Purpose

ThistutorialintroducesthenewlambdaexpressionsincludedinJavaPlatformStandardEdition8(JavaSE8).

TimetoComplete

Approximately1hour

Introduction

LambdaexpressionsareanewandimportantfeatureincludedinJavaSE8.Theyprovideaclearandconcisewaytorepresentonemethodinterfaceusinganexpression.LambdaexpressionsalsoimprovetheCollectionlibrariesmakingiteasiertoiteratethrough,filter,andextractdatafromaCollection.Inaddition,newconcurrencyfeaturesimproveperformanceinmulticoreenvironments.

ThisOraclebyExample(OBE)providesanintroductiontolambdaexpressionsincludedinJavaSE8.Anintroductiontoanonymousinnerfunctionsisprovided,followedbyadiscussionoffunctionalinterfacesandthenewlambdasyntax.Then,examplesofcommonusagepatternsbeforeandafterlambdaexpressionsareshown.

ThenextsectionreviewsacommonsearchusecaseandhowJavacodecanbeimprovedwiththeinclusionoflambdaexpressions.Inaddition,someofthecommonfunctionalinterfaces,PredicateandFunction,providedinjava.util.functionareshowninaction.

TheOBEfinishesupwithareviewofhowtheJavacollectionhasbeenupdatedwithlambdaexpressions.

Thesourcecodeforallexamplesisprovidedtoyou.

HardwareandSoftwareRequirements

Thefollowingisalistofhardwareandsoftwarerequirements:

JavaDevelopmentKit(JDK8)earlyaccess

Overview

JavaSE8:LambdaQuickStart

JavaDevelopmentKit(JDK8)earlyaccessNetBeans7.4

PrerequisitesToruntheexamples,youmusthaveanearlyaccessversionofJDK8andacopyofNetBeans7.4orlater.LinkscanbefoundatthemainLambdasite.Foryourconvenience,herearethedirectlinks.

JavaDevelopmentKit8(JDK8)EarlyAccessRecommended:DownloadtheJDK8APIDocsfromthesamepage.

NetBeans7.4orlater.Version7.4ofNetBeansincludessupportforJDK8.

Note:Buildsareprovidedforallmajoroperatingsystems.ThisOBEwasdevelopedusingLinuxMint13(Ubuntu/Debian).

InstallJDK8andNetBeansfollowingtheinstructionsprovidedwiththedownloads.AddthebindirectoriesforJavaandNetBeanstoyourPATH.

Note:ThisOBEwaslastupdatedDecember2013.

AnonymousInnerClass

InJava,anonymousinnerclassesprovideawaytoimplementclassesthatmayoccuronlyonceinanapplication.Forexample,inastandardSwingorJavaFXapplicationanumberofeventhandlersarerequiredforkeyboardandmouseevents.Ratherthanwritingaseparateeventhandlingclassforeachevent,youcanwritesomethinglikethis.

16 JButton testButton = new JButton("Test Button");17 testButton.addActionListener(new ActionListener(){18 @Override public void actionPerformed(ActionEvent ae){19 System.out.println("Click Detected by Anon Class");20 }21 });

Otherwise,aseparateclassthatimplementsActionListenerisrequiredforeachevent.Bycreatingtheclassinplace,whereitisneeded,thecodeisalittleeasiertoread.Thecodeisnotelegant,becausequiteabitofcodeisrequiredjusttodefineonemethod.

FunctionalInterfaces

ThecodethatdefinestheActionListenerinterface,lookssomethinglikethis:

Background

https://jdk8.java.net/download.htmlhttp://openjdk.java.net/projects/lambda/http://netbeans.org/

this:

1 package java.awt.event; 2 import java.util.EventListener; 3 4 public interface ActionListener extends EventListener { 5 6 public void actionPerformed(ActionEvent e); 7 8 }

TheActionListenerexampleisaninterfacewithonlyonemethod.WithJavaSE8,aninterfacethatfollowsthispatternisknownasa"functionalinterface."

Note:Thistypeofinterface,waspreviouslyknownasaSingleAbstractMethodtype(SAM).

UsingfunctionalinterfaceswithanonymousinnerclassesareacommonpatterninJava.InadditiontotheEventListenerclasses,interfaceslikeRunnableandComparatorareusedinasimilarmanner.Therefore,functionalinterfacesareleveragedforusewithlambdaexpressions.

LambdaExpressionSyntax

Lambdaexpressionsaddressthebulkinessofanonymousinnerclassesbyconvertingfivelinesofcodeintoasinglestatement.Thissimplehorizontalsolutionsolvesthe"verticalproblem"presentedbyinnerclasses.

Alambdaexpressioniscomposedofthreeparts.

