Java 8 Stream API. A different way to process collections.

Java8 Stream API
A different way to process collections
David Gómez G.
@dgomezg
dgomezg@autentia.com

A Stream is…
An convenience method to iterate over
collections in a declarative way
List<Integer> numbers = new ArrayList<Integer>(); 
for (int i= 0; i < 100 ; i++) { 
numbers.add(i); 
}
List<Integer> evenNumbers = new ArrayList<>(); 
for (int i : numbers) { 
if (i % 2 == 0) { 
evenNumbers.add(i); 
} 
}
@dgomezg

A Stream is…
An convenience method to iterate over
collections in a declarative way
List<Integer> numbers = new ArrayList<Integer>(); 
for (int i= 0; i < 100 ; i++) { 
numbers.add(i); 
}
List<Integer> evenNumbers = numbers.stream() 
.filter(n -> n % 2 == 0) 
.collect(toList());
@dgomezg

So… Streams are collections?
Not Really
Collections Streams
Sequence of elements
Computed at construction
In-memory data structure
Sequence of elements
Computed at iteration
Traversable only Once
External Iteration Internal Iteration
Finite size Infinite size
@dgomezg

Iterating a Collection
List<Integer> evenNumbers = new ArrayList<>(); 
for (int i : numbers) { 
if (i % 2 == 0) { 
evenNumbers.add(i); 
} 
}
External Iteration
- Use forEach or Iterator
- Very verbose
Parallelism by manually using Threads
- Concurrency is hard to be done right!
- Lots of contention and error-prone
- Thread-safety@dgomezg

Iterating a Stream
List<Integer> evenNumbers = numbers.stream() 
.filter(n -> n % 2 == 0) 
.collect(toList());
Internal Iteration
- No manual Iterators handling
- Concise
- Fluent API: chain sequence processing
Elements computed only when needed
@dgomezg

Iterating a Stream
List<Integer> evenNumbers = numbers.parallelStream() 
.filter(n -> n % 2 == 0) 
.collect(toList());
Easily Parallelism
- Concurrency is hard to be done right!
- Uses ForkJoin
- Process steps should be
- stateless
- independent
@dgomezg

@FunctionalInterface
@FunctionalInterface 
public interface Predicate<T> {
 
boolean test(T t);
!
!
!
!
!
}
An interface with exactly one abstract method
!
!
@dgomezg

@FunctionalInterface
 
boolean test(T t);
!
default Predicate<T> negate() { 
return (t) -> !test(t); 
}
 
!
}
An interface with exactly one abstract method
Could have default methods, though!
!
@dgomezg

Lambda Types
Based on abstract method signature from
@FunctionalInterface:
(Arguments) -> <return type>
 
boolean test(T t);
}
T -> boolean
@dgomezg

Lambda Types
public interface Runnable {
 
void run();
}
() -> void
@dgomezg

Lambda Types
public interface Supplier<T> {
 
T get();
}
() -> T
@dgomezg

Lambda Types
public interface BiFunction<T, U, R> {
 
R apply(T t, U t);
}
(T, U) -> R
@dgomezg

Lambda Types
public interface Comparator<T> {
 
int compare(T o1, T o2);
}
(T, T) -> int
@dgomezg

Method References
Allows to use a method name as a lambda
Usually better readability
!
Syntax:
<TargetReference>::<MethodName>
!
TargetReference: Instance or Class
@dgomezg

Method References
phoneCall -> phoneCall.getContact()
Method ReferenceLambda
PhoneCall::getContact
() -> Thread.currentThread() Thread::currentThread
(str, c) -> str.indexOf(c) String::indexOf
(String s) -> System.out.println(s) System.out::println
@dgomezg

Characteristics of A Stream
• Interface to Sequence of elements
• Focused on processing (not on storage)
• Elements computed on demand
(or extracted from source)
• Can be traversed only once
• Internal iteration
• Parallel Support
• Could be Infinite
@dgomezg

Anatomy of a Stream
Source
Intermediate
Operations
filter
map
order
function
Final
operation
pipeline
@dgomezg

Anatomy of Stream Iteration
1. Start from the DataSource (Usually a
collection) and create the Stream
List<Integer> numbers =
Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
 
Stream<Integer> numbersStream = numbers.stream(); 
@dgomezg

2. Add a chain of intermediate Operations
(Stream Pipeline)
Stream<Integer> numbersStream = numbers.stream() 
.filter(new Predicate<Integer>() { 
@Override 
public boolean test(Integer number) { 
return number % 2 == 0; 
} 
})
!
.map(new Function<Integer, Integer>() { 
@Override 
public Integer apply(Integer number) { 
return number * 2; 
} 
});
@dgomezg

