Lambdas and Stream

Streams and Lambdas
in Java 8
STREAMS AND LAMBDAS IN JAVA 8 | © PROF. DR. DANIEL JOBST | WS 2015/16 | VERSION 1.0
1
Working Environment
-  Integrated Development Environment (IDE)
-  JDK 8
STREAMS AND LAMBDAS IN JAVA 8 | © PROF. DR. DANIEL JOBST | WS 2015/16 | VERSION 1.0 5
WORKING ENVIRONMENT
Integrated Development Environments

NetBeans by Oracle 
https://netbeans.org/downloads/
NetBeans Any bundle will do 
current version: 8.1
by Eclipse Foundation (formerly by IBM) 

http://www.eclipse.org/downloads/ 
Eclipse current version: Eclipse Mars (4.5); with Neon available 
Choose bundle „...Java EE Developers“  
or „...Java Developers“
Intellij IDEA by JetBrains (commercial) 
https://www.jetbrains.com/idea/ 
IDEA current version: 15 
Students can get „Ultimate Edition“ for free 
See https://www.jetbrains.com/student/
WORKING ENVIRONMENT
JDK 1.8 (JDK 8)

$ java -‐version
Java
java version "1.8.0_65"
Java(TM) SE Runtime
... JLS JDK Environment (build 1.8.0_65-‐
b17)
Java HotSpot(TM) 64-‐Bit
JRE Tools Server VM (build 25.65-‐b01,
javac.exe mixed mode)
JVM API
java.exe java.*
WORKING ENVIRONMENT
Dependency Management with Maven

Manage dependencies to 3rd party recursively and have your project
build automatically
e.g. use Google Guava Libraries by simply adding one dependency.
Basics Revisited
-  Interfaces
-  Collections Framework
-  Anonymous Inner Classes
BASICS REVISITED
Interfaces
“Ordinary” interfaces
Marker interfaces (e.g. Serializable or Runnable)
Functional interfaces (annotated with @FunctionalInterface)
@interface classes (Annotations)
New in Java 8:
static methods
default methods
Default methods1
“A default method is a method that is declared in an interface with the default modifier; its body is always represented by a block. It
provides a default implementa=on for any class that implements the interface without overriding the method. Default methods are
dis=nct from concrete methods (§8.4.3.1), which are declared in classes.”
1[Gosling 2015, p. 288]
BASICS REVISITED
Generics with <>

Generic class: Generic method:
T
<<interface>> Collec,ons
Comparable
+ <<T>> emptySet() : Set<T>
Definition
+ compareTo( other : T ) : int + <<U>> singleton( : U ) : Set<U>
public interface Comparable<T> { public class Collections {

public int compareTo(T other); public <T> Set<T> singleton(T obj)
} { ... }
}
class Student Set<String> name =

implements Comparable<Person> Collections.singleton("Max");
{
Usage
public int compareTo(Person other) Set<Person> persons =

{ Collections.<Person>emptySet();
// ...
}
}
BASICS REVISITED
Collections Framework
Two things to remember:
1.  There are Lists, Sets, and Maps: 
 
List<T>: ArrayList<T> or LinkedList<T>
Set<T>: TreeSet<T> or HashSet<T>
Map<K, V>: TreeMap<K, V> or HashMap<K, V>

2.  Use “loosly coupled” references: 
 
List<String> = new ArrayList<>();
Collections Framework1
“The collec=ons framework is a unified architecture for represen=ng and manipula=ng collec=ons, enabling them to be manipulated
independently of the details of their representa=on. It reduces programming effort while increasing performance. […]”
1[Oracle Corp. 2016]
BASICS REVISITED
Collection interfaces (abridged)

<<interface>>
E <<interface>>
E
Collec,on Iterable
<<interface>>
E E <<interface>>
E <<interface>>
E
<<interface>>
List Queue Set SortedSet
<<interface>> <<interface>>
E
Deque NavigableSet

E
AbstractCollec,on
E

T
AbstractSet

AbstractList

E
AbstractSequen,alList

E
E
E
E
E
ArrayList Vector LinkedList HashSet TreeSet
BASICS REVISITED
Map interfaces (abridged)
K,V <<interface>>
K,V
<<interface>>
Map SortedMap

