Use streams to filter and sum numbers

Description: From the numbers 1 to 10: keep the even ones, square them and print the list and the sum. Then group the words "kiwi", "fig", "apple", "pear", "plum" by their length.

Hint:

• IntStream.rangeClosed, filter, map, sum and Collectors.groupingBy.

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Solution

import java.util.List;
import java.util.Map;
import java.util.TreeMap;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class Main {
    public static void main(String[] args) {
        List<Integer> squares = IntStream.rangeClosed(1, 10)
                .filter(n -> n % 2 == 0)
                .map(n -> n * n)
                .boxed()
                .toList();
        System.out.println(squares);
        System.out.println(squares.stream().mapToInt(Integer::intValue).sum());

        Map<Integer, List<String>> byLength = List.of("kiwi", "fig", "apple", "pear", "plum").stream()
                .collect(Collectors.groupingBy(String::length, TreeMap::new, Collectors.toList()));
        System.out.println(byLength);
    }
}

How it works

• Each step takes the result of the one before it: filter, then map, then sum.
• groupingBy makes a map from the length to the list of words.

Read Java Streams.

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