Introductory Java 8 Lambda and collection handling

As a disclaimer, I have coded C# for several years now and am familiar with .NET technology, but my experience with Java 8 is more limited, so this article is just introductory level Java 8 Lambda and collection handling. I will mainly present code examples for handling collections and using lambda expressions in Java 8, using the Eclipse Luna IDE. I also downloaded Jdk 8 to get the necessary Java Development Kit and Java 8 runtime required. The code presented here is heavily based on the tutorial Oracle provides on their websites: Lambda Expressions in Java Let's look at some code right away, defining some classes and enums:

import java.time.LocalDate;
import java.time.chrono.IsoChronology;
import java.util.ArrayList;
import java.util.List;

public class Person {

 String name;  
 Sex gender; 
 LocalDate birthday; 
 String emailAddress; 

 Person(String nameArg, LocalDate birthdayArg, Sex genderArg, String emailArg){
  name = nameArg;
  birthday = birthdayArg; 
  gender = genderArg; 
  emailAddress = emailArg;   
 }
 
 public int getAge(){
  return birthday.until(IsoChronology.INSTANCE.dateNow()).getYears();  
 }
 
 public void printPerson(){
  System.out.println(name + ", " + this.getAge());   
 }
 
 public Sex getGender(){
  return gender;
 }
 
 public String getName(){
  return name;
 }
 
 public String getEmailAddress(){
  return emailAddress;
 }
 
 public LocalDate getBirthday(){
  return birthday;
 }
 
 public static int compareByAge(Person a, Person b){
  return a.birthday.compareTo(b.birthday); 
 }
 
 public static List createRoster(){
  List roster = new ArrayList(); 
  roster.add(new Person(
    "Fred", 
    IsoChronology.INSTANCE.date(1991, 6, 20),
    Sex.MALE,
    "fred@example.com")); 
  roster.add(new Person(
    "Jane", 
    IsoChronology.INSTANCE.date(1990, 7, 15),
    Sex.FEMALE,
    "jane@example.com")); 
  roster.add(new Person(
    "George", 
    IsoChronology.INSTANCE.date(1993, 8, 13),
    Sex.MALE,
    "george@example.com")); 
  roster.add(new Person(
    "Bob", 
    IsoChronology.INSTANCE.date(2000, 9, 12),
    Sex.MALE,
    "bob@example.com"));   
  return roster;
 }
 

}



public class SimplePerson {

 public SimplePerson(String nameArg, int ageArg) {
  name = nameArg;
  age = ageArg;
 }
 
 String name;
 int age;
 
 public String getName(){
  return name;
 }
 
 public int getAge(){
  return age;
 }
 
}


public enum Sex {
 
 MALE,
 
 FEMALE

}



public interface ICheckPerson {
 
 boolean test(Person p);

}

The following code makes use of these definitions and demonstrates use of lambda and collections in Java 8:

import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collector;
import java.util.stream.Collectors;


public class RosterTest {
 
 //Approach 1: Create methods that Search for Persons that match one characteristics
 
 public static void printPersonsOlderThan(List<Person> roster, int age){
  for (Person p : roster){
   if (p.getAge() >= age){
    p.printPerson();
   }
  }  
 }

 
 //Approach 2: Create more generalized search methods 
 
 public static void printPersonsWithinAgeRange(
   List<Person> roster, int low, int high){
  for (Person p : roster){
   if (low <= p.getAge() && p.getAge() < high){
    p.printPerson();
   }
  }
 }
 
 
 //Approach 3: Specify search criteria code in a local class
 //Approach 4: Specify Search criteria code in an anonymous class
 //Approach 5: Specify search criteria code with a lambda expression
 public static void printPersons(
   List<Person> roster, ICheckPerson tester){
  for (Person p : roster){
   if (tester.test(p)){
    p.printPerson();       
   }
  }
 }
 
 
 //Approach 6: Use standard functional interfaces with lambda expressions
 public static void processPersonsWithPredicate(
   List<Person> roster,
   Predicate<Person> tester){
  for (Person p : roster){
   if (tester.test(p)){
    p.printPerson();
   }
  }  
 }
 
 
 //Approach 7, second example
 
 public static void processPersonsWithFunction(
   List<Person> roster,
   Predicate<Person> tester,
   Function<Person, String> mapper,
   Consumer<String> block){
  for (Person p : roster){
   if (tester.test(p)){
    String data = mapper.apply(p);
    block.accept(data); 
   }
  }
 }
 