ArgumentList ArrowToken Body

(intx,inty)

> x+y

Thebodycanbeeitherasingleexpressionorastatementblock.Intheexpressionform,thebodyissimplyevaluatedandreturned.Intheblockform,thebodyisevaluatedlikeamethodbodyandareturnstatementreturnscontroltothecalleroftheanonymousmethod.Thebreakandcontinuekeywordsareillegalatthetoplevel,butarepermittedwithinloops.Ifthebodyproducesaresult,everycontrolpathmustreturnsomethingorthrowanexception.

Takealookattheseexamples:

(int x, int y) -> x + y

() -> 42

(String s) -> { System.out.println(s); }

Thefirstexpressiontakestwointegerarguments,namedxandy,anduses

theexpressionformtoreturnx+y.Thesecondexpressiontakesnoargumentsandusestheexpressionformtoreturnaninteger42.Thethirdexpressiontakesastringandusestheblockformtoprintthestringtotheconsole,andreturnsnothing.

Withthebasicsofsyntaxcovered,let'slookatsomeexamples.

Herearesomecommonusecasesusingthepreviouslycoveredexamples.

RunnableLambda

HereishowyouwriteaRunnableusinglambdas.

6 public class RunnableTest { 7 public static void main(String[] args) { 8 9 System.out.println("=== RunnableTest ===");10 11 // Anonymous Runnable12 Runnable r1 = new Runnable(){13 14 @Override15 public void run(){16 System.out.println("Hello world one!");17 }18 };19 20 // Lambda Runnable21 Runnable r2 = () -> System.out.println("Hello world two!");22 23 // Run em!24 r1.run();25 r2.run();26 27 }28 }

Inbothcases,noticethatnoparameterispassedandisreturned.TheRunnablelambdaexpression,whichusestheblockformat,convertsfivelinesofcodeintoonestatement.

ComparatorLambda

InJava,theComparatorclassisusedforsortingcollections.Inthefollowingexample,anArrayListconsistingofPersonobjectsissortedbasedon

LambdaExamples

surName.ThefollowingarethefieldsincludedinthePersonclass.

9 public class Person {

10 private String givenName;11 private String surName;12 private int age;13 private Gender gender;14 private String eMail;15 private String phone;16 private String address;17

ThefollowingcodeappliesaComparatorbyusingananonymousinnerclassandacouplelambdaexpressions.

10 public class ComparatorTest {11 12 public static void main(String[] args) {13 14 List personList = Person.createShortList();15 16 // Sort with Inner Class17 Collections.sort(personList, new Comparator(){18 public int compare(Person p1, Person p2){19 return p1.getSurName().compareTo(p2.getSurName());20 }21 });22 23 System.out.println("=== Sorted Asc SurName ===");24 for(Person p:personList){25 p.printName();26 }27 28 // Use Lambda instead29 30 // Print Asc31 System.out.println("=== Sorted Asc SurName ===");32 Collections.sort(personList, (Person p1, Person p2) -> p1.getSurName().compareTo(p2.getSurName()));33 34 for(Person p:personList){35 p.printName();36 }37 38 // Print Desc39 System.out.println("=== Sorted Desc SurName ===");40 Collections.sort(personList, (p1, p2) -> p2.getSurName().compareTo(p1.getSurName()));41 42 for(Person p:personList){43 p.printName();44 }45 46 }47 }

Lines1721areeasilyreplacedbythelambdaexpressiononline32.Noticethatthefirstlambdaexpressiondeclarestheparametertypepassedtotheexpression.However,asyoucanseefromthesecondexpression,thisis

expression.However,asyoucanseefromthesecondexpression,thisisoptional.Lambdasupports"targettyping"whichinferstheobjecttypefromthecontextinwhichitisused.BecauseweareassigningtheresulttoaComparatordefinedwithageneric,thecompilercaninferthatthetwoparametersareofthePersontype.

ListenerLambda

Finally,let'srevisittheActionListenerexample.

13 public class ListenerTest {14 public static void main(String[] args) {15 16 JButton testButton = new JButton("Test Button");17 testButton.addActionListener(new ActionListener(){18 @Override public void actionPerformed(ActionEvent ae){19 System.out.println("Click Detected by Anon Class");20 }21 });22 23 testButton.addActionListener(e -> System.out.println("Click Detected by Lambda Listner"));24 25 // Swing stuff26 JFrame frame = new JFrame("Listener Test");27 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);28 frame.add(testButton, BorderLayout.CENTER);29 frame.pack();30 frame.setVisible(true);31 32 }33 }

Noticethatthelambdaexpressionispassedasaparameter.Targettypingisusedinanumberofcontextsincludingthefollowing:

VariabledeclarationsAssignmentsReturnstatementsArrayinitializersMethodorconstructorargumentsLambdaexpressionbodiesConditionalexpressions?:Castexpressions

Resources

ANetBeansprojectcontainingthesourcefilesfortheexamplescoveredtothispointisinthefollowingzipfile.