T
K,V <<interface>>
K,V
Dic,onary AbstractMap
NavigableMap
E

K,V

K,V

Hashtable HashMap TreeMap
E
E
Proper,es LinkedHashMap
COLLECTIONS
Utilities in class Collections

public static void reverse(List<?> list)
public static <E> Collection<E> checkedCollection(Collection<E> c,
Class<E> type)
public static <T> List<T> nCopies(int n, T o)
public static int frequency(Collection<?> c, Object o)
public static void shuffle(List<?> list)
public static void rotate(List<?> list, int distance)
public static void reverse(List<?> list)
public static <T> boolean replaceAll(List<T> list, T oldVal, T newVal)
public static <T> T min(Collection<? extends T> coll,
Comparator<? super T> comp)
// others:
Arrays.asList(Object... o)
Arrays.stream(T[] array)
Stream.of(T[] array)

BASICS REVISITED
Anonymous Inner Classes

public interface Comparator<T> { int compare (T obj1, T obj2); }
public class Collections {

public static <T> void sort(List<T> l, Comparator<? super T> c)
{...}
}
List<String> names = Arrays.asList("John", "Andrew", "Eve");

Collections.sort(names, new Comparator<String>() {
@Override public int compare(String s1, String s2) {
return Integer.compare(s1.length(), s2.length());
}
});

for(String name : names) System.out.print(name + " ");
// Eve John Andrew
Streams
STREAMS
Streams
A stream is “[…] a sequence of elements from a source that supports
aggregate operations”:1
§  “Sequence of elements: A stream provides an interface to a sequenced set

of values of a specific element type. However, streams don’t actually store
elements; they are computed on demand.
§  Source: Streams conamounte from a data-providing source such as
collections, arrays, or I/O resources.
§  Aggregate operations: Streams support SQL-like operations and common
operations from functional programing languages, such as filter, map,
reduce, find, match, sorted, and so on.”
1[Urma (2014a), section “Getting Started With Streams”]
STREAMS
Streams
§  are not data structure
§  do not contain storage for data
§  are “pipelines” for streams of data (i.e. of objects)
§  while in the pipeline data undergo transformation 
(without changing the data structure holding it)
§  wrap collections (lists, set, maps)
§  read data from it
§  work on copies
STREAMS
How do data get into streams?

Streams are mainly generated based on collections:
List<String> names = Arrays.asList("John", "George", "Sue");

Stream<String> stream1 = names.stream();

Stream<String> stream2 = Stream.of("Tom", "Rita", "Mae");

Stream<String> stream3;
stream2 = Arrays.stream( new String[]{"Lisa", "Max", "Anna"} );

Or with builder pattern:

Stream<String> stream4 = Stream.<String>builder()
.add("Mike")
.add("Sandra").build();
STREAMS
How do data get out of streams?

§  The Streaming API provides so called “finalizing” methods  forEach
(i.e. methods that do not return stream objects) toArray
collect
reduce
min
max
count
anyMatch
noneMatch
findFirst
findAny
STREAMS
Streaming example
“Take all names from the stream that start with the letter “J”, map the names
into capital letters, skip one, and collect them into a new set”
List<String> names = Stream.of("John","George","Joe","Sue","James");

Stream<String> stream1 = names.stream();

______ = stream1.filter( ___________ )
.map( ___________ )
.skip( ___________ )
.collect( ___________ );

Lambda Expressions and
Functional Interfaces
-  Lambdas
-  Functional Interfaces
LAMBDA EXPRESSIONS
Lambdas or Closures
“Lambda” = “closure” = record storing a function (functionality,
method) and its environment (but without a class or method name)
Roughly: anonymous method
Lambdas represent source code - not data and not object state!
Syntax:
( parameter list ) -‐> { expression(s) }
Examples:
(int x, int y) -‐> { return x + y; }
(long x) -‐> { return x * 2; }
() -‐> { String msg = "Lambda"; System.out.println(msg); }

For details on “functional programming” cf. [Huges 1984] or [Turner 2013]
LAMBDA EXPRESSIONS
Lambdas and functional interfaces

Functional interfaces are so called SAM types (single abstract method)
A functional interface has exactly one abstract method, e.g. Runnable,
Comparator<T>, or Comparable<T>
Functional interfaces can define 0..* default methods and 0..* static
methods
Using the @FunctionalInterface annotation on classes the compiler
is required to generate an error message if it is no interface and it
doesn’t define exactly one SAM.
@FunctionalInterface
public interface Counter<T> {
int count(T obj);
}
Task: Create a first lambda expression that counts the letters within a String
Counter<String> strCount = (String s) -‐> { return s.length(); };
[https://docs.oracle.com/javase/8/docs/api/java/lang/FunctionalInterface.html]
LAMBDA EXPRESSIONS
Your first lambda with Comparator<T>

public interface Comparator<T> { int compare(T obj1, T obj2); }
public class Collections {

public static <T> void sort(List<T> l, Comparator<? super T> c)
{...}
}
Your task:
Prepare your first lambda expression
to compare two String objects by length
List<String> names = Arrays.asList("John", "Andrew", "Eve");

