Functional Programming in Java - Code for Maintainability
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Functional Programming is nothing new to the JVM. But with Java 8 it was the big change in paradigm for Java developers. Learn how switching from imperative to functional results in more maintainable code. The presentation was given at tech meetup KrkDataLink, The missing link between Functional Programming and Data Science.
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Functional Programming in Java - Code for Maintainability
- 1. Functional Programming in Java Code For Maintainability 1 KrkDataLink Kraków 2017-02-15 @marcinstepien www.smart.biz.pl
- 3. What is here 3 Code for maintainability Functional Programing intro Object vs Functional Code examples
- 4. So the next developer doesn’t hate you Code For Maintainability 4
- 5. Maintainable Code 5 Reusable Expressive Clear > Clever Separate of concerns
- 6. Closure Lisp Frege kind of Haskell Scala Java 8, Kotlin, Groovy Functional Programming on JVM 6
- 8. ➔ Streams ➔ Lambda expressions ➔ Functions as first class citizens ➔ Functional Interfaces 8 Java 8 FP
- 9. ➔ No side effects ➔ Final variables ➔ y = f(x) always same y for given x 9 (pure) function
- 10. Function<Integer,Integer> increase = x -> x + 1; #parameter -> body 10 Java 8 function
- 11. 11 Functional interface Function descriptor Predicate<T> T -> boolean Consumer<T> T -> void Function<T,R> T -> R Supplier<T> () -> T UnaryOperator<T> T -> T BinaryOperator<T> (T,T) -> T BiPredicate<L,R> (L,R) -> boolean BiConsumer<T,U> (T,U) -> void BiFunction<T,U,R> (T,U) -> R
- 12. #instead of Runnable task = new Runnable() { public void run() { System.out.println(”no arg consumer” ) } } 12 Replace ceremony with Lambda Runnable task = () -> { System.out.println(”no arg consumer” ); };
- 13. 13 Internal iterators #instead of loops for(String name : names) { System.out.println(name) } #use internal iterators names.forEach(e -> System.out.println(e)) #with method reference names.forEach(System.out::println)
- 14. public int getSumOfEvens(List<Integer> nums){ Integer sum = 0; for(Integer num : nums) { if(num != null && num % 2 == 0) { sum += num; } } return sum; } 14 Fight verbosity with streams nums.stream() .filter(Objects::nonNull) .filter( number -> number % 2 == 0) .reduce(0, Integer::sum);
- 15. 15 Passing functions public int getSum(List<Integer>nums,Predicate<Integer> filter, Bifunction<Integer,Integer,Integer> accumulator) { return nums.stream() .filter(filter) .filter(Objects::nonNull) .reduce(0, accumulator); } Predicate<Integer> isEven = n -> n % 2 == 0; int sum = getSum(numbers, isEven, Integer::sum);
- 16. 16 Map reduce, lazy evaluation public int getTranactions(List<User> users) { return users.stream() # or parallelStream() .filter(u -> u.isActivated()) .map( u -> u.getTranTotal()) .reduce(0, Integer::sum);} #terminal op public Stream<Integer> get(List<User> users) { #no computation is happening here return users.stream() .filter(u -> u.isActivated()) .map( u -> u.getTranTotal()); }
- 17. 17 Lazy evaluation, infinite series Stream<Integer> infiniteSeries = Stream.iterate(1, e -> e + 1)
- 18. 18 Slimmer patterns: decorator ← https://en.wikipedia.org/wiki/Decorator_pattern#Second_example_.28coffee_making_scenario.29 ← Hierarchy of types can be replaced with Function Composition: ● .compose(Function f) ● .andThan(Function f)
- 19. 19 Function composition class Barista { public static Coffee makeCoffee (Coffee baseCoffee, Function<Coffee, Coffee>... addIngredients) { return Stream.of(addIngredients) .reduce( Function.<Coffee>identity(), //or Function::andThan (addIngr1, addIngr2) -> addIngr1.andThan(addIngr2)) Coffee coffee = Barista.makeCoffee( new Arabica(), Coffee::withMilk, Coffee::withSprinkles));
- 20. Object vs Functional 20 ● Imperative ● Mutability ● Lower abstraction ● Reveals impl. details ● Eager by default ● nulls ● Hierarchy of types ● Declarative ● Favors Immutability ● Expressive ● Hides impl. details ● Lazy by default ● Optionals ● Function composition