LambdaExamples01.zip

http://www.oracle.com/webfolder/technetwork/tutorials/obe/java/Lambda-QuickStart/examples/LambdaExamples01.zip

Thissectionbuildsuponthepreviousexamplestoshowyouhowlambdaexpressionscanimproveyourcode.LambdasshouldprovideameanstobettersupporttheDon'tRepeatYourself(DRY)principleandmakeyourcodesimplerandmorereadable.

ACommonQueryUseCase

Acommonusecaseforprogramsistosearchthroughacollectionofdatatofinditemsthatmatchaspecificcriteria.Intheexcellent"JumpStartingLambda"presentationatJavaOne2012,StuartMarksandMikeDuigouwalkthoughjustsuchausecase.Givenalistofpeople,variouscriteriaareusedtomakerobocalls(automatedphonecalls)tomatchingpersons.Thistutorialfollowsthatbasicpremisewithslightvariations.

Inthisexample,ourmessageneedstogetouttothreedifferentgroupsintheUnitedStates:

Drivers:Personsovertheageof16Draftees:Malepersonsbetweentheagesof18and25Pilots(specificallycommercialpilots):Personsbetweentheagesof23and65

Theactualrobotthatdoesallthisworkhasnotyetarrivedatourplaceofbusiness.Insteadofcalling,mailingoremailing,amessageisprintedtotheconsole.Themessagecontainsaperson'sname,age,andinformationspecifictothetargetmedium(forexample,emailaddresswhenemailingorphonenumberwhencalling).

PersonClass

EachpersoninthetestlistisdefinedbyusingthePersonclasswiththefollowingproperties:

10 public class Person {11 private String givenName;12 private String surName;13 private int age;14 private Gender gender;15 private String eMail;16 private String phone;17 private String address;18

ThePersonclassusesaBuildertocreatenewobjects.AsamplelistofpeopleiscreatedwiththecreateShortListmethod.Hereisashortcodefragmentofthatmethod.Note:AllsourcecodeforthistutorialisincludedinaNetBeansprojectthatislinkedattheendofthissection.

ImprovingCodewithLambdaExpressions

http://www.youtube.com/watch?v=bzO5GSujdqI

128 public static List createShortList(){129 List people = new ArrayList();130 131 people.add(132 new Person.Builder()133 .givenName("Bob")134 .surName("Baker")135 .age(21)136 .gender(Gender.MALE)137 .email("bob.baker@example.com")138 .phoneNumber("201-121-4678")139 .address("44 4th St, Smallville, KS 12333")140 .build() 141 );142 143 people.add(144 new Person.Builder()145 .givenName("Jane")146 .surName("Doe")147 .age(25)148 .gender(Gender.FEMALE)149 .email("jane.doe@example.com")150 .phoneNumber("202-123-4678")151 .address("33 3rd St, Smallville, KS 12333")152 .build() 153 );154 155 people.add(156 new Person.Builder()157 .givenName("John")158 .surName("Doe")159 .age(25)160 .gender(Gender.MALE)161 .email("john.doe@example.com")162 .phoneNumber("202-123-4678")163 .address("33 3rd St, Smallville, KS 12333")164 .build()165 );166

AFirstAttempt

WithaPersonclassandsearchcriteriadefined,youcanwriteaRoboContactclass.Onepossiblesolutiondefinesamethodforeachusecase:

RoboContactsMethods.java

1 package com.example.lambda; 2 3 import java.util.List; 4 5 /** 6 * 7 * @author MikeW 8 */ 9 public class RoboContactMethods { 10 11 public void callDrivers(List pl){ 12 for(Person p:pl){

12 for(Person p:pl){ 13 if (p.getAge() >= 16){ 14 roboCall(p); 15 } 16 } 17 } 18 19 public void emailDraftees(List pl){ 20 for(Person p:pl){ 21 if (p.getAge() >= 18 && p.getAge() = 23 && p.getAge()

RoboContactMethods2.java

1 package com.example.lambda; 2 3 import java.util.List; 4 5 /** 6 * 7 * @author MikeW 8 */ 9 public class RoboContactMethods2 { 10 11 public void callDrivers(List pl){ 12 for(Person p:pl){ 13 if (isDriver(p)){ 14 roboCall(p); 15 } 16 } 17 } 18 19 public void emailDraftees(List pl){ 20 for(Person p:pl){ 21 if (isDraftee(p)){ 22 roboEmail(p); 23 } 24 } 25 } 26 27 public void mailPilots(List pl){ 28 for(Person p:pl){ 29 if (isPilot(p)){ 30 roboMail(p); 31 } 32 } 33 } 34 35 public boolean isDriver(Person p){ 36 return p.getAge() >= 16; 37 } 38 39 public boolean isDraftee(Person p){ 40 return p.getAge() >= 18 && p.getAge() = 23 && p.getAge()

57 } 58 59 }

Thesearchcriteriaareencapsulatedinamethod,animprovementoverthepreviousexample.Thetestconditionscanbereusedandchangesflowbackthroughouttheclass.Howeverthereisstillalotofrepeatedcodeandaseparatemethodisstillrequiredforeachusecase.Isthereabetterwaytopassthesearchcriteriatothemethods?