Collections.sort(names, ???? );

for(String name : names) System.out.print(name + " ");
// Eve John Andrew
LAMBDA EXPRESSIONS
Functional interfaces in JDK

Selection of most used interfaces from package java.util.function:
// Computes a single input with no result
public interface Consumer<T> {
void accept(T t);
default Consumer<T> andThen(Consumer<? super T> after) {...}
}

// Represents a supplier of results.
interface Supplier<T> {
T get();
}

// Computes a single output, prodeces output of different type
interface Function<T,R> {
R apply(T t);
default <V> Function<T,V> andThen(Function<? super R,? extends V> after) {...}
default <V> Function<V,R> compose(Function<? super V,? extends T> before) {...}
}

// Represents a predicate (boolean-‐valued function) of one argument
interface Predicate<T> {
boolean test(T t);
default Predicate<T> and(Predicate<? super T> other) {...}
default Predicate<T> negate() {...}
// ...
}
[https://docs.oracle.com/javase/8/docs/api/java/util/function/package-‐amountmary.html]
LAMBDA EXPRESSIONS
Type inference
Lambda expressions allow for minimal syntax if compiler can deduct
type information (so called type inference), e.g.: 
public interface BiFunction<T, U, R> {
R = Integer could be inferred from return type of
R apply(T t, U u);
expression t.length() + u.length() à Integer 
} (e.g. when used directly in typed method call)
BiFunction<String, Object, Integer> bf;

compiler can infer types from declaration of f
bf = (String txt, Object obj) -‐> { return t.length() + u.hashCode() ); }
// can be even shorter:
bf = (txt, obj) -‐> t.length() + u.hashCode(); // se below
for single return statements keyword return

Further syntax shortening examples: together with {}-pair can be dropped
(int x, int y) -‐> { return x * y; } // shorter: (x, y) -‐> x * y
(long x) -‐> { return x * 2; } // shortest: x -‐> x * 2

( )-pair can be dropped with only one parameter
LAMBDA EXPRESSIONS
Lambda as parameters and return types

Further examples for type inference using Comparator<T> as
functional interface:
List<String> names = Arrays.asList("Ahab", "George", "Sue");
Collections.sort(names, (s1, s2) -‐> s1.length() -‐ s2.length() );
T :: String can be deducted from names being a List<String>
Signature of Collections::sort method is:

public static <T> void sort(List<T> list, Comparator<? super T> c)
Collections.sort(names, createComparator() );
public Comparator<String> createComparator() {

return (s1, s2) -‐> s1.length() -‐ s2.length();
}
Statement returned here is of functional
interface type Comparator<String>
LAMBDA EXPRESSIONS
Method references (“function pointers”)

Syntax: Classname::methodName objectReferenceName::methodName
Lambdas can be replaced by method references whenever there
would not further actions within the the lambda
Examples:
Reference Method reference… …replacing Lambda

sta=c method String::valueOf obj -‐> String.valueOf(obj)
instance method String::compareTo (s1, s2) -‐> s1.comapreTo(s2)
(via class)
instance method person::getName () -‐> person.getName()
(via object ref)
Constructor ArrayList::new () -‐> new ArrayList<>()
[Table taken from Inden 2015, p. 812]
Streaming API
-  Creating Streams
-  Fluently working with streams
-  Finalize Streams
STREAMING API
Stream ops I use most
STREAMING API
Stream operations
Source => STREAM => OP1 => OP2 => ... => OPn => Result
Create “lazy” Intermediate Terminal

