2. Lambdas & Streams Laboratory
Stuart Marks
Simon Ritter
Angela Caicedo
Oracle Corp.
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3. Lambdas and Functions
Library Review

4. Lambda Expressions Lambda expression is an anonymous function
Think of it like a method
But not associated with a class
Can be used wherever you would use an anonymous inner class
Single abstract method type
Syntax
([optional-parameters]) -> body
Types can be inferred (parameters and return type)

5. Lambda Examples
SomeList<Student> students = ... double highestScore = students.stream(). filter(Student s -> s.getGradYear() == 2011). map(Student s -> s.getScore()). max();
Question: how are we going to get there with real collections?

6. Method References
Method references let us reuse a method as a lambda expression
FileFilter x = new FileFilter() {
public boolean accept(File f) {
return f.canRead(); } };
FileFilter x = (File f) -> f.canRead();
FileFilter x = File::canRead;

7. The Stream Class java.util.stream Stream<T>
A sequence of elements supporting sequential and parallel operations
A Stream is opened by calling:
Collection.stream()
Collection.parallelStream()
Many Stream methods return Stream objects
Very simple (and logical) method chaining

8. Stream Basics Using a Stream means having three things A source
Something that creates a Stream of objects
Zero or more intermediate objects
Take a Stream as input, produce a Stream as output
Potentially modify the contents of the Stream (but don’t have to)
A terminal operation
Takes a Stream as input
Consumes the Stream, or generates some other type of output

9. Stream Usage Multiple operations available
collect, filter, count, skip, limit, sorted
map (and map to types, e.g. mapToInt)
flatMap maps each element in a Stream to possibly multiple elements
e.g. flatMap(line -> Stream.of(line.split(REGEXP));
List<String> names = Arrays.asList(“Bob”, “Alice”, “Charlie”);
System.out.println(names.
stream().
filter(e -> e.getLength() > 4).
findFirst().
get());

10. java.util.function Package
Predicate<T>
Determine if the input of type T matches some criteria
Consumer<T>
Accept a single input argumentof type T, and return no result
Function<T, R>
Apply a function to the input type T, generating a result of type R
Plus several more

11. The iterable Interface
Used by most collections
One method
forEach()
The parameter is a Consumer
wordList.forEach(s -> System.out.println(s));
wordList.forEach(System.out::println);

12. Files and Lines of Text BufferedReader has new method
Stream<String> lines()
HINT: Test framework creates a BufferedReader for you

13. Maps and FlatMaps Map FlatMap Map Values in a Stream 1 to 1 mapping
Input Stream
Output Stream
Map
1 to many mapping
Input Stream
Output Stream
FlatMap

14. Useful Stream Methods collect (terminal) filter (intermediate)
count (terminal)
skip, limit (intermediate)
max (terminal)
getAsInt (terminal)

15. Getting Started Open the LambdasHOL project in NetBeans
The exercises are configured as tests
Edit the tests
Remove annotation
Run the tests (Ctrl F6, or from the menu)
Make the tests pass
Simple!

16. Let’s Go!