2. Add a chain of intermediate Operations
(Stream Pipeline) - Better using lambdas
Stream<Integer> numbersStream = numbers.stream() 
.filter(number -> number % 2 == 0) 
.map(number -> number * 2);
@dgomezg

3. Close with a Terminal Operation
List<Integer> numbersStream = numbers.stream() 
.filter(number -> number % 2 == 0) 
.map(number -> number * 2)
.collect(Collectors.toList());
•The terminal operation triggers Stream Iteration
•Before that, nothing is computed.
•Depending on the terminal operation, the
stream could be fully traversed or not.
@dgomezg

Operation Types
Intermediate operations
• Always return a Stream
• Chain as many as needed (Pipeline)
• Guide processing of data
• Does not start processing
• Can be Stateless or Stateful
Terminal operations
• Can return an object, a collection, or void
• Start the pipeline process
• After its execution, the Stream can not be
revisited

Intermediate Operations
// T -> boolean
Stream<T> filter(Predicate<? super T> predicate);
!
//T -> R 
<R> Stream<R> map(Function<? super T, ? extends R> mapper);
 
//(T,T) -> int 
Stream<T> sorted(Comparator<? super T> comparator);
Stream<T> sorted();
!
//T -> void 
Stream<T> peek(Consumer<? super T> action);
!
Stream<T> distinct(); 
Stream<T> limit(long maxSize); 
Stream<T> skip(long n);
@dgomezg

Final Operations
Object[] toArray();
void forEach(Consumer<? super T> action); //T -> void 
<R, A> R collect(Collector<? super T, A, R> collector); 
!
!
java.util.stream.Collectors.toList();
java.util.stream.Collectors.toSet();
java.util.stream.Collectors.toMap();
java.util.stream.Collectors.joining(CharSequence);
!
!
!
@dgomezg

Final Operations (II)
//T,U -> R
Optional<T> reduce(BinaryOperator<T> accumulator);
//(T,T) -> int 
Optional<T> min(Comparator<? super T> comparator); 
//(T,T) -> int
Optional<T> max(Comparator<? super T> comparator); 
long count(); 
!
@dgomezg

Final Operations (y III)
//T -> boolean
boolean anyMatch(Predicate<? super T> predicate); 
boolean allMatch(Predicate<? super T> predicate); 
boolean noneMatch(Predicate<? super T> predicate); 
!
@dgomezg

Usage examples - Context
public class Contact { 
private final String name; 
private final String city; 
private final String phoneNumber; 
private final LocalDate birth; 
 
 
public int getAge() { 
return Period.between(birth, LocalDate.now()) 
.getYears(); 
} 
//Constructor and getters omitted 
!
} 
@dgomezg

Usage examples - Context
public class PhoneCall { 
private final Contact contact; 
private final LocalDate time; 
private final Duration duration; 
!
//Constructor and getters omitted
} 
Contact me = new Contact("dgomezg", "Madrid", "555 55 55 55", LocalDate.of(1975, Month.MARCH, 26)); 
Contact martin = new Contact("Martin", "Santiago", "666 66 66 66", LocalDate.of(1978, Month.JANUARY, 17)); 
Contact roberto = new Contact("Roberto", "Santiago", "111 11 11 11", LocalDate.of(1973, Month.MAY, 11)); 
Contact heinz = new Contact("Heinz", "Chania", "444 44 44 44", LocalDate.of(1972, Month.APRIL, 29)); 
Contact michael = new Contact("michael", "Munich", "222 22 22 22", LocalDate.of(1976, Month.DECEMBER, 8)); 
 
List<PhoneCall> phoneCallLog = Arrays.asList( 
new PhoneCall(heinz, LocalDate.of(2014, Month.MAY, 28), Duration.ofSeconds(125)), 
new PhoneCall(martin, LocalDate.of(2014, Month.MAY, 30), Duration.ofMinutes(5)), 
new PhoneCall(roberto, LocalDate.of(2014, Month.MAY, 30), Duration.ofMinutes(12)), 
new PhoneCall(michael, LocalDate.of(2014, Month.MAY, 28), Duration.ofMinutes(3)), 
new PhoneCall(michael, LocalDate.of(2014, Month.MAY, 29), Duration.ofSeconds(90)), 
new PhoneCall(heinz, LocalDate.of(2014, Month.MAY, 30), Duration.ofSeconds(365)), 
new PhoneCall(heinz, LocalDate.of(2014, Month.JUNE, 1), Duration.ofMinutes(7)), 
new PhoneCall(martin, LocalDate.of(2014, Month.JUNE, 2), Duration.ofSeconds(315)) 
) ;
@dgomezg

