I've been wanting to check the location of my Generator and use streams to check if the location is valid.
The idea was as follows;
public Generator getGeneratorFromLocation(final Location location) {
for (List<Generator> generator : playerGeneratorMap.values()) {
for (Generator generator1 : generator) {
if (generator1.getGenLocation().equals(location)) {
return generator1;
}
}
}
return null;
}
I'm wanting to return a Generator from this using streams instead to try and learn more ways of doing it.
Current map:
public final Map<UUID, List<Generator>> playerGeneratorMap = new HashMap<>();
Any help would be greatly appreciated.
You can use AtomicRef object to init a retVal and then assign the wanted Generator to it in the lambda expression because regular vars can't be assigned in lambdas, only final or effectivly final can be used inside arrow functions.
This function should solve the problem :)
public Generator getGeneratorFromLocation(final Location location) {
AtomicReference<Generator> retVal = new AtomicReference<>(null);
playerGeneratorMap.values().stream().forEach(generators -> {
generators.forEach(generator -> {
if (generator.getLocation().equals(location)) {
retVal.set(generator);
}
});
});
return retVal.get();
}
By the way, streams are unnecessary because you have Collection.forEach instead of Stream.forEach, streams are used for more 'exotic' types of iterations like, filter, anyMatch, allMatch, reduce and such functionalities, you can read about Streams API on Oracle's website,
I'll link in the docs for you for future usage, important for functional proggraming.
Related
I suspect that my understanding of Streams in Dart might have a few holes in it...
I have a situation in which I'd like a Dart app to respond to intermittent input (which immediately suggests the use of Streamss -- or Futures, maybe). I can implement the behavior I want with listener functions but I was wondering how to do this in a better, more Dartesque way.
As a simple example, the following (working) program listens to keyboard input from the user and adds a div element to the document containing what has been typed since the previous space, whenever the space bar is hit.
import 'dart:html';
main() {
listenForSpaces(showInput);
}
void listenForSpaces(void Function(String) listener) {
var input = List<String>();
document.onKeyDown.listen((keyboardEvent) {
var key = keyboardEvent.key;
if (key == " ") {
listener(input.join());
input.clear();
} else {
input.add(key.length > 1 ? "[$key]" : key);
}
});
}
void showInput(String message) {
document.body.children.add(DivElement()..text = message);
}
What I'd like to be able to do is to create a new Stream from the Stream that I'm listening to (in the example above, to create a new Stream from onKeyDown). In other words, I might set the program above out as:
var myStream = ...
myStream.listen(showInput);
I suspect that there is a way to create a Stream and then, at different times and places, insert elements to it or call for it to emit a value: it feels as though I am missing something simple. In any case, any help or direction to documentation would be appreciated.
Creating a new stream from an existing stream is fairly easy with an async* function.
For a normal stream, I would just do:
Stream<String> listenForSpaces() async* {
var input = <String>[];
await for (var keyboardEvent in document.onKeyDown) {
var key = keyboardEvent.key;
if (key == " ") {
yield input.join();
input.clear();
} else {
input.add(key.length > 1 ? "[$key]" : key);
}
}
}
The async* function will propagate pauses through to the underlying stream, and it may potentially pause the source during the yield.
That may or may not be what you want, since pausing a DOM event stream can cause you to miss events. For a DOM stream, I'd probably prefer to go with the StreamController based solution above.
There are several methods and there is a whole package rxdart to allow all kinds of things.
Only the final consumer should use listen and only if you need to explicitly want to unsubscribe, otherwise use forEach
If you want to manipulate events like in your example, use map.
I wasn't originally planning to answer my own question but I have since found a very simple answer to this question in the dartlang creating streams article; in case it's helpful to others:
Specifically, if we'd like to create a stream that we can insert elements into at arbitrary times and places in the code, we can do so via the StreamController class. Instances of this class have an add method; we can simply use the instance's stream property as our stream.
As an example, the code in my question could be rewritten as:
import 'dart:html';
import 'dart:async';
main() async {
// The desired implementation stated in the question:
var myStream = listenForSpaces();
myStream.listen(showInput);
}
Stream<String> listenForSpaces() {
// Use the StreamController class.
var controller = StreamController<String>();
var input = List<String>();
document.onKeyDown.listen((keyboardEvent) {
var key = keyboardEvent.key;
if (key == " ") {
// Add items to the controller's stream.
controller.add(input.join());
input.clear();
} else {
input.add(key.length > 1 ? "[$key]" : key);
}
});
// Listen to the controller's stream.
return controller.stream;
}
void showInput(String message) {
document.body.children.add(DivElement()..text = message);
}
(As mentioned in the article, we need to be careful if we want to set up a stream from scratch like this because there is nothing to stop us from inserting items to streams that don't have associated, active subscribers; inserted items would in that case be buffered, which could result in a memory leak.)
