stream().map(), stream().mapToInt()

mapはFunction
mapToIntはToIntFunction
ToIntFunctionはreturn int

package sampleproject;

import java.util.List;

import java.util.function.Function;
import java.util.function.ToIntFunction;
import java.util.stream.IntStream;
import java.util.stream.Stream;

class Item {
    int i;
    Item(int i){
        this.i = i;
    }
    int getI() {
        return this.i;
    }
}
public class SampleProject {
    public static void main(String[] args) {
        List<Integer> l = List.of(1,2,3,4,5);
        List<String> l2 = List.of("1","2","3","4","5");
        int[][] array = {{1,2},{3,4},{5,6}}; 
        Function<Integer,String> f = a -> String.valueOf(a) + "xxx";
        l.stream().<String>map(f).forEach(System.out::println);
        ToIntFunction<String> tf = s -> Integer.valueOf(s);
        List<Item> l3 = List.of(
                new Item(1),
                new Item(2),
                new Item(3),
                new Item(4),
                new Item(5)
        );
        l2.stream().mapToInt(tf).forEach(System.out::println);
        Stream<int[]> st = Stream.of(array);
        st.flatMapToInt(x -> IntStream.of(x)).forEach(System.out::println);
        System.out.println(l2.stream().mapToInt(tf).average().getAsDouble());
        l3.stream().mapToInt(Item::getI).average().getAsDouble();
    }    
}

1xxx
2xxx
3xxx
4xxx
5xxx
1
2
3
4
5
1
2
3
4
5
6
3.0