JDK8 Lambdas and Streams: Changing The Way You Think When Developing Java

JDK8 Lambdas and Streams:
Changing The Way You Think
When Developing Java
Simon Ritter
Head of Java Technology Evangelism
Oracle Corp.
Twitter: @speakjava
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.

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information purposes only, and may not be incorporated into any contract. It is not a
commitment to deliver any material, code, or functionality, and should not be relied upon
in making purchasing decisions. The development, release, and timing of any features or
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2

Lambda & Streams
Primer

Lambda Expressions In JDK8
• Old style, anonymous inner classes
• New style, using a Lambda expression
4
Simplified Parameterised Behaviour
new Thread(new Runnable {
public void run() {
doSomeStuff();
}
}).start();
new Thread(() -> doSomeStuff()).start();

Lambda Expressions
• Lambda expressions represent anonymous functions
– Same structure as a method
• typed argument list, return type, set of thrown exceptions, and a body
– Not associated with a class
• We now have parameterised behaviour, not just values
Some Details
double highestScore = students
.filter(Student s -> s.getGradYear() == 2011)
.map(Student s -> s.getScore())
.max();
What
How

Lambda Expression Types
• Single-method interfaces are used extensively in Java
– Definition: a functional interface is an interface with one abstract method
– Functional interfaces are identified structurally
– The type of a lambda expression will be a functional interface
• Lambda expressions provide implementations of the abstract method
interface Comparator<T> { boolean compare(T x, T y); }
interface FileFilter { boolean accept(File x); }
interface Runnable { void run(); }
interface ActionListener { void actionPerformed(…); }
interface Callable<T> { T call(); }

Local Variable Capture
• Lambda expressions can refer to effectively final local variables from the
surrounding scope
– Effectively final: A variable that meets the requirements for final variables (i.e.,
assigned once), even if not explicitly declared final
– Closures on values, not variables
void expire(File root, long before) {
root.listFiles(File p -> p.lastModified() <= before);
}

What Does ‘this’ Mean In A Lambda
• ‘this’ refers to the enclosing object, not the lambda itself
• Think of ‘this’ as a final predefined local
• Remember the Lambda is an anonymous function
– It is not associated with a class
– Therefore there can be no ‘this’ for the Lambda

Referencing Instance Variables
Which are not final, or effectively final
class DataProcessor {
private int currentValue;
public void process() {
DataSet myData = myFactory.getDataSet();
dataSet.forEach(d -> d.use(currentValue++));
}
}

Referencing Instance Variables
The compiler helps us out
class DataProcessor {
private int currentValue;
public void process() {
DataSet myData = myFactory.getDataSet();
dataSet.forEach(d -> d.use(this.currentValue++);
}
}
‘this’ (which is effectively final)
inserted by the compiler

Type Inference
• The compiler can often infer parameter types in a lambda expression
 Inferrence based on the target functional interface’s method signature
• Fully statically typed (no dynamic typing sneaking in)
– More typing with less typing
List<String> list = getList();
Collections.sort(list, (String x, String y) -> x.length() - y.length());
Collections.sort(list, (x, y) -> x.length() - y.length());
static T void sort(List<T> l, Comparator<? super T> c);

Method References
• Method references let us reuse a method as a lambda expression
FileFilter x = File f -> f.canRead();
FileFilter x = File::canRead;

Method References
13
Rules For Construction
Lambda
Method Ref
Lambda
Method Ref
Lambda
Method Ref
(args) -> ClassName.staticMethod(args)
(arg0, rest) -> arg0.instanceMethod(rest)
(args) -> expr.instanceMethod(args)
ClassName::staticMethod
ClassName::instanceMethod
expr::instanceMethod
instanceOf

Constructor References
• Same concept as a method reference
– For the constructor
Factory<List<String>> f = ArrayList<String>::new;
Factory<List<String>> f = () -> return new ArrayList<String>();
Replace with

Useful Methods That Can Use Lambdas
• Iterable.forEach(Consumer c)
List<String> myList = ...
myList.forEach(s -> System.out.println(s));
myList.forEach(System.out::println);

• Collection.removeIf(Predicate p)
myList.removeIf(s -> s.length() == 0)

• List.replaceAll(UnaryOperator o)
myList.replaceAll(s -> s.toUpperCase());
myList.replaceAll(String::toUpper);

• List.sort(Comparator c)
• Replaces Collections.sort(List l, Comparator c)
myList.sort((x, y) -> x.length() – y.length());

• Logger.finest(Supplier<String> msgSupplier)
• Overloads Logger.finest(String msg)
logger.finest(produceComplexMessage());
logger.finest(() -> produceComplexMessage());

Functional Interface Definition
• An interface
• Must have only one abstract method
– In JDK 7 this would mean only one method (like ActionListener)
• JDK 8 introduced default methods
– Adding multiple inheritance of types to Java
– These are, by definition, not abstract (they have an implementation)
• JDK 8 also now allows interfaces to have static methods
– Again, not abstract
• @FunctionalInterface can be used to have the compiler check
20

