Collectors: counting, mapping, reducing

counting: https://stackabuse.com/guide-to-java-8-collectors-counting/

Guide to Java 8 Collectors: counting()

mapping: https://www.javabrahman.com/java-8/java-8-how-to-use-collectors-mapping-collector-with-examples/

Java 8 – How to use Collectors.mapping Collector with examples

reducing: https://codechacha.com/ko/java8-stream-reduction/

Java8의 Stream reduction 사용 방법 및 예제

 

1. counting을 groupingBy function과 함께 사용하면 통계를 내는데 유용하다.

package com.code.test;

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

public class Main {

    public static void main(String[] args) {
        List<Book> books = Arrays.asList(
                new Book("The Fellowship of the Ring", "J.R.R. Tolkien", 1954, 30),
                new Book("The Hobbit", "J.R.R. Tolkien", 1937, 40),
                new Book("Animal Farm", "George Orwell", 1945, 37),
                new Book("Nineteen Eighty-Four", "George Orwell", 1949, 55),
                new Book("The Road to Wigan Pier", "George Orwell", 1937, 25),
                new Book("Lord of the Flies", "William Golding", 1954, 44)
        );

        Map<String, Long> soldCopiesStats = books.stream()
                .collect(Collectors.groupingBy(Book::getAuthor, Collectors.counting()));

        System.out.println(soldCopiesStats);
    }

    public static class Book {
        private String title;
        private String author;
        private int releaseYear;
        private int soldCopies;


        public Book(String title, String author, int releaseYear, int soldCopies) {
            this.title = title;
            this.author = author;
            this.releaseYear = releaseYear;
            this.soldCopies = soldCopies;
        }

        public String getTitle() {
            return title;
        }

        public void setTitle(String title) {
            this.title = title;
        }

        public String getAuthor() {
            return author;
        }

        public void setAuthor(String author) {
            this.author = author;
        }

        public int getReleaseYear() {
            return releaseYear;
        }

        public void setReleaseYear(int releaseYear) {
            this.releaseYear = releaseYear;
        }

        public int getSoldCopies() {
            return soldCopies;
        }

        public void setSoldCopies(int soldCopies) {
            this.soldCopies = soldCopies;
        }
    }
}

결과

{J.R.R. Tolkien=2, William Golding=1, George Orwell=3}

 

2. downstream기준으로 mapper를 mapping시킨다. 

package com.code.test;

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

public class Main {

    public static void main(String[] args) {
        List<Book> books = Arrays.asList(
                new Book("The Fellowship of the Ring", "J.R.R. Tolkien", 1954, 30),
                new Book("The Hobbit", "J.R.R. Tolkien", 1937, 40),
                new Book("Animal Farm", "George Orwell", 1945, 37),
                new Book("Nineteen Eighty-Four", "George Orwell", 1949, 55),
                new Book("The Road to Wigan Pier", "George Orwell", 1937, 25),
                new Book("Lord of the Flies", "William Golding", 1954, 44)
        );
        List<String > titles = books.stream()
                .collect(Collectors.mapping(Book::getTitle, Collectors.toList()));
        System.out.println(titles);

        Optional<Integer> maxSoldCopies = books.stream()
                .collect(Collectors.mapping(Book::getSoldCopies, Collectors.maxBy(Integer::compareTo)));
        System.out.println(maxSoldCopies);
    }

    public static class Book {
        private String title;
        private String author;
        private int releaseYear;
        private int soldCopies;


        public Book(String title, String author, int releaseYear, int soldCopies) {
            this.title = title;
            this.author = author;
            this.releaseYear = releaseYear;
            this.soldCopies = soldCopies;
        }

        public String getTitle() {
            return title;
        }

        public void setTitle(String title) {
            this.title = title;
        }

        public String getAuthor() {
            return author;
        }

        public void setAuthor(String author) {
            this.author = author;
        }

        public int getReleaseYear() {
            return releaseYear;
        }

        public void setReleaseYear(int releaseYear) {
            this.releaseYear = releaseYear;
        }

        public int getSoldCopies() {
            return soldCopies;
        }

        public void setSoldCopies(int soldCopies) {
            this.soldCopies = soldCopies;
        }
    }
}

결과

[The Fellowship of the Ring, The Hobbit, Animal Farm, Nineteen Eighty-Four, The Road to Wigan Pier, Lord of the Flies] Optional[55]

 

3. 데이터를 더하거나 빼는 등의 연산을 수행한다.

package com.code.test;

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

public class Main {

    public static void main(String[] args) {
        List<Book> books = Arrays.asList(
                new Book("The Fellowship of the Ring", "J.R.R. Tolkien", 1954, 30),
                new Book("The Hobbit", "J.R.R. Tolkien", 1937, 40),
                new Book("Animal Farm", "George Orwell", 1945, 37),
                new Book("Nineteen Eighty-Four", "George Orwell", 1949, 55),
                new Book("The Road to Wigan Pier", "George Orwell", 1937, 25),
                new Book("Lord of the Flies", "William Golding", 1954, 44)
        );

        Optional<Integer> sum = books.stream()
                .collect(Collectors.mapping(Book::getSoldCopies, Collectors.toList()))
                .stream()
                .reduce((x, y) -> x + y);
        System.out.println(sum);
    }

    public static class Book {
        private String title;
        private String author;
        private int releaseYear;
        private int soldCopies;


        public Book(String title, String author, int releaseYear, int soldCopies) {
            this.title = title;
            this.author = author;
            this.releaseYear = releaseYear;
            this.soldCopies = soldCopies;
        }

        public String getTitle() {
            return title;
        }

        public void setTitle(String title) {
            this.title = title;
        }

        public String getAuthor() {
            return author;
        }

        public void setAuthor(String author) {
            this.author = author;
        }

        public int getReleaseYear() {
            return releaseYear;
        }

        public void setReleaseYear(int releaseYear) {
            this.releaseYear = releaseYear;
        }

        public int getSoldCopies() {
            return soldCopies;
        }

        public void setSoldCopies(int soldCopies) {
            this.soldCopies = soldCopies;
        }
    }
}

결과

Optional[231]