Practice›Java Fundamentals›Module 11. Modern Java›11.4 Lambdas and streams

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11.4 Lambdas and streams — transforming and summing

Lesson

Turning elements into numbers and adding them

To add up values, turn the stream into a stream of ints with mapToInt(lambda), then call sum(). The lambda transforms each element; sum() adds the results. This one line replaces a loop with an accumulator.

Worked example

List<Integer> nums = List.of(1, 2, 3, 4);

int total = nums.stream().mapToInt(n -> n).sum();        // 10
int sumSquares = nums.stream().mapToInt(n -> n * n).sum();// 1+4+9+16 = 30

System.out.println(total);
System.out.println(sumSquares);

mapToInt(n -> n) just passes the values through; mapToInt(n -> n * n) squares them first.

▶ Try it: run this — a plain sum and a sum of squares, each in one line.

import java.util.List;
public class Main {
    public static void main(String[] args) {
        List<Integer> nums = List.of(1, 2, 3, 4);
        System.out.println(nums.stream().mapToInt(n -> n).sum());
        System.out.println(nums.stream().mapToInt(n -> n * n).sum());
    }
}

mapToInt(lambda) is used to...

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After mapToInt, calling sum() gives...

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Type exactly what this program prints.

import java.util.List;
public class Main { public static void main(String[] a){
    List<Integer> xs = List.of(2, 3, 4);
    System.out.println(xs.stream().mapToInt(n -> n * 10).sum());
}}

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The stream pipeline mapToInt(n -> n * n).sum() replaces...

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Match each stream operation to what it does.

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True or false?

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