AnonymousClasses

Beforelambdaexpressions,anonymousinnerclasseswereanoption.Forexample,aninterface(MyTest.java)writtenwithonetestmethodthatreturnsaboolean(afunctionalinterface)isapossiblesolution.Thesearchcriteriacouldbepassedwhenthemethodiscalled.Theinterfacelookslikethis:

6 public interface MyTest {7 public boolean test(T t);8 }

Theupdatedrobotclasslookslikethis:

RoboContactsAnon.java

9 public class RoboContactAnon {10 11 public void phoneContacts(List pl, MyTest aTest){12 for(Person p:pl){13 if (aTest.test(p)){14 roboCall(p);15 }16 }17 }18 19 public void emailContacts(List pl, MyTest aTest){20 for(Person p:pl){21 if (aTest.test(p)){22 roboEmail(p);23 }24 }25 }26 27 public void mailContacts(List pl, MyTest aTest){28 for(Person p:pl){29 if (aTest.test(p)){30 roboMail(p);31 }32 }33 } 34

34 35 public void roboCall(Person p){36 System.out.println("Calling " + p.getGivenName() + " " + p.getSurName() + " age " + p.getAge() + " at " + p.getPhone());37 }

38 39 public void roboEmail(Person p){40 System.out.println("EMailing " + p.getGivenName() + " " + p.getSurName() + " age " + p.getAge() + " at " + p.getEmail());41 }42 43 public void roboMail(Person p){44 System.out.println("Mailing " + p.getGivenName() + " " + p.getSurName() + " age " + p.getAge() + " at " + p.getAddress());45 }46 47 }

Thatisdefinitelyanotherimprovement,becauseonlythreemethodsareneededtoperformroboticoperations.However,thereisaslightproblemwithuglinesswhenthemethodsarecalled.Checkoutthetestclassusedforthisclass:

RoboCallTest03.java

1 package com.example.lambda; 2 3 import java.util.List; 4 5 /** 6 * @author MikeW 7 */ 8 public class RoboCallTest03 { 9 10 public static void main(String[] args) { 11 12 List pl = Person.createShortList(); 13 RoboContactAnon robo = new RoboContactAnon(); 14 15 System.out.println("\n==== Test 03 ===="); 16 System.out.println("\n=== Calling all Drivers ==="); 17 robo.phoneContacts(pl, 18 new MyTest(){ 19 @Override 20 public boolean test(Person p){ 21 return p.getAge() >=16; 22 } 23 } 24 ); 25 26 System.out.println("\n=== Emailing all Draftees ==="); 27 robo.emailContacts(pl, 28 new MyTest(){ 29 @Override 30 public boolean test(Person p){ 31 return p.getAge() >= 18 && p.getAge()

34 ); 35 36 37 System.out.println("\n=== Mail all Pilots ==="); 38 robo.mailContacts(pl,

39 new MyTest(){ 40 @Override 41 public boolean test(Person p){ 42 return p.getAge() >= 23 && p.getAge()

10 public class RoboContactLambda { 11 public void phoneContacts(List pl, Predicate pred){ 12 for(Person p:pl){ 13 if (pred.test(p)){ 14 roboCall(p); 15 } 16 } 17 } 18 19 public void emailContacts(List pl, Predicate pred){ 20 for(Person p:pl){ 21 if (pred.test(p)){ 22 roboEmail(p); 23 } 24 } 25 } 26 27 public void mailContacts(List pl, Predicate pred){ 28 for(Person p:pl){ 29 if (pred.test(p)){ 30 roboMail(p); 31 } 32 } 33 } 34 35 public void roboCall(Person p){ 36 System.out.println("Calling " + p.getGivenName() + " " + p.getSurName() + " age " + p.getAge() + " at " + p.getPhone()); 37 } 38 39 public void roboEmail(Person p){ 40 System.out.println("EMailing " + p.getGivenName() + " " + p.getSurName() + " age " + p.getAge() + " at " + p.getEmail()); 41 } 42 43 public void roboMail(Person p){ 44 System.out.println("Mailing " + p.getGivenName() + " " + p.getSurName() + " age " + p.getAge() + " at " + p.getAddress()); 45 } 46 47 }

Withthisapproachonlythreemethodsareneeded,oneforeachcontactmethod.ThelambdaexpressionpassedtothemethodselectsthePersoninstancesthatmeetthetestconditions.

VerticalProblemSolved

Lambdaexpressionssolvetheverticalproblemandallowtheeasyreuseofanyexpression.Takealookatthenewtestclassupdatedforlambdaexpressions.

