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JAVA 8 - Cheat Sheet

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 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 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"));

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)

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();

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();
}


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