say I desire to collect a list of doubles[] from a lambda function, I have created the following example, however it do not compile given the type. What could I do as a workaround?
public class CollectingStreams {
public static double [] methodReturnArray(int n){
return new double[] {1*n,2*n};
}
public static void main(String[] args){
List<double[]> listArray= IntStream.range(0,5).parallel().forEach(
(n) -> {methodReturnArray(n).collect(Collectors.toList());
});
}
}
Here is the compilation error:
java: cannot find symbol
symbol: method collect(java.util.stream.Collector<java.lang.Object,capture#1 of ?,java.util.List<java.lang.Object>>)
location: class double[]
As an attempt I have tried the following
List<double[]> listArray= IntStream.range(0,5).parallel().forEach(
(n) -> {
Arrays.stream(methodReturnArray(n)).collect(Collectors.toList());
});
However it prompts this compilation error:
java: method collect in interface java.util.stream.DoubleStream cannot be applied to given types;
required: java.util.function.Supplier<R>,java.util.function.ObjDoubleConsumer<R>,java.util.function.BiConsumer<R,R>
found: java.util.stream.Collector<java.lang.Object,capture#1 of ?,java.util.List<java.lang.Object>>
reason: cannot infer type-variable(s) R
(actual and formal argument lists differ in length)
Moreover I have seen (here)[https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html] that fiven side effects it is best to avoid arrays favoring collecting to lists, therefore I must use Arrays with care.
I am not sure exactly how to properly cast or modify my attempts do that I can collect my list of double[].
Any thoughts?
Thanks
CodePudding user response:
Instead of the .forEach()
use .mapToObject()
, as forEach doesn't return anything that could be processed further.
This is the code I would try:
List<double[]> listArray = IntStream.range(0,5)
.mapToObj(CollectingStreams::methodReturnArray)
.collect(Collectors.toList());
CodePudding user response:
List<int> numbers = new ArrayList<>();
List<double> numbers.stream().map(number -> (double) number).toList();