RoboCallTest04.java

1 package com.example.lambda;

1 package com.example.lambda; 2 3 import java.util.List; 4 import java.util.function.Predicate; 5

6 /** 7 * 8 * @author MikeW 9 */ 10 public class RoboCallTest04 { 11 12 public static void main(String[] args){ 13 14 List pl = Person.createShortList(); 15 RoboContactLambda robo = new RoboContactLambda(); 16 17 // Predicates 18 Predicate allDrivers = p -> p.getAge() >= 16; 19 Predicate allDraftees = p -> p.getAge() >= 18 && p.getAge() p.getAge() >= 23 && p.getAge()

Thejava.util.functionPackage

Ofcourse,PredicateisnottheonlyfunctionalinterfaceprovidedwithJavaSE8.Anumberofstandardinterfacesaredesignedasastartersetfor

developers.

Predicate:ApropertyoftheobjectpassedasargumentConsumer:AnactiontobeperformedwiththeobjectpassedasargumentFunction:TransformaTtoaUSupplier:ProvideaninstanceofaT(suchasafactory)UnaryOperator:AunaryoperatorfromT>TBinaryOperator:Abinaryoperatorfrom(T,T)>T

Inaddition,manyoftheseinterfacesalsohaveprimitiveversions.Thisshouldgiveyouagreatstartingpointforyourlambdaexpressions.

EasternStyleNamesandMethodReferences

Whenworkingonthepreviousexample,IdecideditwouldbenicetohaveaflexibleprintingsystemforthePersonclass.Onefeaturerequirementistodisplaynamesinbothawesternstyleandaneasternstyle.IntheWest,namesaredisplayedwiththegivennamefirstandthesurnamesecond.Inmanyeasterncultures,namesaredisplayedwiththesurnamefirstandthegivennamesecond.

AnOldStyleExample

HereisanexampleofhowtoimplementaPersonprintingclasswithoutlambdasupport.

Person.java

128 public void printWesternName(){129 130 System.out.println("\nName: " + this.getGivenName() + " " + this.getSurName() + "\n" +131 "Age: " + this.getAge() + " " + "Gender: " + this.getGender() + "\n" +132 "EMail: " + this.getEmail() + "\n" + 133 "Phone: " + this.getPhone() + "\n" +134 "Address: " + this.getAddress());135 }136 137 138 139 public void printEasternName(){140 141 System.out.println("\nName: " + this.getSurName() + " " + this.getGivenName() + "\n" +142 "Age: " + this.getAge() + " " + "Gender: " + this.getGender() + "\n" +143 "EMail: " + this.getEmail() + "\n" + 144 "Phone: " + this.getPhone() + "\n" +

144 "Phone: " + this.getPhone() + "\n" +145 "Address: " + this.getAddress());146 }

Amethodexistsforeachstylethatprintsoutaperson.

TheFunctionInterface

TheFunctioninterfaceisusefulforthisproblem.Ithasonlyonemethodapplywiththefollowingsignature:

publicRapply(Tt){}

IttakesagenericclassTandreturnsagenericclassR.Forthisexample,passthePersonclassandreturnaString.Amoreflexibleprintmethodforpersoncouldbewrittenlikethis:

Person.java

123 public String printCustom(Function f){124 return f.apply(this);125 }126

Thatisquiteabitsimpler.AFunctionispassedtothemethodandastringreturned.TheapplymethodprocessesalambdaexpressionwhichdetermineswhatPersoninformationisreturned.

HowaretheFunctionsdefined?Hereisthetestcodethatcallsthepreviousmethod.

NameTestNew.java

9 public class NameTestNew {10 11 public static void main(String[] args) {12 13 System.out.println("\n==== NameTestNew02 ===");14 15 List list1 = Person.createShortList();16 17 // Print Custom First Name and e-mail18 System.out.println("===Custom List===");19 for (Person person:list1){20 System.out.println(21 person.printCustom(p -> "Name: " + p.getGivenName() + " EMail: " + p.getEmail())22 );23 }24 25 26 // Define Western and Eastern Lambdas

26 // Define Western and Eastern Lambdas27 28 Function westernStyle = p -> {29 return "\nName: " + p.getGivenName() + " " + p.getSurName() + "\n" +

30 "Age: " + p.getAge() + " " + "Gender: " + p.getGender() + "\n" +31 "EMail: " + p.getEmail() + "\n" + 32 "Phone: " + p.getPhone() + "\n" +33 "Address: " + p.getAddress();34 };35 36 Function easternStyle = p -> "\nName: " + p.getSurName() + " " 37 + p.getGivenName() + "\n" + "Age: " + p.getAge() + " " + 38 "Gender: " + p.getGender() + "\n" +39 "EMail: " + p.getEmail() + "\n" + 40 "Phone: " + p.getPhone() + "\n" +41 "Address: " + p.getAddress(); 42 43 // Print Western List44 System.out.println("\n===Western List===");45 for (Person person:list1){46 System.out.println(47 person.printCustom(westernStyle)48 );49 }50 51 // Print Eastern List52 System.out.println("\n===Eastern List===");53 for (Person person:list1){54 System.out.println(55 person.printCustom(easternStyle)56 );57 }58 59 60 }61 }

Thefirstloopjustprintsgivennameandtheemailaddress.ButanyvalidexpressioncouldbepassedtotheprintCustommethod.Easternandwesternprintstylesaredefinedwithlambdaexpressionsandstoredinavariable.Thevariablesarethenpassedtothefinaltwoloops.ThelambdaexpressionscouldveryeasilybeincorporatedintoaMaptomaketheirreusemucheasier.Thelambdaexpressionprovidesagreatdealofflexibility.

