A Brief Conceptual Introduction to Functional Java 8 and its API


Functional Extensions in Java 8
Jörn Guy Süß
October 2016

Agenda
Motivation
Basic Principle (API Blocks)
Streams
Impedance Problems (Nulls, IO)
Questions
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3
4
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Why add functional to Java?
1. Syntax Improvement: Inner Classes are a syntactic pain
2. Performance for all: Fork/Join framework is for experts
3. Cloud Languages: „new“ languages have closures
3

An Example
 IntStream.iterate(0, n -> (n+1)%2)
.distinct()
.limit(10)
.forEach(System.out::println);
 Typical example: stream, closure, terminal operation
4
Create Stream Apply Mapping Export Outcomes
Invocation is lazy from the terminal operation

Attaching Functional Constructs to Java Infrastructure
• Change the parser
• Retrofit collections to work with functional
• Type inference and Functions
• Method References
5

What can you write
Lambda expression Meaning Interface
()->System.out.println() It takes no arguments and displays a single line Consumer<Void> ???
x->System.out.println(x) It takes a single argument and displays it on a
line
Consumer<T>
x->2*x It takes a single argument and returns its double Function<T>
(x,y)->x+y It takes two arguments and returns their sum BiFunction<T,U>
x -> {
int y = 2*x;
return y;
}
It takes a single argument and returns its double
using multiple statements
Function<T>
x -> y -> x + y Curried Function Function<T,
Function<U,V>>
System.out::println Method Reference Consumer<T>
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... Typical Functional Expressions

Retrofit Collections
 Collections need to be a source of closures streams with no hassle
 We cannot break the collection implementations interface to add
methods
 Lets add them to the interface then
 Interfaces now have default methods …
 All collections now have Spliterator that produces the stream
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Or: How interfaces got implementations

Type Check and Functions
 If a Lambda is an inner class,
how do we know if that class fits as a parameter to a method?
 We can use an interface to describe the requirement.
 Ok, but if we try to fit the lambda to the method signatures in the interface, which should
we pick if there are several fits?
 Well than we have to make that impossible.
 Interfaces with only one method?
 One functional method. The others can be default remember?
Lets call these @FunctionalInterface
 Completely clear.
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Method References
 If I just want to invoke a method, does that mean I have to always
write a lambda and implicitly produce an inner class?
 No, we could use invokeDynamic and Method Handles from Java 7,
and you can write something like System.out::println
 In fact, we can use invokeDynamic for all lambda expressions when
we store the bytecode.
 That seems complex. Luckily stack overflow has more details:
http://stackoverflow.com/questions/30002380/why-are-java-8-
lambdas-invoked-using-invokedynamic
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API Blocks – Sources and Sinks
 Supplier<T>s: Allow you to T get() items for streams
 Function<T,U>: You can U apply(T) them to items
 Consumers<T>s: Will accept(T) items from streams
 Predicate<T>: You can boolean test(T) items
 And lots of ...
 versions for Primitive Types, like IntSupplier and ObjDoubleConsumer
 conversion functions like ToIntFunction
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These are the essential concepts
of stream processing

API Blocks – Special Functions and Constructs
 (Unary|Binary)Operator: Functions with homogenous argument
types
 Bi(Supplier|Function|Consumer|Predicate): Operate on
pairs:
stream = Arrays.stream(rectangles);
total = stream
.map(r -> r.scale(0.25))
.mapToInt(Rectangle::getArea)
.reduce(0,(r, s) -> r + s);
(Pair versions of the conversion functions as seen before)
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Streams – Creating one
1. Use a .stream() method from some glue code
2. Call of(T ...) and convert the varargs array
3. Call builder(), add items, and pump it out
4. Call generate(Supplier<T>) e.g. for making endless streams
5. Call empty()
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A stream is a sequential process abstraction
for mutable objects that is not reusable

Streams – Consuming one
Caveat:
Streams are lazy in the functional sense.
Unless and until a terminal operation is called, no processing occurs.
To terminate your stream you can:
1. Use a forEach(Ordered)() to produce a side-effect
2. Call toArray(T ...) to convert the result to an array
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Eventually, every stream leaves the
functional zone to interface with IO

Stream: Getting a single result
 count() the elements
 min|max() biggest, smallest element
 find(First|Any)() obtain an element
 (none|all|any)Match() the logical quantifiers ∄, ∀, ∃
 sorted() find out if the stream is sorted
 reduce|collect(): aggregate elements (functional fold operation)
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Streams are often used to „teles-
cope fold“ data into a single output

Getting a stream from a stream
 filter(): remove anything that does not match
 limit()/skip(): take only / skip N elements
 map(): apply a transformation function
 peek(): run an output on each passing item – Progress bars and loggers!
 concat(): append one stream to another
 sort() : ensure elements are ordered
 distinct(): ensure elements are distinct
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Streams can be „conveyor belts“
for manipulating items sequentially
Do not work on
endless streams!

Numeric Streams – First Class API citizens
 Suppliers, Consumers, Predicates
 Operators on numeric values*
 Functions to convert between other types the numeric value: from, to*
 Numeric Streams with special functions for boxing
 Collectors over streams for Sums, Summaries (Hashes) and
Averages
* A version with two arguments (BiFunction) is also available
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int, long and double have ready-made implementations of Stream Interfaces

Stream Collectors
 group sort the elements into a number of groups using a function
 partition split the elements into disjunct collections using a predicate
 joining Strings with prefix, seperator and postfix
 Conversion to Collection, List, Set, Map
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Collectors are SQL-like domain functions to run in collect() calls

Stream, Spliterator and Parallel Processing
Each Stream is based on a Spliterator which can
 invoke a terminal operation on one (tryAdvance) or all (forEachRemaining) of its contents
 attempt to split another Spliterator of (trySplit) for parallel processing
 estimate its content size (estimateSize)
 provide its Comparator if its source is sorted
 Provide its characteristics()
based on theses Characteristics the streams framework decides if parallel processing is possible, and
enables it if desired and/or opportune.
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parallel()/sequential() change the parallel stream processing advice

Spliterator Characteristics
IMMUTABLE the element source cannot be structurally modified; that is, elements cannot be added, replaced, or removed, so such
changes cannot occur during traversal.
CONCURRENT the element source may be safely concurrently modified (allowing additions, replacements, and/or removals) by multiple
threads without external synchronization.
NONNULL the source guarantees that encountered elements will not be null.
SIZED the value returned from estimateSize() prior to traversal or splitting represents a finite size that, in the absence of
structural source modification, represents an exact count of the number of elements that would be encountered by a
complete traversal.
SUBSIZED all Spliterators resulting from trySplit() will be both SIZED and SUBSIZED. This means that all child Spliterators,
whether direct or indirect, will be SIZED.
ORDERED an encounter order is defined for elements.
DISTINCT for each pair of encountered elements x, y, !x.equals(y).
SORTED encounter order follows a defined sort order.
Confidential – Oracle Internal/Restricted/Highly Restricted
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Impedance Problems (Nulls, IO)
 Optional<T> is a monad that works with null Values
 empty()
 of(Nullable)(T)
 boolean isPresent() is used with filter()
and T get() is used with map()
 Streams are AutoCloseable,
create them in try-with-resources if you use IO
 Example: Stream<JarItem> JarFile.stream()
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Facilities to handle exceptional
flow and null values

A Brief Conceptual Introduction to Functional Java 8 and its API

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