not performed
until
// interface Collection // stateless forEach

stream() filter toArray
parallelStream() map collect
flatMap reduce
// Utiliy class peek min Check the
Arrays.stream(...) //stateful max factory
Arrays.stream(...).parallel() distinct methods in
count
Collectors for
sorted anyMatch a set of
limit noneMatch Collector-
skip findFirst Operations
Return an object findAny
reference of Type
Stream<E> Check the java doc for more information on parameter types
and return types;
intermediate operations return (of course) Stream<E> object
references [Inden 2015, p. 825]
STREAMING API
Class Optional<T>
The class Optional<T> is a container wrapped around an object and is useful
if you do not know whether its content is null or not (e.g. when using in
fluent programming style).
T
<<interface>>
Optional
+ get() : T
+ isPresent() : boolean
+ ifPresent( : Consumer<? super T> )
+ orElse( other : T ) : T
STREAMING API
Example
List<Transaction> groceryTransactions = new Arraylist<>();
for(Transaction t : transactions){
if(t.getType() == Transaction.GROCERY) {
groceryTransactions.add(t);
}
“old fashioned”
}
Collections.sort(groceryTransactions, new Comparator(){
@Override
public int compare(Transaction t1, Transaction t2){
return t2.getValue().compareTo(t1.getValue());
}
});
List<Integer> transactionIds = new ArrayList<>();
for(Transaction t : groceryTransactions) {
transactionsIds.add(t.getId());
}
List<Integer> transactionsIds =
Stream like
transactions.stream()
.filter(t -‐> t.getType() == Transaction.GROCERY)
.sorted(comparing(Transaction::getValue).reversed())
.map(Transaction::getId)
.collect(toList());
[taken from Urma (2014a)]
Bibliography
Gosling, J. et al. (2015): The Java® Language Specification – Java SE 8 Edition, March
2015, http://docs.oracle.com/javase/specs/jls/se8/jls8.pdf
Huges, J. (1984): Why Functional Programming Matters, rev. ed., http://
www.cse.chalmers.se/~rjmh/Papers/whyfp.pdf
Oracle Corp. (2016): The Collections Framework, Website, http://docs.oracle.com/
javase/8/docs/technotes/guides/collections/
Turner, D. (2013): Some History of Functional Programming Languages. In: Loidl, H.-
W. and Pena, R.: Trends in Functional Programming: 13th International Symposium,
TFP 2012, St. Andrews, UK, June 12-14, 2012, Revised Selected Papers, p. 1-20.
Springer: Berlin and Heidelberg
Urma, R.-G. (2014a): Processing Data with Java SE 8 Streams, Part 1, Java Magazine,
March/April 2014, http://www.oracle.com/technetwork/articles/java/ma14-‐
java-‐se-‐8-‐streams-‐2177646.html
Urma, R.-G. (2014b): Part 2: Processing Data with Java SE 8 Streams, Java Magazine,
May/June 2014, http://www.oracle.com/technetwork/articles/java/architect-‐
streams-‐pt2-‐2227132.html

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Streams and Lambdas

Integrated Development Environments

by Eclipse Foundation (formerly by IBM)

JDK 1.8 (JDK 8)

Dependency Management with Maven

Generics with <>

+ compareTo( other : T ) : int + <<U>> singleton( : U ) : Set<U>

public interface Comparable<T> { public class Collections {

class Student Set<String> name =

public int compareTo(Person other) Set<Person> persons =

Collection interfaces (abridged)

Map interfaces (abridged)

Hashtable HashMap TreeMap

Utilities in class Collections

Anonymous Inner Classes

public class Collections {

List<String> names = Arrays.asList("John", "Andrew", "Eve");

§ “Sequence of elements: A stream provides an interface to a sequenced set

1[Urma (2014a), section “Getting Started With Streams”]

How do data get into streams?

List<String> names = Arrays.asList("John", "George", "Sue");

Or with builder pattern:

How do data get out of streams?

List<String> names = Stream.of("John","George","Joe","Sue","James");

(long x) -­‐> { return x * 2; }

() -­‐> { String msg = "Lambda"; System.out.println(msg); }

Lambdas and functional interfaces

Your first lambda with Comparator<T>

public class Collections {

List<String> names = Arrays.asList("John", "Andrew", "Eve");

Functional interfaces in JDK

BiFunction<String, Object, Integer> bf;

for single return statements keyword return

Lambda as parameters and return types

T :: String can be deducted from names being a List<String>

Signature of Collections::sort method is:

Collections.sort(names, createComparator() );

public Comparator<String> createComparator() {

Method references (“function pointers”)

Reference Method reference… …replacing Lambda

[Table taken from Inden 2015, p. 812]

Stream ops I use most

Create “lazy” Intermediate Terminal

// interface Collection // stateless forEach

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by Eclipse Foundation (formerly by IBM) 

§  “Sequence of elements: A stream provides an interface to a sequenced set

(long x) -‐> { return x * 2; }

() -‐> { String msg = "Lambda"; System.out.println(msg); }