SampleOutput

Hereissomesampleoutputfromtheprogram.

==== NameTestNew02 ======Custom List===Name: Bob EMail: bob.baker@example.comName: Jane EMail: jane.doe@example.comName: John EMail: john.doe@example.comName: James EMail: james.johnson@example.comName: Joe EMail: joebob.bailey@example.com

Name: Joe EMail: joebob.bailey@example.comName: Phil EMail: phil.smith@examp;e.comName: Betty EMail: betty.jones@example.com

===Western List===

Name: Bob BakerAge: 21 Gender: MALEEMail: bob.baker@example.comPhone: 201-121-4678Address: 44 4th St, Smallville, KS 12333

Name: Jane DoeAge: 25 Gender: FEMALEEMail: jane.doe@example.comPhone: 202-123-4678Address: 33 3rd St, Smallville, KS 12333

Name: John DoeAge: 25 Gender: MALEEMail: john.doe@example.comPhone: 202-123-4678Address: 33 3rd St, Smallville, KS 12333

Name: James JohnsonAge: 45 Gender: MALEEMail: james.johnson@example.comPhone: 333-456-1233Address: 201 2nd St, New York, NY 12111

Name: Joe BaileyAge: 67 Gender: MALEEMail: joebob.bailey@example.comPhone: 112-111-1111Address: 111 1st St, Town, CA 11111

Name: Phil SmithAge: 55 Gender: MALEEMail: phil.smith@examp;e.comPhone: 222-33-1234Address: 22 2nd St, New Park, CO 222333

Name: Betty JonesAge: 85 Gender: FEMALEEMail: betty.jones@example.comPhone: 211-33-1234Address: 22 4th St, New Park, CO 222333

===Eastern List===

Name: Baker BobAge: 21 Gender: MALEEMail: bob.baker@example.comPhone: 201-121-4678Address: 44 4th St, Smallville, KS 12333

Name: Doe JaneAge: 25 Gender: FEMALEEMail: jane.doe@example.comPhone: 202-123-4678Address: 33 3rd St, Smallville, KS 12333

Name: Doe JohnAge: 25 Gender: MALEEMail: john.doe@example.comPhone: 202-123-4678Address: 33 3rd St, Smallville, KS 12333

Address: 33 3rd St, Smallville, KS 12333

Name: Johnson JamesAge: 45 Gender: MALEEMail: james.johnson@example.comPhone: 333-456-1233Address: 201 2nd St, New York, NY 12111

Name: Bailey JoeAge: 67 Gender: MALEEMail: joebob.bailey@example.comPhone: 112-111-1111Address: 111 1st St, Town, CA 11111

Name: Smith PhilAge: 55 Gender: MALEEMail: phil.smith@examp;e.comPhone: 222-33-1234Address: 22 2nd St, New Park, CO 222333

Name: Jones BettyAge: 85 Gender: FEMALEEMail: betty.jones@example.comPhone: 211-33-1234Address: 22 4th St, New Park, CO 222333

Resources

TheNetBeansprojectwiththesourcecodefortheexamplesinthissectionisincludedinthefollowingzipfile.

LambdaFunctionExamples.zip

TheprevioussectionintroducedtheFunctioninterfaceandfinishedupexamplesofbasiclambdaexpressionsyntax.ThissectionreviewshowlambdaexpressionsimprovetheCollectionclasses.

LambdaExpressionsandCollections

Intheexamplescreatedsofar,thecollectionsclasseswereusedquiteabit.However,anumberofnewlambdaexpressionfeatureschangethewaycollectionsareused.Thissectionintroducesafewofthesenewfeatures.

ClassAdditions

Thedrivers,pilots,anddrafteessearchcriteriahavebeenencapsulatedintheSearchCriteriaclass.

SearchCriteria.java

LambdaExpressionsandCollections

http://www.oracle.com/webfolder/technetwork/tutorials/obe/java/Lambda-QuickStart/examples/LambdaFunctionExamples.zip

SearchCriteria.java

1 package com.example.lambda; 2 3 import java.util.HashMap; 4 import java.util.Map; 5 import java.util.function.Predicate; 6 7 /** 8 * 9 * @author MikeW10 */11 public class SearchCriteria {12 13 private final Map searchMap = new HashMap();14 15 private SearchCriteria() {16 super();17 initSearchMap();18 }19 20 private void initSearchMap() {21 Predicate allDrivers = p -> p.getAge() >= 16;22 Predicate allDraftees = p -> p.getAge() >= 18 && p.getAge() p.getAge() >= 23 && p.getAge()

ThefirstfeaturetolookatisthenewforEachmethodavailabletoanycollectionclass.HereareacoupleofexamplesthatprintoutaPersonlist.

