Java 8 Cheatsheet by Nikolche Mihajlovski, Google

Lambda expression - easy way to implement single-method (aka functional) interface
Lambda syntax
(parameters) -> expression
(parameters) -> statement
(parameters) -> { statements }
Lambda expression examples
(int x, int y) -> x + y
() -> System.out.println("hi " + s);
(String s) -> { int n = s.length(); return n; }
Run a Runnable
Runnable r = () -> System.out.println("Hello!");
r.run();
The PI function
Callable<Double> pi = () -> 3.14;
Double p = pi.call();
Sort strings by length
String[] words = {"aaa", "b", "cc"};
Arrays.sort(words, (s1, s2) -> s1.length() - s2.length());

// equivalent to:
Arrays.sort(words, (String s1, String s2) -> s1.length() - s2.length());
Effectively final variables can be referenced in lambdas
// s is effectively final (not changed anywhere)
String s = "foo";

// s can be referenced in the lambda
Runnable r = () -> System.out.println(s);
Method reference - easy way to use existing method in a functional way
Static method reference
// Class::staticMethod syntax
Arrays.sort(items, Util::compareItems);

// equivalent to:
Arrays.sort(items, (a, b) -> Util.compareItems(a, b));
Instance method reference
// instance::instanceMethod syntax
items.forEach(System.out::print);

// equivalent to:
items.forEach((x) -> System.out.print(x));
Reference to a method of arbitrary instance
// Class::instanceMethod syntax
items.forEach(Item::publish);

// equivalent to:
items.forEach((x) -> { x.publish(); });
Constructor reference
ConstructorReference cref = Item::new;
Item item = cref.constructor();
Default method - interface method with default implementation
Defining default methods in interfaces
interface Descriptive {

  default String desc() {
    return "fantastic";
  }

}
Implementing interface with default method
class Item implements Descriptive { }

Item x = new Item();

// prints "fantastic"
System.out.println(x.desc());
Stream - sequence of values
Count the non-empty strings
List<String> strings = ...;
long n = strings.stream().filter(x -> !x.isEmpty()).count();
Join item titles
List<Item> items = ...;
String names = items.stream().map((x) -> x.getTitle()).collect(Collectors.joining(", "));
Get distinct countries from cities
List<City> cities = ...;
List<Country> countries = cities.stream().map((c) -> c.getCountry()).distinct().collect(Collectors.toList());
Get count, min, max, sum, and average statistics for items rating
List<Item> items = ...;
IntSummaryStatistics stats = items.stream().mapToInt((x) -> x.getRating()).summaryStatistics();