JAVA 8 Cheat Sheet

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

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

(x, y) -> {
    int sum = x + y;
    int avg = sum / 2;
    return avg;
}

interface MyMath {
    int getDoubleOf(int a);
}

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

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

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

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

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]

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

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);

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

// Method Reference:
getPrimes(numbers, StaticMethod::isPrime);

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

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);

// 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(", "));

// 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 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

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

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

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

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

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

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

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

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

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

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

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

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

// 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));

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