Core Java
Yet another Java 8 Lamdbas and Streams example
I’ve been lagging behind with what Java 8 features exercising concerns, so in this post I will briefly present my initial experience with lambdas and streams.
As usual, I will focus on a Podcast class:
001 002 003 004 005 006 007 008 009 010 011 012 013 014 015 016 017 018 019 020 021 022 023 024 025 026 027 028 029 030 031 032 033 034 035 036 037 038 039 040 041 042 043 044 045 046 047 048 049 050 051 052 053 054 055 056 057 058 059 060 061 062 063 064 065 066 067 068 069 070 071 072 073 074 075 076 077 078 079 080 081 082 083 084 085 086 087 088 089 090 091 092 093 094 095 096 097 098 099 100 101 102 103 | package org.codingpedia.learning.java.core; import java.util.Comparator; public class Podcast { int id; String title; String producer; int subscriptionsNumber; /** number of up votes(likes) */ int upVotes; /** number of down votes*/ int downVotes; public Podcast() { this .subscriptionsNumber = 0 ; } public Podcast( int id, String title, String producer, int subscriptionsNumber, int upVotes, int downVotes) { this .id = id; this .title = title; this .producer = producer; this .subscriptionsNumber = subscriptionsNumber; this .upVotes = upVotes; this .downVotes = downVotes; } public static final Comparator<Podcast> BY_POSITIVE_VOTES_DIFFERENCE = (left, right) -> (right.getUpVotes()-right.getDownVotes()) - (left.getUpVotes()-left.getDownVotes()); @Override public String toString() { return "Podcast{" + "title='" + title + '\ '' + ", producer='" + producer + '\ '' + ", upVotes=" + upVotes + ", downVotes=" + downVotes + ", subscriptionsNumber=" + subscriptionsNumber + '}' ; } public static String toJSON(Podcast p) { return "{" + "title:'" + p.title + '\ '' + ", producer:'" + p.producer + '\ '' + ", upVotes:" + p.upVotes + ", downVotes:" + p.downVotes + ", subscriptionsNumber:" + p.subscriptionsNumber + "}" ; } public int getUpVotes() { return upVotes; } public void setUpVotes( int upVotes) { this .upVotes = upVotes; } public int getDownVotes() { return downVotes; } public void setDownVotes( int downVotes) { this .downVotes = downVotes; } public String getTitle() { return title; } public void setTitle(String title) { this .title = title; } public String getProducer() { return producer; } public void setProducer(String producer) { this .producer = producer; } public int getSubscriptionsNumber() { return subscriptionsNumber; } public void setSubscriptionsNumber( int subscriptionsNumber) { this .subscriptionsNumber = subscriptionsNumber; } public int getId() { return id; } public void setId( int id) { this .id = id; } } |
which I will use in different operations, built with lambdas and streams. But I will let the code speak for itself this time:
Lambdas and streams example
001 002 003 004 005 006 007 008 009 010 011 012 013 014 015 016 017 018 019 020 021 022 023 024 025 026 027 028 029 030 031 032 033 034 035 036 037 038 039 040 041 042 043 044 045 046 047 048 049 050 051 052 053 054 055 056 057 058 059 060 061 062 063 064 065 066 067 068 069 070 071 072 073 074 075 076 077 078 079 080 081 082 083 084 085 086 087 088 089 090 091 092 093 094 095 096 097 098 099 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 | package org.codingpedia.learning.java.core; import java.util.*; import java.util.function.Predicate; import java.util.stream.Collectors; public class LambdasAndStreams { public static void main(String[] args) { List<Podcast> podcasts = Arrays.asList( //new Podcast(podcastId, title, producer, subscriptionsNumber, upVotes, downVotes), new Podcast( 1 , "Quarks&Co" , "wdr" , 50 , 18 , 1 ), new Podcast( 2 , "Angeklickt - zum Mitnehmen" , "wdr" , 10 , 5 , 1 ), new Podcast( 3 , "Leonardo im WDR 5-Radio zum Mitnehmen" , "wdr" , 12 , 10 , 5 ), new Podcast( 4 , "L'ESPRIT PUBLIC" , "France culture" , 3 , 10 , 1 ), new Podcast( 5 , "LA FABRIQUE DE L'HISTOIRE" , "France culture" , 10 , 4 , 1 ), new Podcast( 6 , "LES MATINS DE FRANCE CULTURE" , "France culture" , 46 , 12 , 8 ) ); System.out.println( "*********** Display initial podcasts with forEach ************" ); podcasts.forEach(podcast -> System.out.println(podcast)); System.out.println( "\n\n********************** Sorting with lambdas ***********************" ); // Sort by title System.out.println( "\n*********** Sort by title (default alphabetically) - highlight comparator ************" ); Collections.sort(podcasts, Comparator.comparing(Podcast::getTitle)); podcasts.forEach(podcast -> System.out.println(podcast)); System.out.println( "\n*********** Sort by numbers of subscribers DESCENDING - highlight reversed ************" ); Collections.sort(podcasts, Comparator.comparing(Podcast::getSubscriptionsNumber).reversed()); podcasts.forEach(podcast -> System.out.println(podcast)); System.out.println( "\n*********** Sort by producer and then by title - highlight composed conditions************" ); Collections.sort(podcasts, Comparator.comparing(Podcast::getProducer).thenComparing(Podcast::getTitle)); podcasts.forEach(podcast -> System.out.println(podcast)); System.out.println( "\n*********** Sort by difference in positive votes DESCENDING ************" ); Collections.sort(podcasts, Podcast.BY_POSITIVE_VOTES_DIFFERENCE); podcasts.forEach(podcast -> System.out.println(podcast)); System.out.println( "\n\n******************** Streams *************************" ); System.out.println( "\n*********** Filter podcasts with more than 21 subscribers - highlight filters ************" ); podcasts.stream() .filter((podcast)-> podcast.getSubscriptionsNumber() >= 21 ) .forEach((podcast)-> System.out.println(podcast)); System.out.println( "\n********* Filter podcasts from producer with more than 21 subscribers - highlight predicate **************" ); Predicate<Podcast> hasManySubscribers = (podcast) -> podcast.getSubscriptionsNumber() >= 21 ; Predicate<Podcast> wdrProducer = (podcast) -> podcast.getProducer().equals( "wdr" ); podcasts.stream() .filter(hasManySubscribers.and(wdrProducer)) .forEach((podcast) -> System.out.println(podcast)); System.out.println( "\n********* Display popular podcasts - highlight \"or\" in predicate **************" ); Predicate<Podcast> hasManyLikes = (podcast) -> (podcast.getUpVotes()-podcast.getDownVotes()) > 8 ; podcasts.stream() .filter(hasManySubscribers.or(hasManyLikes)) .forEach((podcast) -> System.out.println(podcast)); System.out.println( "\n********* Collect subscription numbers - highlight \"mapToInt\" **************" ); int numberOfSubscriptions = podcasts.stream() .mapToInt(Podcast::getSubscriptionsNumber).sum(); System.out.println( "Number of all subscriptions : " + numberOfSubscriptions); System.out.println( "\n********* Display podcast with most subscriptions -highlight \"map reduce\" capabilities **************" ); Podcast podcastWithMostSubscriptions; podcastWithMostSubscriptions = podcasts.stream() .map(podcast -> new Podcast(podcast.getId(), podcast.getTitle(), podcast.getProducer(), podcast.getSubscriptionsNumber(), podcast.getUpVotes(), podcast.getDownVotes())) .reduce( new Podcast(), (pod1, pod2) -> (pod1.getSubscriptionsNumber() > pod2.getSubscriptionsNumber()) ? pod1 : pod2); System.out.println(podcastWithMostSubscriptions); System.out.println( "\n********* Display podcasts titles in XML format -highlight \"map reduce\" capabilities **************" ); String titlesInXml = "<podcasts data='titles'>" + podcasts.stream() .map(podcast -> "<title>" + podcast.getTitle() + "</title>" ) .reduce( "" , String::concat) + "</podcasts>" ; System.out.println(titlesInXml); System.out.println( "\n********* Display podcasts in JSON format -highlight \"map reduce\" capabilities **************" ); String json = podcasts.stream() .map(Podcast::toJSON) .reduce( "[" , (l, r) -> l + (l.equals( "[" ) ? "" : "," ) + r) + "]" ; System.out.println(json); System.out.println( "\n********* Display sorted podcasts by title in JSON format -highlight \"map collect\" capabilities **************" ); String jsonViaCollectors = podcasts.stream() .sorted(Comparator.comparing(Podcast::getTitle)) .map(Podcast::toJSON) .collect(Collectors.joining( "," , "[" , "]" )); System.out.println(jsonViaCollectors); System.out.println( "\n********* Select first 3 podcasts with most subscribers -highlight \"map collect\" capabilities **************" ); List<Podcast> podcastsWithMostSubscribers = podcasts.stream() .sorted(Comparator.comparing(Podcast::getSubscriptionsNumber).reversed()) .limit( 3 ) .collect(Collectors.toList()); System.out.println(podcastsWithMostSubscribers); System.out.println( "\n********* Get podcasts grouped by producer -highlight \"collector\" capabilities **************" ); Map<String, List<Podcast>> podcastsByProducer = podcasts.stream() .collect(Collectors.groupingBy(podcast -> podcast.getProducer())); System.out.println(podcastsByProducer); } } |
Resources
Reference: | Yet another Java 8 Lamdbas and Streams example from our JCG partner Adrian Matei at the Codingpedia.org blog. |