Productive Programming in Java 8 - with Lambdas and Streams

Productive
Programming
in Java 8
Covering Lambdas and Streams
ocpjava.wordpress.com
Ganesh Samarthyam (ganesh.samarthyam@gmail.com)

❖ Programming examples are
from our book:
❖ Oracle Certiﬁed
Professional Java SE 8
Programmer Exam 1Z0-809:
A Comprehensive OCPJP 8
Certiﬁcation Guide, S.G.
Ganesh, Hari Kiran Kumar,
Tushar Sharma, Apress, 2016.
❖ Website: ocpjava.wordpress.com

He he, but lambdas
are fun, not scary

Java meets functional
programming (with lambdas)

How different is lambdas?
if
while
for
switch

Introducing our star
feature - lambda
functions

List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo");
Consumer<String> printString = string -> System.out.println(string);
strings.forEach(printString);
Lambda
functions!

Lambdas is just a fancy
name for functions
without a name!

What are lambdas?
❖ (Java 8) One way to think about lambdas is
“anonymous function” or “unnamed function” - they
are functions without a name and are not associated
with any class
❖ Functions don’t change external state

Arrays.asList("eeny", "meeny", "miny", “mo”)
.forEach(string -> System.out.println(string));
Internal Iteration
for(String string : strings) {
System.out.println(string);
}
External Iteration
Lambda
expression

You can use lambdas for
some amazing stuff

sediment
pre-
carbon
ultra-ﬁlter
post-
carbon
Filtered
water
E.g., you can compose lambda
functions as in pipes-and-ﬁlters

$ cat limerick.txt
There was a young lady of Niger
Who smiled as she rode on a tiger.
They returned from the ride
With the lady inside
And a smile on the face of the tiger.

$ cat limerick.txt | tr -cs "[:alpha:]" "n" | awk '{print
length(), $0}' | sort | uniq
1 a
2 as
2 of
2 on
3 And
3 Who
3 she
3 the
3 was
4 They
4 With
4 face
4 from
4 lady
4 ride
4 rode
5 Niger
5 There
5 smile
5 tiger
5 young
6 inside
6 smiled
8 returned

List<String> lines
= Files.readAllLines(Paths.get("./limerick.txt"), Charset.defaultCharset());
Map<Integer, List<String>> wordGroups
= lines.stream()
.map(line -> line.replaceAll("W", "n").split("n"))
.flatMap(Arrays::stream)
.sorted()
.distinct()
.collect(Collectors.groupingBy(String::length));
wordGroups.forEach( (count, words) -> {
words.forEach(word -> System.out.printf("%d %s %n", count, word));
});
1 a
2 as
2 of
2 on
3 And
3 Who
3 she
3 the
3 was
4 They
4 With
4 face
4 from
4 lady
4 ride
4 rode
5 Niger
5 There
5 smile
5 tiger
5 young
6 inside
6 smiled
8 returned

Lambdas & streams help in
productive programming!

public static void main(String []file) throws Exception {
// process each file passed as argument
// try opening the file with FileReader
try (FileReader inputFile = new FileReader(file[0])) {
int ch = 0;
while( (ch = inputFile.read()) != -1) {
// ch is of type int - convert it back to char
System.out.print( (char)ch );
}
}
// try-with-resources will automatically release FileReader object
}
public static void main(String []file) throws Exception {
Files.lines(Paths.get(file[0])).forEach(System.out::println);
}
Existing APIs are enriched with
lambdas and streams support

So, lets get our hands dirty
and start coding

interface LambdaFunction {
void call();
}
class FirstLambda {
public static void main(String []args) {
LambdaFunction lambdaFunction = () -> System.out.println("Hello world");
lambdaFunction.call();
}
}
Functional interface - provides
signature for lambda functions
Lambda function/expression
Call to the lambda
Prints “Hello world” on the console when executed

@FunctionalInterface
interface LambdaFunction {
void call();
}
Functional interface
Abstract method providing the signature of the
lambda function
Annotation to explicitly state that it is a functional
interface

Old functional interfaces
// in java.lang package
interface Runnable { void run(); }
// in java.util package
interface Comparator<T> { boolean compare(T x, T y); }
// java.awt.event package:
interface ActionListener { void actionPerformed(ActionEvent e) }
// java.io package
interface FileFilter { boolean accept(File pathName); }

