model. Since your model object is null, it cannot be used to derive the type either.
In this case Wicket assumes a String model object type :/.
Either provide the type to the NumberTextField explicitly, or use a model that keeps its generic information, e.g. a PropertyModel.
There is a way to tell wicket about the type you want, it is by adding the type in the constructor. More here.
In Java it looks like this:
new NumberTextField<Integer>("housenumberModel", housenumberModel, Integer.class);
I'm fairly new to lua and have the following problem with an assignment from a class:
We currently extend lua to support objects and inheritance. The Syntax for that is
Class{'MyClass',
attribute1 = String,
attribute2 = Number
}
Class{'MySubClass', MyClass,
attribute3 = Number
}
This works perfectly fine. The real problem lies within the next task: We should support "recursive types", that means a call like
Class{'MyClass', attribute = MyClass}
should result in an class with a field of the same type as the class. When this "class-constructor" is called the variable MyClass is nil, thats why the parameter table doesnt't have an entry attribute. How is it possible to access this attribute?
My first thought was using some kind of nil-table which gets returned every time the global __index is called with an unset key. This nil-table should behave like the normal nil, but can be checked for in the "class-constructor". The problem with this approach are comparisons like nil == unknown. This should return true, but as the __eq meta method of the nil-table is never called we cannot return true.
Is there another approach I'm currently just ignoring? Any hint is greatly appreciated.
Thanks in advance.
Edit:
Here the relevant part of the "testfile". The test by which the code is rated in class is another one and gets published later.
three = 3
print( three == 3 , "Should be true")
print( unknown == nil , "Should be true" )
Class{'AClass', name = String, ref = AClass}
function AClass:write()
print("AClass:write(), name of AClass:", self.name)
end
aclass = AClass:create("A. Class")
aclass:write()
Since MyClass is just a lookup in the global table (_G), you could mess with its metatable's __index to return a newly-defined MyClass object (which you would later need to fill with the details).
However, while feasible, such an implementation is
wildly unsafe, as you could end up with an undefined class (or worse, you may end up inadvertantly creating an infinite lookup loop. Trust me, I've been there)
very hard to debug, as every _G lookup for a non-existing variable will now return a newly created class object instead of nil (this problem could somewhat be reduced by requiring that class names start with an uppercase character)
If you go that route, be sure to also override __newindex.
How about providing the argument in string form?
Class{'MyClass', attribute = 'MyClass'}
Detect strings inside the implementation of Class and process them with _G[string] after creating the class
Or alternatively, use a function to delay the lookup:
Class{'MyClass', attribute = function() return MyClass end}
One question I have about current Scala couchdb drivers is whether they can work with "partial" schemas". I'll try to explain what I mean: the libraries I've see seem to all want to do a complete conversion from JSON docs in the database to a Scala object, handle the Scala object, and convert it back to JSON. This is is fine if your application knows everything about that type of object --- especially if it is the sole piece of software interacting with that database. However, what if I want to write a little application that only knows about part of the JSON object: for example, what if I'm only interested in a 'mybook' component embedded like this:
{
_id: "0ea56a7ec317138700743cdb740f555a",
_rev: "2-3e15c3acfc3936abf10ea4f84a0aeced",
type: "user",
profiles: {
mybook: {
key: "AGW45HWH",
secret: "g4juh43ui9hg929gk4"
},
.. 6 or 7 other profiles
},
.. lots of other stuff
}
I really don't want to convert the whole JSON AST to a Scala object. On the other hand, in couchdb, you must save back the entire JSON doc, so this needs to be preserved somehow. I think what I really what is something like this:
class MyBook {
private val userJson: JObject = ... // full JSON retrieved from the database
lazy val _id: String = ... // parsed from the JSON
lazy val _rev: String = ... // parsed from the JSON
lazy val key: String = ... // parsed from the JSON
lazy val secret: String = ... // (ditto)
def withSecret(secret: String): MyBook = ... // new object with altered userJson
def save(db: CouchDB) = ... // save userJson back to couchdb
}
Advantages:
computationally cheaper to extract only needed fields
don't have to sync with database evolution except for 'mybook' part
more suitable for development with partial schemas
safer, because there is less change as inadvertently deleting fields if we didn't keep up with the database schema
Disadavantages:
domain objects in Scala are not pristinely independent of couch/JSON
more memory use per object
Is this possible with any of the current Scala drivers? With either of scouchdb or the new Sohva library, it seems not.
As long as you have a good JSON library and a good HTTP client library, implementing a schemaless CouchDB client library is really easy.
Here is an example in Java: code, tests.
My couchDB library uses spray-json for (de)serialization, which is very flexible and would enable you to ignore parts of a document but still save it. Let's look at a simplified example:
Say we have a document like this
{
dontcare: {
...
},
important: "foo"
}
Then you could declare a class to hold information from this document and define how the conversion is done:
case class Dummy(js:JsValue)
case class PartialDoc(dontcare: Dummy, important: String)
implicit object DummyFormat extends JsonFormat[Dummy] {
override def read(js:JsValue):Dummy = Dummy(js)
override def write(d:Dummy):JsValue = d.js
}
implicit val productFormat = jsonFormat2(PartialDoc)
This will ignore anything in dontcare but still safe it as a raw JSON AST. Of course this example is not as complex as the one in your question, but it should give you an idea how to solve your problem.