 hamdiank
7/6/2018 - 2:03 PM

# 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 ReferenceLambda 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"));
``````

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

``````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
``````

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>>` :

• `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
``````

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