 
 //Approach 8: Use generics more extensively
 
 public static <X,Y> void processElements(
   Iterable<X> source,
   Predicate<X> tester,
   Function<X,Y> mapper,
   Consumer<Y> block) {
  for (X p : source){
   if (tester.test(p)){
    Y data = mapper.apply(p);
    block.accept(data);
   }   
  }
 }
 
 
 
 public static void main(String... args){
  
  List<Person> roster = Person.createRoster(); 
  
  for (Person p : roster){
   p.printPerson();
  }
  
  //Approach 1: Create methods that Search for Persons that match one characteristic
  
  System.out.println("Persons older than 20:"); 
     printPersonsOlderThan(roster, 20);
     System.out.println();
     
     //Approach 2 : Create more generalized search methods 
     
     System.out.println("Persons between the ages of 14 and 30:"); 
     printPersonsWithinAgeRange(roster, 14, 30);
     System.out.println();
     
     //Approach 3 : Specify search criteria code in local class 
     
     System.out.println("Persons who are eligible for Selective Service:"); 
     
     class CheckPersonEligibleForSelectiveService implements ICheckPerson{

   @Override
   public boolean test(Person p) {
    return p.getGender() == Sex.MALE
      && p.getAge() >= 18 
      && p.getAge() <= 25;
   }      
     }
     
     printPersons(roster, new CheckPersonEligibleForSelectiveService());
     
     System.out.println();
     
     //Approach 4: Specify search Criteria code in an Anonymous class 
     
     System.out.println("Persons who are eligible for Selective Service " + 
      "(anonymous class)");
     
     printPersons(roster, new ICheckPerson() {   
   public boolean test(Person p) {
    return p.getGender() == Sex.MALE
      && p.getAge() >= 18
      && p.getAge() <= 25;
   }
     }); 
  
     //Approach 6: Use standard functional interfaces with lambda expressions 
     
     System.out.println("Persons who are eligible for Selective Service " + 
     "(with Predicate parameter):"); 
     
     processPersonsWithPredicate(roster,
       p -> p.getGender() == Sex.MALE
         && p.getAge() >= 18
         && p.getAge() <= 25
         );
     
     //Approach 7, second example 
     
     System.out.println();
     
     processPersonsWithFunction(roster, p -> 
     p.getGender() == Sex.MALE
     && p.getAge() >= 18 
     && p.getAge() <= 25, 
     p -> p.getEmailAddress(), 
     email -> System.out.println(email));
     
     System.out.println();
     
     //Approach 8: Use generics more extensively 
     
     System.out.println("Persons who are eligible for Selective Service " + 
     "(generic version):"); 
     processElements(roster, p -> 
     p.getGender() == Sex.MALE
     && p.getAge() >= 18 
     && p.getAge() <= 25, 
     p -> p.getEmailAddress(), 
     email -> System.out.println(email));
     
     //Approach 9: Bulk data operations that accept lambda expressions as parameters 
     
     System.out.println("Persons who are eligible for Selective Service " + 
      "(with bulk data operations):"); 
     
     roster.stream().filter(
       p -> p.getGender() == Sex.MALE
       && p.getAge() >= 18
       && p.getAge() <= 25)
       .map(p -> p.getEmailAddress())
       .forEach(email -> System.out.println(email));
     
     
     //Approach 10: Creating a new collection from a query 
     
     System.out.println("Persons who are eligible for selective Service " + 
     "(with collect to create new collection"); 
     
     List<Person> selectiveServicePersons = roster.stream().filter(
       p -> p.getGender() == Sex.MALE
       && p.getAge() >= 18 
       && p.getAge() <= 25)
       .collect(Collectors.<Person>toList()); 
     
     for(Person p: selectiveServicePersons){
      p.printPerson();
     }     
        
     
     System.out.println();
     
     //Approach 11: Using map to transform a collection to another type of collection    
     
     List<SimplePerson> personsTransformed = (List<SimplePerson>) roster.stream()
       .map(p->new SimplePerson(p.getName(), p.getAge()))
    .collect(Collectors.<SimplePerson>toList()); 
     
     for(SimplePerson p: personsTransformed){
      System.out.println(p.getName() + ", " + p.getAge()); 
     }
     