Default methods in
interfaces
public interface Iterator<E> {
boolean hasNext();
E next();
default void remove() {
throw new UnsupportedOperationException("remove");
}
default void forEachRemaining(Consumer<? super E> action) {
Objects.requireNonNull(action);
while (hasNext())
action.accept(next());
}
}

Diamond inheritance
problem?
interface Interface1 {
default public void foo() { System.out.println("Interface1’s foo"); }
}
}
public class Diamond implements Interface1, Interface2 {
new Diamond().foo();
}
}
Error:(9, 8) java: class Diamond inherits unrelated defaults for foo()
from types Interface1 and Interface2

Diamond inheritance
problem?
}
}
public class Diamond implements Interface1, Interface2 {
public void foo() { Interface1.super.foo(); }
new Diamond().foo();
}
}
Add this deﬁnition
to resolve the
ambiguity

Effectively final variables
import java.util.Arrays;
import java.util.List;
class PigLatin {
String suffix = "ay";
List<String> strings = Arrays.asList("one", "two", "three", "four");
strings.forEach(string -> System.out.println(string + suffix));
}
} Accessing “local variable” suffix
here; hence it is considered
“effectively final”

Effectively ﬁnal variables
class PigLatin {
String suffix = "ay";
List<String> strings = Arrays.asList("one", "two", "three", “four");
suffix = "e"; // assign to suffix variable
}
}
PigLatinAssign.java:9: error: local variables referenced from a
lambda expression must be final or effectively final
^
1 error

arg -> System.out.println(arg)
System.out::println
Method references - “syntactic sugar” for
lambda functions
They “route” function parameters

Using built-in functional interfaces
Consumer<String> printString = string -> System.out.println(string);
strings.forEach(string -> System.out.println(string));

Built-in functional interfaces are
a part of the java.util.function
package (in Java 8)

Predicate<T> Checks a condition and returns a
boolean value as result
In filter() method in
java.util.stream.Stream
which is used to remove
elements in the stream that
don’t match the givenConsumer<T> Operation that takes an argument
but returns nothing
In forEach() method in
collections and in
java.util.stream.Stream;
this method is used for
traversing all the elements inFunction<T,
R>
Functions that take an argument
and return a result
In map() method in
java.util.stream.Stream to
transform or operate on the
passed value and return a
result.Supplier<T> Operation that returns a value to
the caller (the returned value
could be same or different
values)
In generate() method in
java.util.stream.Stream to
create a inﬁnite stream of
elements.

Predicate interface
Stream.of("hello", "world")
.filter(str -> str.startsWith("h"))
.forEach(System.out::println);
The ﬁlter() method takes a Predicate
as an argument (predicates are
functions that check a condition and
return a boolean value)

Predicate interface
A Predicate<T> “afﬁrms” something as true or
false: it takes an argument of type T, and returns a
boolean value. You can call test() method on a
Predicate object.
public interface Predicate<T> {
boolean test(T t);
// other methods elided
}

Predicate interface: example
import java.util.function.Predicate;
public class PredicateTest {
Predicate<String> nullCheck = arg -> arg != null;
Predicate<String> emptyCheck = arg -> arg.length() > 0;
Predicate<String> nullAndEmptyCheck = nullCheck.and(emptyCheck);
String helloStr = "hello";
System.out.println(nullAndEmptyCheck.test(helloStr));
String nullStr = null;
System.out.println(nullAndEmptyCheck.test(nullStr));
}
}
Prints:
true
false

Predicate interface: example
import java.util.ArrayList;
public class RemoveIfMethod {
List<String> greeting = new ArrayList<>();
greeting.add("hello");
greeting.add("world");
greeting.removeIf(str -> !str.startsWith("h"));
greeting.forEach(System.out::println);
}
}
Prints:
hello

Consumer interface
Stream.of("hello", "world")
// void forEach(Consumer<? super T> action);
Prints:
hello
world

Consumer interface
A Consumer<T> “consumes” something: it takes
an argument (of generic type T) and returns
nothing (void). You can call accept() method on a
Consumer object.
public interface Consumer<T> {
void accept(T t);
// the default andThen method elided
}

Consumer interface:
Example
Consumer<String> printUpperCase =
str -> System.out.println(str.toUpperCase());
printUpperCase.accept("hello");
Prints:
HELLO

