Practice›Java Fundamentals›Module 11. Modern Java›11.5 Stream pipelines

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11.5 Stream pipelines — transforming with map

Lesson

Превратить каждый элемент в другой

mapToInt turned elements into numbers to add them up. The general version is map: it applies a lambda to every element and passes on the results, whatever their type.

Worked example

List<String> words = List.of("pear", "fig", "apple");

words.stream().map(w -> w.toUpperCase()).collect(Collectors.toList());
// [PEAR, FIG, APPLE]

With records this gets genuinely useful: filter the objects you want, then map them to the field you need.

students.stream()
        .filter(s -> s.score() >= 70)
        .map(s -> s.name())
        .collect(Collectors.toList());   // [Ada, Bo]

Read it aloud: take the students, keep those scoring 70 or more, take their names, collect them. Three lines that would have been a loop, a condition and an accumulator.

▶ Try it: run this, then change something and run it again.

import java.util.List;
import java.util.stream.Collectors;

record Student(String name, int score) { }

public class Main {
    public static void main(String[] args) {
        List<String> words = List.of("pear", "fig", "apple");
        System.out.println(words.stream().map(w -> w.toUpperCase()).collect(Collectors.toList()));
        List<Student> students = List.of(new Student("Ada", 90), new Student("Sam", 55), new Student("Bo", 70));
        System.out.println(students.stream().filter(s -> s.score() >= 70).map(s -> s.name()).collect(Collectors.toList()));
    }
}

map does what?

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In the pipeline filter(...).map(...), which runs first?

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Type exactly what this program prints.

import java.util.List;
import java.util.stream.Collectors;

public class Main { public static void main(String[] a){
    List<String> w = List.of("ab", "cde", "f");
    System.out.println(w.stream().map(s -> s.length()).collect(Collectors.toList()));
}}

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Why do records work well with streams?

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True or false?

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