I have multiple subscriptions from Cloud PubSub to read based on certain prefix pattern using Apache Beam. I extend PTransform class and implement expand() method to read from multiple subscriptions and do Flatten transformation to the PCollectionList (multiple PCollection on from each subscription). I have a problem to pass subscription prefix as ValueProvider into the expand() method, since expand() is called on template creation time, not when launching the job. However, if I only use 1 subscription, I can pass ValueProvider into PubsubIO.readStrings().fromSubscription().
Here's some sample code.
public class MultiPubSubIO extends PTransform<PBegin, PCollection<PubsubMessage>> {
private ValueProvider<String> prefixPubsub;
public MultiPubSubIO(#Nullable String name, ValueProvider<String> prefixPubsub) {
super(name);
this.prefixPubsub = prefixPubsub;
}
#Override
public PCollection<PubsubMessage> expand(PBegin input) {
List<String> myList = null;
try {
// prefixPubsub.get() will return error
myList = PubsubHelper.getAllSubscription("projectID", prefixPubsub.get());
} catch (Exception e) {
LogHelper.error(String.format("Error getting list of subscription : %s",e.toString()));
}
List<PCollection<PubsubMessage>> collectionList = new ArrayList<PCollection<PubsubMessage>>();
if(myList != null && !myList.isEmpty()){
for(String subs : myList){
PCollection<PubsubMessage> pCollection = input
.apply("ReadPubSub", PubsubIO.readMessagesWithAttributes().fromSubscription(this.prefixPubsub));
collectionList.add(pCollection);
}
PCollection<PubsubMessage> pubsubMessagePCollection = PCollectionList.of(collectionList)
.apply("FlattenPcollections", Flatten.pCollections());
return pubsubMessagePCollection;
} else {
LogHelper.error(String.format("No subscription with prefix %s found", prefixPubsub));
return null;
}
}
public static MultiPubSubIO read(ValueProvider<String> prefixPubsub){
return new MultiPubSubIO(null, prefixPubsub);
}
}
So I'm thinking of how to use the same way PubsubIO.read().fromSubscription() to read from ValueProvider. Or am I missing something?
Searched links:
extract-value-from-valueprovider-in-apache-beam - Answer talked about using DoFn, while I need PTransform that receives PBegin.
Unfortunately this is not possible currently:
It is not possible for the value of a ValueProvider to affect transform expansion - at expansion time, it is unknown; by the time it is known, the pipeline shape is already fixed.
There is currently no transform like PubsubIO.read() that can accept a PCollection of topic names. Eventually there will be (it is enabled by Splittable DoFn), but it will take a while - nobody is working on this currently.
You can use MultipleReadFromPubSub from apache beam io module https://beam.apache.org/releases/pydoc/2.27.0/_modules/apache_beam/io/gcp/pubsub.html
topic_1 = PubSubSourceDescriptor('projects/myproject/topics/a_topic')
topic_2 = PubSubSourceDescriptor(
'projects/myproject2/topics/b_topic',
'my_label',
'my_timestamp_attribute')
subscription_1 = PubSubSourceDescriptor(
'projects/myproject/subscriptions/a_subscription')
results = pipeline | MultipleReadFromPubSub(
[topic_1, topic_2, subscription_1])
I have implemented a dummy reactive repository but I am struggling with the update method:
#Override
public Mono<User> updateUser(int id, Mono<User> updateMono) {
return //todo with getUser
}
#Override
public Mono<User> getUser(int id) {
return Mono.justOrEmpty(this.users.get(id));
}
From one hand I have incoming publisher Mono<User> updateMono, from the other hand I have another publisher during Mono.justOrEmpty(this.users.get(id)).
How to combine it together, make update, and give back just one publisher?
The only thing come to my mind is:
#Override
public Mono<User> updateUser(int id, Mono<User> updateMono) {
return getUser(id).doOnNext(user -> {
updateMono.subscribe(update -> {
users.put(id, new User(id, update.getName(), update.getAge()));
System.out.format("Updated user with id %d to %s%n", id, update);
});
});
}
Is it correct?
