JAVA 8 - Cheat Sheet

Lambda Expression

(inta) -> a * 2; // Calculate the double of aa -> a * 2; // or simply without type

(a, b) -> a + b; // Sum of 2 parameters

If the lambda is more than one expression we can use { } and return

(x, y) -> {
intsum = x + y;
intavg = sum / 2;
returnavg;
}

A lambda expression cannot stand alone in Java, it need to be associated to a functional interface.

interfaceMyMath {
intgetDoubleOf(inta);
}
MyMathd = a -> a * 2; // associated to the interfaced.getDoubleOf(4); // is 8

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All examples with "list" use :

List<String> list = [Bohr, Darwin, Galilei, Tesla, Einstein, Newton]

Collections

sortsort(list, comparator)

list.sort((a, b) -> a.length() - b.length())
list.sort(Comparator.comparing(n -> n.length())); // samelist.sort(Comparator.comparing(String::length)); // same//> [Bohr, Tesla, Darwin, Newton, Galilei, Einstein]

removeIf

list.removeIf(w -> w.length() < 6);
//> [Darwin, Galilei, Einstein, Newton]

mergemerge(key, value, remappingFunction)

Map<String, String> names = newHashMap<>();
names.put("Albert", "Ein?");
names.put("Marie", "Curie");
names.put("Max", "Plank");
// Value "Albert" exists// {Marie=Curie, Max=Plank, Albert=Einstein}names.merge("Albert", "stein", (old, val) -> old.substring(0, 3) + val);
// Value "Newname" don't exists// {Marie=Curie, Newname=stein, Max=Plank, Albert=Einstein}names.merge("Newname", "stein", (old, val) -> old.substring(0, 3) + val);

Method Expressions Class::staticMethod

Allows to reference methods (and constructors) without executing them

// Lambda Form:getPrimes(numbers, a -> StaticMethod.isPrime(a));
// Method Reference:getPrimes(numbers, StaticMethod::isPrime);

Method Reference	Lambda Form
StaticMethod::isPrime	n -> StaticMethod.isPrime(n)
String::toUpperCase	(String w) -> w.toUpperCase()
String::compareTo	(String s, String t) -> s.compareTo(t)
System.out::println	x -> System.out.println(x)
Double::new	n -> new Double(n)
String[]::new	(int n) -> new String[n]

Streams

Similar to collections, but

• They don't store their own data
• The data comes from elsewhere (collection, file, db, web, ...)
• immutable (produce new streams)
• lazy (only computes what is necessary !)

// Will compute just 3 "filter"Stream<String> longNames = list
.filter(n -> n.length() > 8)
.limit(3);

Create a new stream

Stream<Integer> stream = Stream.of(1, 2, 3, 5, 7, 11);
Stream<String> stream = Stream.of("Jazz", "Blues", "Rock");
Stream<String> stream = Stream.of(myArray); // or from an arraylist.stream(); // or from a list// Infinit stream [0; inf[Stream<Integer> integers = Stream.iterate(0, n -> n + 1);

Collecting results

// Collect into an array (::new is the constructor reference)String[] myArray = stream.toArray(String[]::new);
// Collect into a List or SetList<String> myList = stream.collect(Collectors.toList());
Set<String> mySet = stream.collect(Collectors.toSet());
// Collect into a StringStringstr = list.collect(Collectors.joining(", "));

mapmap(mapper)
Applying a function to each element

// Apply "toLowerCase" for each elementres = stream.map(w -> w.toLowerCase());
res = stream.map(String::toLowerCase);
//> bohr darwin galilei tesla einstein newtonres = Stream.of(1,2,3,4,5).map(x -> x + 1);
//> 2 3 4 5 6

filterfilter(predicate)
Retains elements that match the predicate

// Filter elements that begin with "E"res = stream.filter(n -> n.substring(0, 1).equals("E"));
//> Einsteinres = Stream.of(1,2,3,4,5).filter(x -> x < 3);
//> 1 2

