How to turn a number reading directly into a stream?

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Recently I wrote a reply exploring a little more the API of stream of Java 8, but I must say I was disappointed with some details of my code.

The idea was to read a sequence of numbers and then return what was the highest and lowest value of the readings taken. In this case, the entry ended when the number 0 was entered.

I don’t know how to skillfully use an iterable with the Apis of stream. So I decided to go by the pig method and goal: I can read dice and play in a ArrayList to then use this list quietly to stream. The resulting code was this:

Scanner input = new Scanner(System.in); 
ArrayList<Double> lista = new ArrayList<>();

double valorLido;
while (true) {
  valorLido = input.nextDouble();
  if (valorLido == 0) {
    break;
  }
  lista.add(valorLido);
}
DoubleSummaryStatistics summary = lista.stream().collect(Collectors.summarizingDouble(Double::doubleValue));
double maior = summary.getMax();
double menor = summary.getMin();
System.out.println("maior " + maior);
System.out.println("menor " + menor);

This code did not please me. I would like to be able to directly use the reading result to pass to stream no need to transform into any kind of collection.

For example, if I were to obtain an eternal reading, I would do so:

Iterable<Double> leitorDoubleAteh0(Scanner in) {
  return () -> new Iterator<Double>() {
    boolean conseguiuFetch;
    double valorFetch;
    void fetchNext() {
        valorFetch = in.nextDouble();
        conseguiuFetch = valorFetch != 0;
    }

    @Override
    public boolean hasNext() {
      return conseguiuFetch;
    }

    @Override
    public Double next() {
      if (!conseguiuFetch) {
        throw new NoSuchElementException("fim da leitura");
      }
      double valorRetorno = valorFetch;
      fetchNext();
      return valorRetorno;
    }

    {
      fetchNext();
    }
  };
}

That would at least give me access to foreach, but still it is not my desired stream.

java java-8
  • 1

    I don’t know if I fully understood the question, but if the idea is how to go directly to min and max can do without collecting with lista.stream().max(Double::compare).get() since the max and min of the stream take a Comparator

  • Fair, but that doesn’t explain how to turn a reading sequence into a stream without going through a collection.

  • The big problem here is that reading from the console you don’t get an EOF. A solution with a number of known lines is elegant but helps you?

  • @Julianoalves known and limited number would not be my focus. The intention is to avoid loading an entire list into memory. I would like to work with stream using o(1) memory, no o(n)

1 answer

2


With the method Stream.generate(Supplier<? extends T>), you can read the numbers of Scanner without needing the ArrayList.

I know the tag is . But if you accept to use Java 9 or higher, you can use method Stream.takeWhile(Predicate<? super T>) to make the stop criteria.

That is the result:

Scanner input = new Scanner(System.in);
DoubleSummaryStatistics summary = Stream
        .generate(input::nextDouble)
        .takeWhile(z -> z != 0)
        .collect(Collectors.summarizingDouble(Double::doubleValue));
double maior = summary.getMax();
double menor = summary.getMin();
System.out.println("maior " + maior);
System.out.println("menor " + menor);

  • I have code limitations for Java 8 (GWT and Totalcross support, the versions used only support Java 8), but obviously I accept new suggestions. I’m negotiating with the operations people to try to move up to Java 10 on trial

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