See the reference guide on finding the right operator
Notably, for Mono you have and, when, then (note this last one will become flatMap in 3.1.0, and flatmap will become flatMapMany)
doOnNext is more for side operations like logging or stats gathering. Subscribe inside subscribe is another bad form; generally you want flatMap or similar instead.
I have played Spring 5 Reactive Streams features in these days, and have written down some sample codes(not public via blog or twitter yet, I still need more practice on Reactor).
I have encountered the same problems and finally used a Mono.zip to update the existing item in MongoDB.
https://github.com/hantsy/spring-reactive-sample/blob/master/boot-routes/src/main/java/com/example/demo/DemoApplication.java
public Mono<ServerResponse> update(ServerRequest req) {
return Mono
.zip(
(data) -> {
Post p = (Post) data[0];
Post p2 = (Post) data[1];
p.setTitle(p2.getTitle());
p.setContent(p2.getContent());
return p;
},
this.posts.findById(req.pathVariable("id")),
req.bodyToMono(Post.class)
)
.cast(Post.class)
.flatMap(post -> this.posts.save(post))
.flatMap(post -> ServerResponse.noContent().build());
}
Update: Another working version written in Kotlin.
fun update(req: ServerRequest): Mono<ServerResponse> {
return this.posts.findById(req.pathVariable("id"))
.and(req.bodyToMono(Post::class.java))
.map { it.t1.copy(title = it.t2.title, content = it.t2.content) }
.flatMap { this.posts.save(it) }
.flatMap { noContent().build() }
}
Scenario
If I want to read from a file and store the data in a Map, and if that map is being used multiple times for validation.
Is it possible for me to do this without having to change the return type of all methods, that use the above mentioned map, to Future?
Example:
Map metadata = null
Future readFromFile async {
.... metadata = await File.readingfromFile(...);
}
Future getRegion(..) async {
if(metadata == null) { await readFromFile() }
return metadata["region"]
}
Using the above code if a method(like isValidRegion,etc) that uses and needs getRegion(..) to complete, then the return type of isValidRegion should be converted to Future.
Future<bool> isValidRegion(..) async {
return ((await getRegionData(...)) != null )
}
If that isValidRegion is present within another methods, then the return type of them have to be changed to Future as well.
Future<String> parse(...) async {
....
if(await isValidRegion()) {
...
}
...
}
What is an elegant way to avoid this chain of futures as return types?
Async execution is contagious, there is nothing you can do to get back from async to sync execution.
What you can do is to do the read from the file synchronous to avoid the problem in the first place (if this is possible, if you read it from a network connection, this might not be possible).
I use leucene.net for my site and it Index some of the words fine and correct but it doesn't index some words like "الله"!
I have see the indexed file with Luke and it shows that "الله"is not indexed.
I have used ArabicAnalyzer for indexing.
you can see my site at www.qoranic.com , if you search "مریم" it will be ok but if you search "الله" it shows nothing.
any idea is appreciated in forward.
The ArabicAnalyzer does some transformation to that input; it will transform the input الله to له. This is due to the usage of the ArabicStemFilter (and ArabicStemmer) which is documented with ...
Stemming is defined as:
Removal of attached definite article, conjunction, and prepositions.
Stemming of common suffixes.
This shouldn't be an issue since you should be parsing the user provided query through the same analyzer when searching, producing the same tokens.
Here's the sample code I used to see what terms an analyzer produced from a given input.
using System;
using Lucene.Net.Analysis.AR;
using Lucene.Net.Analysis.Tokenattributes;
using System.IO;
namespace ConsoleApplication {
public static class Program {
public static void Main() {
var luceneVersion = Lucene.Net.Util.Version.LUCENE_30;
var input = "الله";
var analyzer = new ArabicAnalyzer(luceneVersion);
var inputReader = new StringReader(input);
var stream = analyzer.TokenStream("fieldName", inputReader);
var termAttribute = stream.GetAttribute<ITermAttribute>();
while(stream.IncrementToken()) {
Console.WriteLine("Term: {0}", termAttribute.Term);
}
Console.WriteLine("Done.");
Console.ReadLine();
}
}
}
You can overcome this behavior (remove the stemming) by writing a custom Analyzer which uses the ArabicNormalizationFilter, just as ArabicAnalyzer does, but without the call to ArabicStemFilter.
public class CustomAnalyzer : Analyzer {
public override TokenStream TokenStream(String fieldName, TextReader reader) {
TokenStream result = new ArabicLetterTokenizer(reader);
result = new LowerCaseFilter(result);
result = new ArabicNormalizationFilter(result);
return result;
}
}