reduce
Reduce the elements to a single value

Stringreduced = stream
.reduce("", (acc, el) -> acc + "|" + el);
//> |Bohr|Darwin|Galilei|Tesla|Einstein|Newton

limitlimit(maxSize) The n first elements

res = stream.limit(3);
//> Bohr Darwin Galilei

skip Discarding the first n elements

res = strem.skip(2); // skip Bohr and Darwin//> Galilei Tesla Einstein Newton

distinct Remove duplicated elemetns

res = Stream.of(1,0,0,1,0,1).distinct();
//> 1 0

sorted Sort elements (must be Comparable)

res = stream.sorted();
//> Bohr Darwin Einstein Galilei Newton Tesla 

allMatch

// Check if there is a "e" in each elementsbooleanres = words.allMatch(n -> n.contains("e"));

anyMatch: Check if there is a "e" in an element
noneMatch: Check if there is no "e" in elements

parallel Returns an equivalent stream that is parallel

findAny faster than findFirst on parallel streams

Primitive-Type Streams

Wrappers (like Stream) are inefficients. It requires a lot of unboxing and boxing for each element. Better to use IntStream, DoubleStream, etc.

Creation

IntStreamstream = IntStream.of(1, 2, 3, 5, 7);
stream = IntStream.of(myArray); // from an arraystream = IntStream.range(5, 80); // range from 5 to 80Randomgen = newRandom();
IntStreamrand = gen(1, 9); // stream of randoms

Use mapToX (mapToObj, mapToDouble, etc.) if the function yields Object, double, etc. values.

Grouping Results

Collectors.groupingBy

// Groupe by lengthMap<Integer, List<String>> groups = stream
.collect(Collectors.groupingBy(w -> w.length()));
//> 4=[Bohr], 5=[Tesla], 6=[Darwin, Newton], ...

Collectors.toSet

// Same as before but with Set
... Collectors.groupingBy(
w -> w.substring(0, 1), Collectors.toSet()) ...

Collectors.counting Count the number of values in a group

Collectors.summing__summingInt, summingLong, summingDouble to sum group values

Collectors.averaging__averagingInt, averagingLong, ...

// Average length of each element of a groupCollectors.averagingInt(String::length)

PS: Don't forget Optional (like Map<T, Optional<T>>) with some Collection methods (like Collectors.maxBy).

Parallel Streams

Creation

Stream<String> parStream = list.parallelStream();
Stream<String> parStream = Stream.of(myArray).parallel();

unordered Can speed up the limit or distinct

stream.parallelStream().unordered().distinct();

PS: Work with the streams library. Eg. use filter(x -> x.length() < 9) instead of a forEach with an if.

Optional

In Java, it is common to use null to denote absence of result. Problems when no checks: NullPointerException.

// Optional<String> contains a string or nothingOptional<String> res = stream
.filter(w -> w.length() > 10)
.findFirst();
// length of the value or "" if nothingintlength = res.orElse("").length();
// run the lambda if there is a valueres.ifPresent(v -> results.add(v));

Return an Optional

Optional<Double> squareRoot(doublex) {
if (x >= 0) { returnOptional.of(Math.sqrt(x)); }
else { returnOptional.empty(); }
}

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Note on inferance limitations

interfacePair<A, B> {
Afirst();
Bsecond();
}

A steam of type Stream<Pair<String, Long>> :

• stream.sorted(Comparator.comparing(Pair::first)) // ok
• stream.sorted(Comparator.comparing(Pair::first).thenComparing(Pair::second)) // dont work

Java cannot infer type for the .comparing(Pair::first) part and fallback to Object, on which Pair::first cannot be applied.

The required type for the whole expression cannot be propagated through the method call (.thenComparing) and used to infer type of the first part.

Type must be given explicitly.

stream.sorted(
Comparator.<Pair<String, Long>, String>comparing(Pair::first)
.thenComparing(Pair::second)
) // ok

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This cheat sheet was based on the lecture of Cay Horstmann http://horstmann.com/heig-vd/spring2015/poo/