Is This A Functional Interface?
21
@FunctionalInterface
public interface Runnable {
public abstract void run();
}
Yes. There is only
one abstract method

22
public interface Predicate<T> {
default Predicate<T> and(Predicate<? super T> p) {…};
default Predicate<T> negate() {…};
default Predicate<T> or(Predicate<? super T> p) {…};
static <T> Predicate<T> isEqual(Object target) {…};
boolean test(T t);
}
Yes. There is still only
one abstract method

23
public interface Comparator {
// default and static methods elided
int compare(T o1, T o2);
boolean equals(Object obj);
}
The equals(Object)
method is implicit
from the Object class
Therefore only one
abstract method

Stream Overview
• A stream pipeline consists of three types of things
– A source
– Zero or more intermediate operations
– A terminal operation
• Producing a result or a side-effect
Pipeline
int total = transactions.stream()
.filter(t -> t.getBuyer().getCity().equals(“London”))
.mapToInt(Transaction::getPrice)
.sum();
Source
Intermediate operation
Terminal operation

Stream Sources
• From collections and arrays
– Collection.stream()
– Collection.parallelStream()
– Arrays.stream(T array) or Stream.of()
• Static factories
– IntStream.range()
– Files.walk()
• Roll your own
– java.util.Spliterator
Many Ways To Create

Maps and FlatMaps
Map Values in a Stream
Map
FlatMap
Input Stream
Input Stream
1-to-1 mapping
1-to-many mapping
Output Stream
Output Stream

The Optional Class

Optional<T>
Reducing NullPointerException Occurrences
String direction = gpsData.getPosition().getLatitude().getDirection();
String direction = “UNKNOWN”;
if (gpsData != null) {
Position p = gpsData.getPosition();
if (p != null) {
Latitude latitude = p.getLatitude();
if (latitude != null)
direction = latitude.getDirection();
}
}
String direction = gpsData.getPosition().getLatitude().getDirection();

Optional Class
• Terminal operations like min(), max(), etc do not return a direct result
• Suppose the input Stream is empty?
• Optional<T>
– Container for an object reference (null, or real object)
– Think of it like a Stream of 0 or 1 elements
– use get(), ifPresent() and orElse() to access the stored reference
– Can use in more complex ways: filter(), map(), etc
– gpsMaybe.filter(r -> r.lastReading() < 2).ifPresent(GPSData::display);
Helping To Eliminate the NullPointerException

Optional ifPresent()
Do something when set
if (x != null) {
print(x);
}
opt.ifPresent(x -> print(x));
opt.ifPresent(this::print);

Optional filter()
Reject certain values of the Optional
if (x != null && x.contains("a")) {
print(x);
}
opt.filter(x -> x.contains("a"))
.ifPresent(this::print);

Optional map()
Transform value if present
if (x != null) {
String t = x.trim();
if (t.length() > 1)
print(t);
}
opt.map(String::trim)
.filter(t -> t.length() > 1)
.ifPresent(this::print);

Optional flatMap()
Going deeper
public String findSimilar(String s)
Optional<String> tryFindSimilar(String s)
Optional<Optional<String>> bad = opt.map(this::tryFindSimilar);
Optional<String> similar = opt.flatMap(this::tryFindSimilar);

Update Our GPS Code
class GPSData {
public Optional<Position> getPosition() { ... }
}
class Position {
public Optional<Latitude> getLatitude() { ... }
}
class Latitude {
public String getString() { ... }
}

Update Our GPS Code
String direction = Optional.ofNullable(gpsData)
.flatMap(GPSData::getPosition)
.flatMap(Position::getLatitude)
.map(Latitude::getDirection)
.orElse(“None”);

Advanced Lambdas And Streams

Streams and Concurrency

Serial And Parallel Streams
• Collection Stream sources
– stream()
– parallelStream()
• Stream can be made parallel or sequential at any point
– parallel()
– sequential()
• The last call wins
– Whole stream is either sequential or parallel
38

Parallel Streams
• Implemented underneath using the fork-join framework
• Will default to as many threads for the pool as the OS reports processors
– Which may not be what you want
System.setProperty(
"java.util.concurrent.ForkJoinPool.common.parallelism",
"32767");
• Remember, parallel streams always need more work to process
– But they might finish it more quickly
39

When To Use Parallel Streams
• Data set size is important, as is the type of data structure
– ArrayList: GOOD
– HashSet, TreeSet: OK
– LinkedList: BAD
• Operations are also important
– Certain operations decompose to parallel tasks better than others
– filter() and map() are excellent
– sorted() and distinct() do not decompose well
40
Sadly, no simple answer