Consumer interface:
Example
import java.util.stream.Stream;
import java.util.function.Consumer;
class ConsumerUse {
Stream<String> strings = Stream.of("hello", "world");
Consumer<String> printString = System.out::println;
}
}
Prints:
hello
world

Function interface
public class FunctionUse {
Arrays.stream("4, -9, 16".split(", "))
.map(Integer::parseInt)
.map(i -> (i < 0) ? -i : i)
}
}
Prints:
4
9
16

Function interface
A Function<T, R> “operates” on something and
returns something: it takes one argument (of
generic type T) and returns an object (of generic
type R). You can call apply() method on a Function
object.
public interface Function<T, R> {
R apply(T t);
// other methods elided
}

Function interface: example
Function<String, Integer> strLength = str -> str.length();
System.out.println(strLength.apply("supercalifragilisticexpialidocious"));
Prints:
34

Function interface: example
import java.util.function.Function;
public class CombineFunctions {
Function<String, Integer> parseInt = Integer:: parseInt ;
Function<Integer, Integer> absInt = Math:: abs ;
Function<String, Integer> parseAndAbsInt = parseInt.andThen(absInt);
Arrays.stream("4, -9, 16".split(", "))
.map(parseAndAbsInt)
.forEach(System. out ::println);
}
}
Prints:
4
9
16

Supplier interface
import java.util.stream.Stream;
import java.util.Random;
class GenerateBooleans {
Random random = new Random();
Stream.generate(random::nextBoolean)
.limit(2)
}
}
Prints two boolean
values “true” and “false”
in random order

Supplier interface
A Supplier<T> “supplies” takes nothing but
returns something: it has no arguments and
returns an object (of generic type T). You can call
get() method on a Supplier object
public interface Supplier<T> {
T get();
// no other methods in this interface
}

Supplier interface: example
Supplier<String> currentDateTime = () -> LocalDateTime.now().toString();
System.out.println(currentDateTime.get());
Prints current time:
2015-10-16T12:40:55.164

Summary of built-in interfaces
in java.util.function interface
❖ There are only four core functional interfaces in this package:
Predicate, Consumer, Function, and Supplier.
❖ The rest of the interfaces are primitive versions, binary
versions, and derived interfaces such as UnaryOperator
interface.
❖ These interfaces differ mainly on the signature of the abstract
methods they declare.
❖ You need to choose the suitable functional interface based on
the context and your need.

Java 8 streams (and parallel streams):
Excellent example of applying functional
programming in practice

Arrays.stream(Object.class.getMethods())
.map(method -> method.getName())
.distinct()
wait
equals
toString
hashCode
getClass
notify
notifyAll

Method[] objectMethods = Object.class.getMethods();
Stream<Method> objectMethodStream = Arrays.stream(objectMethods);
Stream<String> objectMethodNames
= objectMethodStream.map(method -> method.getName());
Stream<String> uniqueObjectMethodNames = objectMethodNames.distinct();
uniqueObjectMethodNames.forEach(System.out::println);
Arrays.stream(Object.class.getMethods())
.map(method -> method.getName())
.distinct()
Breaking up into separate
(looong) statements for our
understanding

stream pipeline
Stream
source
Intermediate
opera1ons
Terminal
opera1on
stream
stream
Examples:
IntStream.range(),
Arrays.stream()
Examples:
map(), ﬁlter(),
dis1nct(), sorted()
Examples:
sum(), collect(),
forEach(), reduce()

DoubleStream.
of(1.0, 4.0, 9.0)
map(Math::sqrt)
.peek(System.out::
println)
Stream
Source (with
elements 1.0,
4.0, and 9.0)
Intermediate
Opera=on 1
(maps to
element values
1.0, 2.0, and 3.0)
Intermediate
Opera=on 2
(prints 1.0, 2.0,
and 3.0)
.sum();
Terminal
Opera=on
(returns the
sum 6.0)
DoubleStream.of(1.0, 4.0, 9.0)
.map(Math::sqrt)
.peek(System.out::println)
.sum();

IntStream.range(1, 6)
You can use range or iterate
factory methods in the
IntStream interface
IntStream.iterate(1, i -> i + 1).limit(5)

1 2 3 4 5
1 4 9 16 25
map(i -> i * i)
IntStream.range(1, 5).map(i -> i * i).forEach(System.out::println);
Using streams instead of imperative for i = 1 to 5, print i * i

Stream.of (1, 2, 3, 4, 5)
.map(i -> i * i)
.peek(i -> System.out.printf("%d ", i))
.count();
prints: 1 4 9 16 25

stream can be
inﬁnite
IntStream.iterate(0, i -> i + 2).forEach(System.out::println);
This code creates inﬁnite stream of even numbers!

