lambda

StreamsTest.java

package com.golonzovsky.streams;

import lombok.AllArgsConstructor;
import lombok.Data;
import org.junit.Before;
import org.junit.Test;

import java.io.BufferedReader;
import java.io.FileReader;
import java.time.Year;
import java.util.*;
import java.util.function.BinaryOperator;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;

public class StreamsTest {


    private List<Book> library;

    @Before
    public void setUp() throws Exception {
        Book nails = new Book("Fundamentsls of nail image", Arrays.asList("Li", "Fu", "Li"), new int[]{256}, Year.of(2014), 25.2, Topic.MEDICINE);
        Book dragon = new Book("Compilers: Principles, Techniques and Tools", Arrays.asList("Aho", "Lam", "Li"), new int[]{1009}, Year.of(2006), 23.6, Topic.COMPUTING);
        Book voss = new Book("Voss", Arrays.asList("Patrick white"), new int[]{478}, Year.of(1957), 29.8, Topic.FICTION);
        Book lotr = new Book("Lord Of The Rings", Arrays.asList("Tolkien"), new int[]{531, 416, 624}, Year.of(1955), 23.0, Topic.FICTION);
        Book history = new Book("Ancient History", Arrays.asList("Likonov", "Sokolsky"), new int[]{578}, Year.of(1979), 28.0, Topic.HISTORY);

        library = Arrays.asList(nails, dragon, voss, lotr, history);
    }


    @Test
    public void testStreams() throws Exception {
        Stream<Book> computingBooks = library.stream()
                .filter(b -> b.getTopic() == Topic.COMPUTING);

        Stream<String> bookTitles = library.stream()
                .map(Book::getTitle);

        Stream<Book> booksSortedByTitle = library.stream()
                .sorted(Comparator.comparing(Book::getTitle));

        //distinct authors in order of book titles
        Stream<String> authors = library.stream()
                .sorted(Comparator.comparing(Book::getTitle))
                .flatMap(b -> b.getAuthors().stream())
                .distinct();

        Stream<Book> first100inAlphabetical = library.stream().sorted(Comparator.comparing(Book::getTitle))
                .limit(100);
        Stream<Book> rest100inAlphabetical = library.stream().sorted(Comparator.comparing(Book::getTitle))
                .skip(100);

        Optional<Book> firstPublished = library.stream()
                .sorted(Comparator.comparing(Book::getYear))
                .findFirst();

        int totalAuthors = library.stream().mapToInt(b -> b.getAuthors().size()).sum();

        int totalPageCount1 = library.stream().flatMapToInt(b -> Arrays.stream(b.getPageCount())).sum();
        int totalPageCount2 = library.stream().flatMapToInt(b -> IntStream.of(b.getPageCount())).sum();

        List<Book> multipleAuthoredHistories = library.stream()
                .filter(b -> b.getTopic() == Topic.HISTORY)
                .peek(b -> System.out.println(b.getTitle()))
                .filter(b -> b.getAuthors().size() > 1)
                .collect(Collectors.toList());

        Stream<String> sortedTitles = library.stream().map(Book::getTitle).sorted();

        Stream<Book> sortedByAuthorCount = library.stream()
                .sorted(Comparator.comparing(Book::getAuthors, Comparator.comparing(List::size)));
    }

    @Test
    public void testOptionalsAndMatchers() throws Exception {
        // allMatch noneMatch anyMatch
        boolean withinHeight = library.stream().filter(b -> b.getTopic() == Topic.HISTORY)
                .mapToDouble(Book::getHeight).allMatch(i -> i < 19);

        // findAny findFirst
        Optional<Book> anyBook = library.stream().filter(b -> b.getAuthors().contains("Herman Melville")).findAny();

        BufferedReader br = new BufferedReader(new FileReader("mastering.txt"));
        Optional<String> line = br.lines().filter(s -> s.contains("findFirst")).findFirst();

        // optional
        String maybe = Optional.of("something").orElse("nothing");
        String maybe2 = Optional.<String>empty().orElseGet(() -> "nothing");
    }

    @Test
    public void testBasicReduction() throws Exception {
        // statistics
        IntSummaryStatistics intSummaryStatistics = IntStream.rangeClosed(1, 10).summaryStatistics();
        intSummaryStatistics.getMax();
        intSummaryStatistics.getMin();

        IntSummaryStatistics pageCountStatistics = library.stream()
                .mapToInt(b -> IntStream.of(b.getPageCount()).sum())
                .summaryStatistics();

        // object reduction

        Optional<Book> oldestBook = library.stream().min(Comparator.comparing(Book::getTopic));
        Optional<String> firstAlphabeticalTitle = library.stream().map(Book::getTitle).min(Comparator.naturalOrder());
        // Comparator.reverseOrder()

        // collectors
        Set<String> titles = library.stream()
                .map(Book::getTitle)
                .collect(Collectors.toSet());

        Map<String, Year> titleToPubDate = library.stream().collect(Collectors.toMap(Book::getTitle, Book::getYear)); // IllegalStateException on duplicate

    }

    @Test
    public void testCollectors() throws Exception {

        Map<Topic, List<Book>> booksByTopic = library.stream()
                .collect(Collectors.groupingBy(Book::getTopic));


        Map<String, Year> titleByLastPubDate = library.stream()
                .collect(Collectors.toMap(Book::getTitle, Book::getYear, (x, y) -> x.isAfter(y) ? x : y));

        Map<String, Year> titleToLastPubDateSorted = library.stream()
                .collect(Collectors.toMap(Book::getTitle,
                        Book::getYear,
                        BinaryOperator.maxBy(Comparator.naturalOrder()),
                        TreeMap::new));

        Map<Boolean, List<Book>> fictionNonFiction = library.stream()
                .collect(Collectors.partitioningBy(b -> b.getTopic() == Topic.FICTION));

        Map<Topic, Optional<Book>> bookWithMostAuthorsForEachTopic = library.stream()
                .collect(Collectors.groupingBy(Book::getTopic,
                        Collectors.maxBy(Comparator.comparing(b -> b.getAuthors().size()))));

        Map<Topic, Integer> volumeCountByTopic = library.stream()
                .collect(Collectors.groupingBy(Book::getTopic, Collectors.summingInt(b -> b.getPageCount().length)));

        Optional<Topic> mostPopularTopic = library.stream()
                .collect(Collectors.groupingBy(Book::getTopic, Collectors.counting()))
                .entrySet().stream()
                .max(Map.Entry.comparingByValue()) //.max(Comparator.comparing(Map.Entry::getValue))
                .map(Map.Entry::getKey);

        Map<Topic, String> concatTitlesByTopic = library.stream()
                .collect(Collectors.groupingBy(Book::getTopic, Collectors.mapping(Book::getTitle, Collectors.joining(","))));

        String concatTitles = library.stream().map(Book::getTitle).collect(Collectors.joining(", "));
        
        List<String> authorsForBooks = library.stream()
                .map(b -> b.getAuthors().stream().collect(Collectors.joining(", ", "'" + b.getTitle() + "': ", "")))//'Compilers: Principles, Techniques and Tools': Aho, Lam, Li
                .collect(Collectors.toList());
    }
    
    
}


@Data
@AllArgsConstructor
class Book {
    private String title;
    private List<String> authors;
    private int[] pageCount;
    private Year year;
    private double height;
    private Topic topic;
}

enum Topic {HISTORY, COMPUTING, FICTION, MEDICINE}