Restrict CPU usage by java8 parallel stream/High CPU usage by Java 8 parallel stream

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• Error with dx.jar in making an Android app

  I have a problem. I started to learn to make Android apps in Java (I dont have any experience) andenter image description here I cant run my app because of an error which says that it cant load the dx.jar but it exists. Any help?

  
  
• Error:Failed to open zip file

  Error:Failed to open zip file. Gradle 's dependency cache may be corrupt (this sometimes occurs after a network connection timeout.) Re-download dependencies and sync project (requires network) Re-download dependencies and sync project (requires network)
• RxJava2: Problems with retryWhen()

  I'm trying to implement a retryWhen operation for my Completable chain using this blog post as a basis, but I keep getting errors. I'm trying to check whether the error was a HttpException and if so, refresh the token and try again. The chain looks something like this:

  Completable c = ...;
  
  c = c.retryWhen(errors -> errors.flatMap(error -> {
      if (error instanceof HttpExcetion) {
          return authHandler.refreshToken().flatMap(token -> Completable.complete());
      }
      return Completable.error(error);
  }));
  

  However, this won't compile, and I just can't figure out what I'm doing wrong here. Since I'm using RxJava 2 I shouldn't emit null like they do on the article, but that wouldn't seem to fix the issue anyway. I even tried a more simple version, basically copied straight from that article and replaced Observable with Completable:

  c.retryWhen(errors -> errors.flatMap(error -> {
          return Completable.timer(5, TimeUnit.SECONDS);
      }));
  

  This won't compile either. The error is this:

  Error:(31, 51) error: incompatible types: cannot infer type-variable(s) R
  (argument mismatch; bad return type in lambda expression
  Completable cannot be converted to Publisher<? extends R>)
  where R,T are type-variables:
  R extends Object declared in method <R>flatMap(Function<? super T,? extends Publisher<? extends R>>)
  T extends Object declared in class Flowable
  

  Any idea what's going on here?
• Java Stream issue

  I'm training using streams in java.

  That's the request: returns list of strings containing the name of the doctor and the relative number of patients sorted by decreasing number. The string must be formatted as "### : ID SURNAME NAME" where ### represent the number of patients (printed on three characters).

  public Collection<String> doctorsByNumPatients(){
          SortedMap<Integer, List<String>> a= doctors.values().stream().
                  collect(groupingBy(d->d.getPatients().size(), ()->new TreeMap<Integer, List<String>>(reverseOrder()), mapping(Doctor::toString, toList())));
  
          return a.entrySet().stream().
                  ??????????
      }
  

  (Doctor::toString returns ID NAME SURNAME)

  I wrote this code but i can't finish it cause i don't know how to turn the map into the string's list as suggested in the request, using stream. I solved a similar problem but dealing the map with a loop in which for each iteration I initialized an iterator to scan the value (list) and generate a string for each doctor in any list.
• Java 8 Streams parsing to Integer

  Does it exist better way to parse String to Integer using stream than this :

   String line = "1 2 3 4 5";
  List<Integer> elements = Arrays.stream(line.split(" ")).mapToInt(x -> Integer.parseInt(x))
      .boxed().collect(Collectors.toList());
  
• Java 8 Consumer split by Line

  Can someone tell me how i should use consumers to reach the same output, because this doesn't work ?

  String names = "John Alex Peter";
  String[] namesSplitted = names.split(" ");
  for (String s : namesSplitted) {
      System.out.println(s);
  }
  
  Consumer<String> cons = x -> System.out.println(x.split(" "));
  cons.accept(names);
  
• Java flatmap Iterator<Pair<Stream<A>, Stream<B>>> to Pair<Stream<A>, Stream<B>>

  I'm trying to implement a method with the following signature:

  public static <A,B> Pair<Stream<A>, Stream<B>> flatten(Iterator<Pair<Stream<A>, Stream<B>>> iterator);
  

  Where the goal of the method is to flatten each of the stream types into a single stream and wrap the output in a pair. I only have an Iterator (not an Iterable) and I can't alter the method signature, so I have to perform the flattening in a single iteration.

  My current best implementation is

  public static <A,B> Pair<Stream<A>, Stream<B>> flatten(Iterator<Pair<Stream<A>, Stream<B>> iterator) {
      Stream<A> aStream = Stream.empty();
      Stream<B> bStream = Stream.empty();
      while(iterator.hasNext()) {
          Pair<Stream<A>, Stream<B>> elm = iterator.next();
          aStream = Stream.concat(aStream, elm.first);
          bStream = Stream.concat(bStream, elm.second);
      }
      return Pair.of(aStream, bStream);
  }
  

  But while this is technically correct I'm not super happy with this for two reasons:

  1. Stream.concat warns against doing this kind of thing because it may lead to a StackOverflowError.
  2. Stylistically I'd rather it be purely functional if possible instead of having to loop over the iterator and re-assign the streams throughout.

  It feels like Stream#flatMap should be suited here (after transforming the input Iterator to a Stream using Guava's Streams.stream(Iterator), but it seems to not work because of the Pair type in the middle.

  One additional requirement is that any of the iterator/streams may be very large (the input could contain anywhere from a single pair of exceedingly large streams to many of one item streams, for example) so solutions ideally shouldn't contain collecting results into in-memory collections.
• Collect a Stream of strings into a Map

  I have a csv file of user details which I will have to load into a HashMap by reading each line. This is done using a little help from Java 8 streams.

   Map<String,MockUser> users = new HashMap<>();
   try (Stream<String> stream = Files.lines(Paths.get(f.getAbsolutePath()))) {
              stream.forEach(s -> {
                  String[] fields = s.split(",");
                  MockUser user = new MockUser(fields[0], fields[1], fields[2]);
                  users.put(user.getUsername(), user);
              });    
          } catch (IOException e) {
              e.printStackTrace();
          }
  
  
  private class MockUser {
          String username;
          String password;
          String role;
  
          MockUser(String username, String password, String role) {
              this.username = username;
              this.password = password;
              this.role = role;
          }
  
          public String getUsername() {
              return username;
          }
  
          public void setUsername(String username) {
              this.username = username;
          }
  
          public String getPassword() {
              return password;
          }
  
          public void setPassword(String password) {
              this.password = password;
          }
  
          public String getRole() {
              return role;
          }
  
          public void setRole(String role) {
              this.role = role;
          }
      }
  

  This works well. However, I had to write a 3 line code to split and add the stream results into a map. I tried using Stream.collect , But I quite cant get this thing right?

  Is there a way I can simplify the above code using Stream.collect?