     System.out.println();
     
     //Approach 12: Using sorting for Approach 11 
     
     List<SimplePerson> personsTransformedSorted = (List<SimplePerson>) roster.stream()       
       .map(p->new SimplePerson(p.getName(), p.getAge()))
    .collect(Collectors.<SimplePerson>toList()); 
     
     class CustomComparator implements Comparator<SimplePerson>{
      @Override
      public int compare(SimplePerson o1, SimplePerson o2) {
       return o1.getAge() - o2.getAge();
      };
     }
     
     Collections.sort(personsTransformedSorted, new CustomComparator());
     
     for(SimplePerson p: personsTransformedSorted){
      System.out.println(p.getName() + ", " + p.getAge()); 
     }
     
     System.out.println();
     
     //Approach 13: Grouping sorting by gender 
     
     Map<Sex, List<Person>> mapByGenderMap = 
       roster.stream().collect(Collectors.groupingBy(Person::getGender));
     
     for (Map.Entry<Sex, List<Person>> entry : mapByGenderMap.entrySet()){
     
      System.out.println("Persons that are: " + entry.getKey());
      for (Person person : entry.getValue()) {
       person.printPerson();
      }      
     }  

     System.out.println();
     
     //Approach 14: Comparator that is implemented on the fly 
     List<Person> personsSortedByNameList = roster.stream()
       .sorted(new Comparator<Person>() {
        public int compare(Person x, Person y){
         return x.getName().compareTo(y.getName());
        }
    }).collect(Collectors.toList());
     
     System.out.println("Persons sorted by name");
     for (Person person : personsSortedByNameList){
      person.printPerson();
     }  

   //Approach 15: Comparator that is implemented on the fly using a lambda expression 
   List<Person> personsSortedByNameThroughLambdaList  =  roster.stream()
     .sorted((x,y)-> x.getName()
     .compareTo(y.getName())).collect(Collectors.toList()); 
     
    System.out.println("Persons sorted by name through lambda:");
     
    for (Person person : personsSortedByNameThroughLambdaList)
    {
      person.printPerson();
    }        
          
 } 
   

}

First perspectives of the Java 8 support

From looking through the sample code, one can see that Java 8 has got support for most of what LINQ offers, there are also some major drawbacks or shortcomings that are clear. I am not experienced with Java 8, so perhaps some of my considerations here are either wrong or inprecise, but overall the syntax of Java 8 is much more verbose and less fluent than that of LINQ. Also, there are differences between Java 8 and C#. The Consumer concept of Java is great and looks like a missing part in C#. While Java 8 uses ->, C# uses =>, and the rest of the functional syntax is very similar. Runnable also looks like Action of C#. The anonymous class concept of Java is however flawed compared to C# true anonymous functions. To do projections and a requirement to create a new class definition or interface definition is not very flexible. Instantiating interfaces is supported in Java and not in C#, this is however a positive feature of Java that C# lacks. Java supports also default implementations and static implementations in interfaces, which is very practical in many cases - C# does not support these default methods inside interfaces. To do grouping and sorting requires implementing comparators and groupings go via maps. All in all, C# and LINQ is so more flexible and the chaining syntax makes complicated functional programming easy. In Java 8, much functional programming is possible - in fact most functional programming that's supported in LINQ is also supported in Java 8 I guess - it is just so verbose, fragmented and indirect to do basic things such as sorting. I also like the true anonymous functions of C# using thew new { } syntax, which Java 8 seems to lack. The dynamic type inference that the "var" keyword of C# supports seems also to lack when working with Java 8 - you often have to help the compiler by doing specific casts. Again, there might be some misunderstandings here, but all in all Java 8 functional programmings seems "harder" than using C# and LINQ, but I guess this will improve as Java 8 now has got functional programming support. Also, Scala and Clojure are similar languages that are alternatives, where functional programming have been supported for years and have matured. It looks like Java's "marriage" with Oracle has halted progression and while Java 8 has evolved, there are still many parts of the language that can be improved to support functional programming. However, much about Java 8 is positive and it is to be expected that more C# developers (and VB?) will be attracted to Java since it now supports functional programming. But beware, C# programmers - here be dragons! There will be a learning curve and that compact syntax of C# you have learned to love is not the same experience when doing Java 8 functional programming!

Share this article on LinkedIn.