Java 8 map Integer stream to <T> stream

user_2625090 0
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I am a complete noob in Java 8 features and I am trying to generate a stream of Double objects as a result of processing a stream of Integers, something like this:

IntStream.range(0, 10).forEach(integer -> 50.0);

So I end up with a stream that looks like:

{50.0, 50.0, ... 50.0}

Now, I know forEach doesn't work like that and that this specific case can be solved in other simpler ways, but it's just for the sake of exposition.

Basically, what I am looking for is similar to a forEach except that it expects a return value for each element and is able to produce a stream too.

java-stream java-8
3 Answers

Generally speaking, the type of operation you're talking about is a map operation: you want to apply a function to each element of a list and return a new list of those results. Here is an example which converts a stream of integers into a custom class and then collects them into a List:

IntStream.range(1, 10)
    .mapToObj(n -> new MyClass(n))
    .collect(Collectors.toList());

// Alternatively - using a method reference 
IntStream.range(1, 10)
    .mapToObj(MyClass::new)
    .collect(Collectors.toList());

// Alternatively - convert from a stream of primitive ints to a stream 
// of Integer objects (i.e. boxed ints) first so we can use methods not
// available on the IntStream object
IntStream.range(1, 10).boxed()
    .map(MyClass::new)
    .collect(Collectors.toList());

As has been mentioned in another answer, you can also use the mapToDouble method for the specific case you mentioned.

user_3914520

You may use map helper function on your integer stream to do to. Check out this code below.

import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class MapTest {
    public static void main(String[] args) {
        System.out.println(IntStream.range(0, 10).mapToDouble(number -> 50.0).boxed().collect(Collectors.toList()));
    }
}
user_41085900

You can use Stream.generate instead of Stream.range. This is more appropriate for your case, because you don't use values from range:

List<Double> collect = Stream.generate(() -> Double.valueOf(50.0))
                             .limit(10).collect(Collectors.toList());
user_81980560
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