People I phoned in June
phoneCallLog.stream() 
.filter(phoneCall ->
phoneCall.getTime().getMonth() == Month.JUNE) 
.map(phoneCall -> phoneCall.getContact().getName()) 
.distinct() 
.forEach(System.out::println); 
!
@dgomezg

Seconds I talked in May
Long total = phoneCallLog.stream() 
phoneCall.getTime().getMonth() == Month.MAY) 
.map(PhoneCall::getDuration) 
.collect(summingLong(Duration::getSeconds));
@dgomezg

Seconds I talked in May
Optional<Long> total = phoneCallLog.stream() 
.map(PhoneCall::getDuration) 
.reduce(Duration::plus);
 
total.ifPresent(duration ->
{System.out.println(duration.getSeconds());}
); 
!
@dgomezg

Did I phone to Paris?
boolean phonedToParis = phoneCallLog.stream() 
.anyMatch(phoneCall ->
"Paris".equals(phoneCall.getContact().getCity())) 
!
!
@dgomezg

Give me the 3 longest phone calls
.sorted(comparing(PhoneCall::getDuration)) 
.limit(3) 
.forEach(System.out::println);
@dgomezg

Give me the 3 shortest ones
.sorted(comparing(PhoneCall::getDuration).reversed()) 
.limit(3) 
@dgomezg

Streams can be created from
Collections
Directly from values
Generators (infinite Streams)
Resources (like files)
Stream ranges
@dgomezg

From collections
use stream()
List<Integer> numbers = new ArrayList<>(); 
for (int i= 0; i < 10_000_000 ; i++) { 
numbers.add((int)Math.round(Math.random()*100)); 
}
Stream<Integer> evenNumbers = numbers.stream();
or parallelStream()
Stream<Integer> evenNumbers = numbers.parallelStream();
@dgomezg

Directly from Values & ranges
Stream.of("Using", "Stream", "API", "From", “Java8”);
can convert into parallelStream
Stream.of("Using", "Stream", "API", "From", “Java8”)
.parallel(); 
@dgomezg

Generators - Functions
Stream<Integer> integers =
Stream.iterate(0, number -> number + 2);
This is an infinite Stream!,
will never be exhausted!
Stream fibonacci =
Stream.iterate(new int[]{0,1},
t -> new int[]{t[1],t[0]+t[1]});
 
fibonacci.limit(10) 
.map(t -> t[0]) 
@dgomezg

From Resources (Files)
Stream<String> fileContent =
Files.lines(Paths.get(“readme.txt”));
Files.lines(Paths.get(“readme.txt”)) 
.flatMap(line -> Arrays.stream(line.split(" "))) 
.distinct() 
.count()); 
!
Count all distinct words in a file
@dgomezg

Parallel Streams
use stream()
List<Integer> numbers = new ArrayList<>(); 
for (int i= 0; i < 10_000_000 ; i++) { 
numbers.add((int)Math.round(Math.random()*100)); 
}
//This will use just a single thread
Stream<Integer> evenNumbers = numbers.stream();
or parallelStream()
//Automatically select the optimum number of threads
Stream<Integer> evenNumbers = numbers.parallelStream();
@dgomezg

Let’s test it
use stream()
!
for (int i = 0; i < 100; i++) { 
long start = System.currentTimeMillis(); 
List<Integer> even = numbers.stream() 
.filter(n -> n % 2 == 0) 
.sorted() 
.collect(toList());
 
System.out.printf(
"%d elements computed in %5d msecs with %d threadsn”, 
even.size(), System.currentTimeMillis() - start,
Thread.activeCount()); 
}
5001983 elements computed in 828 msecs with 2 threads
@dgomezg

Let’s test it
use stream()
!
for (int i = 0; i < 100; i++) { 
long start = System.currentTimeMillis(); 
List<Integer> even = numbers.parallelStream() 
.filter(n -> n % 2 == 0) 
.sorted() 
.collect(toList());
 
System.out.printf(
"%d elements computed in %5d msecs with %d threadsn”, 
even.size(), System.currentTimeMillis() - start,
Thread.activeCount()); 
}
@dgomezg

Enough, for now,
But this is just the beginning
Thank You.
@dgomezg
dgomezg@gmail.com
www.adictosaltrabajlo.com

Java 8 Stream API. A different way to process collections.

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