Collectors.mapping method

[Alexandru Puiu]

Hi guys,

Can someone please help me understand why do I get an exception here:

import java.util.*;
import java.util.stream.*;

public class Test {


    public static void main(String[] args)  {

        Stream<String> stream = Stream.of("lions", "tigers", "bears");

        Map<Integer, Optional<Character>> map = stream
                .collect(Collectors.groupingBy(
                        String::length,
                        Collectors.mapping(s -> s.charAt(0),
                                Collectors.minBy(Comparator.naturalOrder()))));

        System.out.println(map);
    }

    
}

My compiler doesn't find anything wrong, but I get the following exception:

Error:(33, 26) java: no suitable method found for groupingBy(String::length,java.util.stream.Collector<java.lang.Object,capture#1 of ?,java.util.Optional<T>>)
    method java.util.stream.Collectors.<T,K,D,A,M>groupingBy(java.util.function.Function<? super T,? extends K>,java.util.function.Supplier<M>,java.util.stream.Collector<? super T,A,D>) is not applicable
      (cannot infer type-variable(s) T,K,D,A,M
        (actual and formal argument lists differ in length))
    method java.util.stream.Collectors.<T,K,A,D>groupingBy(java.util.function.Function<? super T,? extends K>,java.util.stream.Collector<? super T,A,D>) is not applicable
      (inferred type does not conform to upper bound(s)
        inferred: java.lang.Object
        upper bound(s): T,java.lang.Comparable<? super T>,java.lang.Object)
    method java.util.stream.Collectors.<T,K>groupingBy(java.util.function.Function<? super T,? extends K>) is not applicable
      (cannot infer type-variable(s) T,K
        (actual and formal argument lists differ in length))

[Alexandru Puiu]

Already found the answer! Thank's in advance!

[Campbell Ritchie]

That isn't an exception; that is a compile time error. Please show us how you corrected it.

[Piet Souris]

What, I wonder, is unclear about the error report?  

@OP
you can get rid of the Optional<Character> if you so wish, by using 'collectingAndThen'. For instance:

class Test {

    public static void main(String[] args) {

        Stream<String> stream = Stream.of("lions", "tigers", "bears");
        Comparator<Character> charComparator = Comparator.naturalOrder();
        Map<Integer, Character> map = stream
            .collect(
                groupingBy(
                    String::length,
                    mapping(
                        s -> s.charAt(0),
                        collectingAndThen(
                            minBy(charComparator),
                            Optional::get
                        )
                    )
                )
            )
        ;
        System.out.println(map);
    }
}