public/subjects/java/piscine/StreamCollect

StreamCollect

Instructions

Create a file StreamCollect.java.

Create a function mapByFirstLetter which maps the first letter (to upper case) to a list of Strings begining with the given letter.
Create a function getMaxByModulo4 which groups the integers by modulo 4 and returns a map associating each module 4 value to the max integer in each group. Create a function orderAndConcatWithSharp which joins the alphabetically sorted Strings with a ' # ' between each. The resulting String starts with a '{' and ends with a '}'.

Expected Functions

public class StreamCollect {
    public static Map<Character, List<String>> mapByFirstLetter(Stream<String> s) {
        // your code here
    }

    public static Map<Integer, Optional<Integer>> getMaxByModulo4(Stream<Integer> s) {
        // your code here
    }

    public static String orderAndConcatWithSharp(Stream<String> s) {
        // your code here
    }
}

Usage

Here is a possible ExerciseRunner.java to test your function :

import java.util.stream.Stream;

public class ExerciseRunner {
    public static void main(String[] args) {
        System.out.println(StreamCollect.mapByFirstLetter(Stream.of("Bonjour", "le", "monde !", "bonsoir")));
        System.out.println(StreamCollect.getMaxByModulo4(Stream.of(5, 12, 32, 4, 9, 17, 98, 424, 97, 5843, 48354)));
        System.out.println(StreamCollect.orderAndConcatWithSharp(Stream.of("Hello", "how are you ?", "where do you live ?", "Bordeaux")));
    }
}

and its output :

$ javac *.java -d build
$ java -cp build ExerciseRunner
{B=[Bonjour, bonsoir], L=[le], M=[monde !]}
{0=Optional[424], 1=Optional[97], 2=Optional[48354], 3=Optional[5843]}
##{Bordeaux # Hello # how are you ? # where do you live ?}
$

Notions

Stream