Lambda Expression

(int a) -> a * 2; // Calculate the double of a
a -> 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) -> {
	int sum = x + y;
	int avg = sum / 2;
	return avg;
}

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

interface MyMath {
    int getDoubleOf(int a);
}
	
MyMath d = a -> a * 2; // associated to the interface
d.getDoubleOf(4); // is 8

All examples with “list” use :

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

Collections

sort sort(list, comparator)

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

removeIf

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

merge merge(key, value, remappingFunction)

Map<String, String> names = new HashMap<>();
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 ReferenceLambda Form
StaticMethod::isPrimen -> StaticMethod.isPrime(n)
String::toUpperCase(String w) -> w.toUpperCase()
String::compareTo(String s, String t) -> s.compareTo(t)
System.out::printlnx -> System.out.println(x)
Double::newn -> new Double(n)
String[]::new(int n) -> new String[n]

Streams

Similar to collections, but

// 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 array
list.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 Set
List<String> myList = stream.collect(Collectors.toList());
Set<String> mySet = stream.collect(Collectors.toSet());

// Collect into a String
String str = list.collect(Collectors.joining(", "));

map map(mapper) Applying a function to each element

// Apply "toLowerCase" for each element
res = stream.map(w -> w.toLowerCase());
res = stream.map(String::toLowerCase);
//> bohr darwin galilei tesla einstein newton

res = Stream.of(1,2,3,4,5).map(x -> x + 1);
//> 2 3 4 5 6

filter filter(predicate) Retains elements that match the predicate

// Filter elements that begin with "E"
res = stream.filter(n -> n.substring(0, 1).equals("E"));
//> Einstein

res = Stream.of(1,2,3,4,5).filter(x -> x < 3);
//> 1 2

reduce Reduce the elements to a single value

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

limit limit(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 elements
boolean res = 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<Integer>) are inefficients. It requires a lot of unboxing and boxing for each element. Better to use IntStream, DoubleStream, etc.

Creation

IntStream stream = IntStream.of(1, 2, 3, 5, 7);
stream = IntStream.of(myArray); // from an array
stream = IntStream.range(5, 80); // range from 5 to 80

Random gen = new Random();
IntStream rand = 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 length
Map<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 group
Collectors.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 nothing
Optional<String> res = stream
   .filter(w -> w.length() > 10)
   .findFirst();

// length of the value or "" if nothing
int length = res.orElse("").length();

// run the lambda if there is a value
res.ifPresent(v -> results.add(v));

Return an Optional

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

Note on inferance limitations

interface Pair<A, B> {
    A first();
    B second();
}

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

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

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