Test01ForEach.java

11 public class Test01ForEach {12 13 public static void main(String[] args) {14 15 List pl = Person.createShortList();16 17 System.out.println("\n=== Western Phone List ===");18 pl.forEach( p -> p.printWesternName() );19 20 System.out.println("\n=== Eastern Phone List ===");21 pl.forEach(Person::printEasternName);22 23 System.out.println("\n=== Custom Phone List ===");24 pl.forEach(p -> { System.out.println(p.printCustom(r -> "Name: " + r.getGivenName() + " EMail: " + r.getEmail())); });25 26 }27 28 }

ThefirstexampleshowsastandardlambdaexpressionwhichcallstheprintWesternNamemethodtoprintouteachpersoninthelist.Thesecondexampledemonstratesamethodreference.Inacasewhereamethodalreadyexiststoperformanoperationontheclass,thissyntaxcanbeusedinsteadofthenormallambdaexpressionsyntax.Finally,thelastexampleshowshowtheprintCustommethodcanalsobeusedinthissituation.Noticetheslightvariationinvariablenameswhenonelambdaexpressionisincludedinanother.

Youcaniteratethroughanycollectionthisway.Thebasicstructureissimilartotheenhancedforloop.However,includinganiterationmechanismwithintheclassprovidesanumberofbenefits.

ChainingandFilters

Inadditiontoloopingthroughthecontentsofacollection,youcanchainmethodstogether.ThefirstmethodtolookatisfilterwhichtakesaPredicateinterfaceasaparameter.

ThefollowingexampleloopsthoughaListafterfirstfilteringtheresults.

Test02Filter.java

9 public class Test02Filter {10 11 public static void main(String[] args) {

11 public static void main(String[] args) {12 13 List pl = Person.createShortList();14 15 SearchCriteria search = SearchCriteria.getInstance();

16 17 System.out.println("\n=== Western Pilot Phone List ===");18 19 pl.stream().filter(search.getCriteria("allPilots"))20 .forEach(Person::printWesternName);21 22 23 System.out.println("\n=== Eastern Draftee Phone List ===");24 25 pl.stream().filter(search.getCriteria("allDraftees"))26 .forEach(Person::printEasternName);27 28 }29 }

ThefirstandlastloopsdemonstratehowtheListisfilteredbasedonthesearchcriteria.Theoutputfromthelastlooplookslikefollowing:

=== Eastern Draftee Phone List ===

Name: Baker BobAge: 21 Gender: MALEEMail: bob.baker@example.comPhone: 201-121-4678Address: 44 4th St, Smallville, KS 12333

Name: Doe JohnAge: 25 Gender: MALEEMail: john.doe@example.comPhone: 202-123-4678Address: 33 3rd St, Smallville, KS 12333

GettingLazy

Thesefeaturesareuseful,butwhyaddthemtothecollectionsclasseswhenthereisalreadyaperfectlygoodforloop?Bymovingiterationfeaturesintoalibrary,itallowsthedevelopersofJavatodomorecodeoptimizations.Toexplainfurther,acoupleoftermsneeddefinitions.

Laziness:Inprogramming,lazinessreferstoprocessingonlytheobjectsthatyouwanttoprocesswhenyouneedtoprocessthem.Inthepreviousexample,thelastloopis"lazy"becauseitloopsonlythroughthetwoPersonobjectsleftaftertheListisfiltered.Thecodeshouldbemoreefficientbecausethefinalprocessingstepoccursonlyontheselectedobjects.Eagerness:Codethatperformsoperationsoneveryobjectinalistisconsidered"eager".Forexample,anenhancedforloopthatiteratesthroughanentirelisttoprocesstwoobjects,isconsideredamore"eager"approach.

Bymakingloopingpartofthecollectionslibrary,codecanbebetteroptimized

Bymakingloopingpartofthecollectionslibrary,codecanbebetteroptimizedfor"lazy"operationswhentheopportunityarises.Whenamoreeagerapproachmakessense(forexample,computingasumoranaverage),eageroperationsarestillapplied.Thisapproachisamuchmoreefficientandflexiblethanalwaysusingeageroperations.

ThestreamMethod

Inthepreviouscodeexample,noticethatthestreammethodiscalledbeforefilteringandloopingbegin.ThismethodtakesaCollectionasinputandreturnsajava.util.stream.Streaminterfaceastheoutput.AStreamrepresentsasequenceofelementsonwhichvariousmethodscanbechained.Bydefault,onceelementsareconsumedtheyarenolongeravailablefromthestream.Therefore,achainofoperationscanoccuronlyonceonaparticularStream.Inaddition,aStreamcanbeserial(default)orparalleldependingonthemethodcalled.Anexampleofaparallelstreamisincludedattheendofthissection.

