2. 2
Lambda expressions
Lambda expressions represent anonymous functions.
Like Methods, they have a typed argument list, a return type, exceptions,
and a body.
Without lambda:
filter(new Predicate<String>(){
@Override
public boolean test(String s){
return s.length() > 5;
}
});
With lambda:
filter(s -> s.length() > 5);
3. 3
Lambda expressions syntax - parameters
A comma-separated list of formal parameters enclosed in parentheses.
● In most cases, the data type can be omitted.
ArrayList<String> strings = new ArrayList<String>();
strings.sort((String s1, String s2) -> s1.compareTo(s2));
strings.sort((s1, s2) -> s1.compareTo(s2));
● If there’s only one parameter, the parentheses can be omitted.
filter(s -> s.length() > 5);
5. 5
Lambda expressions syntax - body
The lambda body.
● If the body is a single expression, it’s value is calculated and returned with no
need of brackets, the return command or ‘;’
filter(s -> s.length() > 5 && s.length() < 10);
● If there’s more than a single expression, the complete syntax is required
filter(s -> {
int len = s.length();
return len > 5 && len < 10;
});
6. 6
Lambda expressions properties
● Lambda expressions don’t create a new scope.
• External variables can be used as final
• Lambda parameter names should be different than external variables
• ‘this’ will refer to the object in the surrounding scope
int limit = 5;
String s = "";
filter(s -> // Produces an error
s.length() <= limit); // refers to the external variable
7. 7
Interfaces
Until now, interfaces could only contain abstract methods.
Now, interfaces can contain code, with Static methods and Default methods.
● Static methods are just like static methods in classes
● Default methods are defined with the ‘default’ keyword, and are
implemented within the interface.
• Overriding default methods is optional, but not mandatory like abstract
methods.
• Can be overridden both in extending interfaces and implementing classes.
• Default methods can be added to existing interfaces without a need to
change implementing classes.
8. 8
Interfaces example
public interface Predicate<T> {
boolean test(T t); // abstract method
default Predicate<T> negate(){
return (t) -> !test(t);
}
}
The negate method is a default method that returns a new predicate which is
negative to the original predicate.
9. 9
Functional interfaces
● A functional interface is an Interface that has only one abstract method.
● Functional interfaces are the type for lambda expressions, and can be
assigned by a lambda expression.
● Can be marked by the @FunctionalInterface annotation
● Java provides ready functional interfaces for different uses, in the package
java.util.function, like:
• Function (Object -> Object)
• Consumer (Object -> void)
• Predicate (Object -> Boolean)
• Supplier (void -> Object)
11. 11
Streams
Stream is a sequence of elements from a source that supports aggregate
operations.
Calculate average salary for employees from London,
Without streams:
for(Employee p : employees){
if (p.getCity().equals("London")){
sum += p.getSalary();
count++;
}
}
average = sum / count;
With streams:
average = employees.stream().
filter(p -> p.getCity().equals("London")).
mapToInt(p -> p.getSalary()).
average().getAsDouble();
12. 12
Streams
Advantages:
● Pipelining – Many stream operations return streams, which enables chaining
and several optimizations such as laziness.
● Internal iteration – Iteration over collections is done internally
● Can make use of multicore architectures
Stream is not a data structure.
13. 13
Stream pipeline
A stream pipeline consists of 3 parts:
1. A source
2. Zero or more intermediate operations, that return a stream
3. A terminal operation, that produces a result or a side effect
14. 14
Intermediate operations
Useful intermediate operations:
● Filter(Predicate) – Returns a stream of elements that match the predicate
● Map(Function) – Apply the function on the stream elements and return a
stream of the results
● FlatMap(Function) – Similar to map, but returns a stream for each element.
All streams are concatenated to a single stream.
● Peek(Consumer) – Perform an action on the stream elements and return the
stream unchanged
● Sorted(Comparator) – Return a sorted stream according to the comparator
● Distinct – Returns a stream of distinct elements, according to the object’s
‘equal’ method
15. 15
Terminal operations
Useful terminal operations:
● forEach – Perform an action for each stream element, returns nothing.
● allMatch, anyMatch, noneMatch – Return true if all, any or none stream
elements match a predicate.
● count – return number of elements
● findFirst, findAny – return the first or any stream element
● max, min – return the max or min element according to a comparator
● collect, reduce – complex operations
● IntStream, LongStream, DoubleStream – contain mathematical operations
like sum, average and summaryStatistics.
16. 16
Stream operations properties
● Stream operations must be non-interfering, which means they shouldn’t
change the source of the stream.
● Intermediate operations are lazy – applying an intermediate operation does
not perform anything until a terminal operation is executed.
● If no terminal operation is executed, the intermediate operations are not
applied at all.
● Intermediate operations can be Stateless or Stateful
• Stateless operations: filter, map
• Stateful operations: sorted, distinct
17. 17
Parallel streams
Streams can be executed in parallel to (hopefully) increase runtime
performance.
To use parallel streams, use the parallelStream() method, or method
parallel() on a given stream.
Reduce and Collect operations need additional information when used in
parallel mode.
Things to consider before using parallel streams:
Java Parallel Streams Are Bad for Your Health!
What's Wrong in Java 8, Part III: Streams and Parallel Streams
18. 18
Method reference
If a lambda expression only calls another method, you can reference that
method immediately instead of calling it.
Method reference is done with the :: operator.
Methods that can be referenced:
● Static methods
● Instance methods
● Methods of particular instances
● Constructors (Person::new)
19. 19
Method reference - example
● Instance method:
ArrayList<String> strings = new ArrayList<String>();
strings.stream().mapToInt(s -> s.length()).sum();
strings.stream().mapToInt(String::length).sum();
● Method of particular instance:
strings.stream().forEach(s -> System.out.println(s));
strings.stream().forEach(System.out::println);
20. 20
Optional
Optional is a value container which might or might not contain a value.
It assists programmers to avoid Null Pointer Exception failure by providing
useful methods:
● isPresent – return true if a value is present
● ifPresent(Consumer) – perform an action if a value is present.
● orElse(T other) – Return the value if present, or a default value otherwise.
● get – return the value, or throw exception if there’s no value
In Java 8 many methods return Optional when null might be returned.
21. 21
DateTime API
A new API that replaces the previous Date and Calendar API.
Main classes:
● Clock
● LocalDate
● LocalTime
● LocalDateTime
● Duration
22. 22
DateTime API – example
LocalDateTime from = LocalDateTime.of(2014, Month.APRIL, 16, 0, 0,
0);
LocalDateTime to = LocalDateTime.of(2015, Month.APRIL, 16, 23, 59,
59);
Duration duration = Duration.between(from, to);
System.out.println("Duration in days: " + duration.toDays());
System.out.println("Duration in hours: " + duration.toHours());
Output:
● Duration in days: 365
● Duration in hours: 8783
23. 23
Javascript – Nashorn engine
Java 8 comes with a new JS engine that allows developing and running JS
code on JVM.
Example:
ScriptEngine engine = new
ScriptEngineManager().getEngineByName("nashorn");
engine.eval("print('Hello World!');");
String jsCode = "function f() { return 1; }; f() + 1;";
System.out.println( "Result:" + engine.eval(jsCode));
Output:
Hello World!
Result:2.0