When To Use Parallel Streams
• N = size of the data set
• Q = Cost per element through the Stream pipeline
• N x Q = Total cost of pipeline operations
• The bigger N x Q is the better a parallel stream will perform
• It is easier to know N than Q, but Q can be estimated
• If in doubt, profile
41
Quantative Considerations

Streams: Pitfalls For The Unwary

Functional v. Imperative
• For functional programming you should not modify state
• Java supports closures over values, not closures over variables
• But state is really useful…
43

Counting Methods That Return Streams
44
Still Thinking Imperatively
Set<String> sourceKeySet = streamReturningMethodMap.keySet();
LongAdder sourceCount = new LongAdder();
sourceKeySet.stream()
.forEach(c ->
sourceCount.add(streamReturningMethodMap.get(c).size()));

Counting Methods That Return Streams
45
Functional Way
sourceKeySet.stream()
.mapToInt(c -> streamReturningMethodMap.get(c).size())
.sum();

Printing And Counting Functional Interfaces
46
Still Thinking Imperatively
LongAdder newMethodCount = new LongAdder();
functionalParameterMethodMap.get(c).stream()
.forEach(m -> {
output.println(m);
if (isNewMethod(c, m))
newMethodCount.increment();
});
return newMethodCount.intValue();

47
More Functional, But Not Pure Functional
int count = functionalParameterMethodMap.get(c).stream()
.mapToInt(m -> {
int newMethod = 0;
output.println(m);
if (isNewMethod(c, m))
newMethod = 1;
return newMethod
})
.sum();
There is still state being
modified in the Lambda

48
Even More Functional, But Still Not Pure Functional
int count = functionalParameterMethodMap.get(nameOfClass)
.stream()
.peek(method -> output.println(method))
.mapToInt(m -> isNewMethod(nameOfClass, m) ? 1 : 0)
.sum();
Strictly speaking printing
is a side effect, which is
not purely functional

The Art Of Reduction
(Or The Need to Think Differently)

A Simple Problem
• Find the length of the longest line in a file
• Hint: BufferedReader has a new method, lines(), that returns a Stream
50
BufferedReader reader = ...
reader.lines()
.mapToInt(String::length)
.max()
.getAsInt();

Another Simple Problem
• Find the length of the longest line in a file
51

Naïve Stream Solution
• That works, so job done, right?
• Not really. Big files will take a long time and a lot of resources
• Must be a better approach
52
String longest = reader.lines().
sort((x, y) -> y.length() - x.length()).
findFirst().
get();

External Iteration Solution
• Simple, but inherently serial
• Not thread safe due to mutable state
53
String longest = "";
while ((String s = reader.readLine()) != null)
if (s.length() > longest.length())
longest = s;

Recursive Approach: The Method
54
String findLongestString(String s, int index, List<String> l) {
if (index >= l.size())
return s;
if (index == l.size() - 1) {
if (s.length() > l.get(index).length())
return s;
return l.get(index);
}
String s2 = findLongestString(l.get(start), index + 1, l);
if (s.length() > s2.length())
return s;
return s2;
}

Recursive Approach: Solving The Problem
• No explicit loop, no mutable state, we’re all good now, right?
• Unfortunately not - larger data sets will generate an OOM exception
55
List<String> lines = new ArrayList<>();
while ((String s = reader.readLine()) != null)
lines.add(s);
String longest = findLongestString("", 0, lines);

A Better Stream Solution
• The Stream API uses the well known filter-map-reduce pattern
• For this problem we do not need to filter or map, just reduce
Optional<T> reduce(BinaryOperator<T> accumulator)
• The key is to find the right accumulator
– The accumulator takes a partial result and the next element, and returns a new
partial result
– In essence it does the same as our recursive solution
– Without all the stack frames
• BinaryOperator is a subclass of BiFunction, but all types are the same
• R apply(T t, U u) or T apply(T x, T y)
56

• Use the recursive approach as an accululator for a reduction
57
String longestLine = reader.lines()
.reduce((x, y) -> {
if (x.length() > y.length())
return x;
return y;
})
.get();

• Use the recursive approach as an accululator for a reduction
58
String longestLine = reader.lines()
.reduce((x, y) -> {
if (x.length() > y.length())
return x;
return y;
})
.get();
x in effect maintains state for
us, by always holding the
longest string found so far

The Simplest Stream Solution
• Use a specialised form of max()
• One that takes a Comparator as a parameter
• comparingInt() is a static method on Comparator
– Comparator<T> comparingInt(ToIntFunction<? extends T> keyExtractor)
59
reader.lines()
.max(comparingInt(String::length))
.get();

Conclusions
• Java needs lambda statements
– Significant improvements in existing libraries are required
• Require a mechanism for interface evolution
– Solution: virtual extension methods
• Bulk operations on Collections
– Much simpler with Lambdas
• Java SE 8 evolves the language, libraries, and VM together

Simon Ritter
Oracle Corporartion
Twitter: @speakjava

JDK8 Lambdas and Streams: Changing The Way You Think When Developing Java

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