MutationandResults

Aspreviouslymentioned,aStreamisdisposedofafteritsuse.Therefore,theelementsinacollectioncannotbechangedormutatedwithaStream.However,whatifyouwanttokeepelementsreturnedfromyourchainedoperations?Youcansavethemtoanewcollection.Thefollowingcodeshowshowtodojustthat.

Test03toList.java

10 public class Test03toList {11 12 public static void main(String[] args) {13 14 List pl = Person.createShortList();15 16 SearchCriteria search = SearchCriteria.getInstance();17 18 // Make a new list after filtering.19 List pilotList = pl20 .stream()21 .filter(search.getCriteria("allPilots"))22 .collect(Collectors.toList());23 24 System.out.println("\n=== Western Pilot Phone List ===");25 pilotList.forEach(Person::printWesternName);26 27 }28 29 }

Thecollectmethodiscalledwithoneparameter,theCollectorsclass.TheCollectorsclassisabletoreturnaListorSetbasedontheresultsofthestream.Theexampleshowshowtheresultofthestreamisassignedtoa

thestream.TheexampleshowshowtheresultofthestreamisassignedtoanewListwhichisiteratedover.

CalculatingwithmapThemapmethodiscommonlyusedwithfilter.Themethodtakesapropertyfromaclassanddoessomethingwithit.Thefollowingexampledemonstratesthisbyperformingcalculationsbasedonage.

Test04Map.java

10 public class Test04Map {11 12 public static void main(String[] args) {13 List pl = Person.createShortList();14 15 SearchCriteria search = SearchCriteria.getInstance();16 17 // Calc average age of pilots old style18 System.out.println("== Calc Old Style ==");19 int sum = 0;20 int count = 0;21 22 for (Person p:pl){23 if (p.getAge() >= 23 && p.getAge() p.getAge())40 .sum();41 42 // Get average of ages43 OptionalDouble averageAge = pl44 .parallelStream()45 .filter(search.getCriteria("allPilots"))46 .mapToDouble(p -> p.getAge())47 .average();48 49 System.out.println("Total Ages: " + totalAge);50 System.out.println("Average Age: " + averageAge.getAsDouble()); 51 52 }53 54 }

Andtheoutputfromtheclassis:

== Calc Old Style ==Total Ages: 150Average Age: 37

== Calc New Style ==Total Ages: 150Average Age: 37.5

Theprogramcalculatestheaverageageofpilotsinourlist.Thefirstloopdemonstratestheoldstyleofcalculatingthenumberbyusingaforloop.Thesecondloopusesthemapmethodtogettheageofeachpersoninaserialstream.NoticethattotalAgeisalong.ThemapmethodreturnsanIntSteamobject,whichcontainsasummethodthatreturnsalong.

Note:Tocomputetheaveragethesecondtime,calculatingthesumofagesisunnecessary.However,itisinstructivetoshowanexamplewiththesummethod.

Thelastloopcomputestheaverageagefromthestream.NoticethattheparallelStreammethodisusedtogetaparallelstreamsothatthevaluescanbecomputedconcurrently.Thereturntypeisabitdifferenthereaswell.

Resources

TheNetBeansprojectwithsourcecodefortheexamplesisinthefollowingzipfile.

LambdaCollectionExamples.zip

Inthistutorial,youlearnedhowtouse:

AnonymousinnerclassesinJava.LambdaexpressionstoreplaceanonymousinnerclassesinJavaSE8.Thecorrectsyntaxforlambdaexpressions.APredicateinterfacetoperformsearchesonalist.AFunctioninterfacetoprocessanobjectandproduceadifferenttypeofobject.NewfeaturesaddedtoCollectionsinJavaSE8thatsupportlambdaexpressions.

Resources

ForfurtherinformationonJavaSE8andlambdaexpressions,seethefollowing:

Java8ProjectLambda

Summary

http://openjdk.java.net/projects/lambda/http://www.oracle.com/webfolder/technetwork/tutorials/obe/java/Lambda-QuickStart/examples/LambdaCollectionExamples.ziphttp://openjdk.java.net/projects/jdk8/

ProjectLambdaStateoftheLambdaStateoftheLambdaCollectionsJumpStartingLambdaJavaOne2012(YouTube)TolearnmoreaboutJavaandrelatedtopicscheckouttheOracleLearningLibrary.

Credits

LeadCurriculumDeveloper:MichaelWilliamsQA:JuanQuesadaNunez

http://www.youtube.com/watch?v=bzO5GSujdqIhttp://www.oracle.com/goto/ollhttp://cr.openjdk.java.net/~briangoetz/lambda/lambda-state-4.htmlhttp://openjdk.java.net/projects/lambda/http://cr.openjdk.java.net/~briangoetz/lambda/sotc3.html