IntStream
.iterate(0, i -> i + 2)
.limit(5)
Using the “limit” function to limit the stream to 5 integers

IntStream chars = "bookkeep".chars();
System.out.println(chars.count());
chars.distinct().sorted().forEach(ch -> System.out.printf("%c ", ch));
Cannot “reuse” a stream; this code
throws IllegalStateException

Files.lines(Paths.get("FileRead.java")).forEach(System.out::println);
This code prints the contents of
the ﬁle “FileRead.java” in the
current directory

Pattern.compile(" ").splitAsStream("java 8 streams").forEach(System.out::println);
This code splits the input string “java 8
streams” based on whitespace and hence
prints the strings “java”, “8”, and
“streams” on the console

new Random().ints().limit(5).forEach(System.out::println);
Generates 5 random integers and prints
them on the console

"hello".chars().sorted().forEach(ch -> System.out.printf("%c ", ch));
Extracts characters in the string “hello”,
sorts the chars and prints the chars

I really really hate
concurrency problems

long numOfPrimes = LongStream.rangeClosed(2, 100_000)
.ﬁlter(PrimeNumbers::isPrime)
.count();
System.out.println(numOfPrimes);
Prints 9592
2.510 seconds

Parallel code
Serial code
Let’s ﬂip the switch by
calling parallel() function

long numOfPrimes = LongStream.rangeClosed(2, 100_000)
.parallel()
.ﬁlter(PrimeNumbers::isPrime)
.count();
System.out.println(numOfPrimes);
Prints 9592
1.235 seconds

Wow! That’s an awesome ﬂip
switch!

Internally, parallel streams make
use of fork-join framework

class StringConcatenator {
public static String result = "";
public static void concatStr(String str) {
result = result + " " + str;
}
}
class StringSplitAndConcatenate {
String words[] = "the quick brown fox jumps over the lazy dog".split(" ");
Arrays.stream(words).forEach(StringConcatenator::concatStr);
System.out.println(StringConcatenator.result);
}
}
Gives wrong results with
with parallel() call

Adapt, learn functional
programming!

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and Design Patterns
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Image credits
• http://www.wetplanetwhitewater.com/images/uploads/adam_mills_elliott82.jpg
• http://s.ecrater.com/stores/321182/53e46c705f68d_321182b.jpg
• http://img.viralpatel.net/2014/01/java-lambda-expression.png
• https://i.ytimg.com/vi/3C0R_fEXcYA/maxresdefault.jpg
• http://farm1.static.flickr.com/74/170765090_53762a686c.jpg
• http://www.monazu.com/wp-content/uploads/2012/06/ask-the-right-questions.jpg
• https://s-media-cache-ak0.pinimg.com/736x/43/42/8a/43428ac2c352166374d851e895ed5db1.jpg
• http://cdn.attackofthecute.com/August-17-2011-12-36-23-peekaboo-
kitty-46f32anul-131384-530-410.jpeg
• https://ludchurchmyblog.files.wordpress.com/2010/02/myths-legend-pictures-083.jpg
• http://www.youramazingplaces.com/wp-content/uploads/2013/06/Mauvoisin-Dam-
Switzerland-620x413.jpg
• http://www.welikeviral.com/files/2014/08/Image-914.jpg
• http://geekandpoke.typepad.com/.a/6a00d8341d3df553ef013485f6be4d970c-800wi
• https://qph.fs.quoracdn.net/main-qimg-56547c0506050206b50b80a268bf2a84
• https://i.ytimg.com/vi/kCVsKsgVhBA/maxresdefault.jpg
• http://i.livescience.com/images/i/000/022/395/original/man-dreams-bed.jpg
• https://static-secure.guim.co.uk/sys-images/Guardian/Pix/pictures/2012/11/26/1353952826063/Alarm-
clock-010.jpg

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Productive Programming in Java 8 - with Lambdas and Streams

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