Functional programming patterns in java.

Reference: https://www.youtube.com/watch?v=YnzisJh-ZNI
Reference: https://www.youtube.com/watch?v=e4MT_OguDKg
Reference: https://www.youtube.com/watch?v=ePXnCezwRuw
Reference: https://www.amazon.com/Modern-Java-Action-functional-programming/dp/1617293563

This repo is a mix of functional design patterns that we have seen in books or on the internet.

1. try to design you API in a composable way (package: composable)

   class ShoppingAPI {
       static Function<List<Item>, Cart> buy() {
           return Cart::new;
       }
   
       static Function<Cart, Order> order() {
           return Order::new;
       }
   
       static Function<Order, Delivery> deliver() {
           return Delivery::new;
       }
   
       static Function<List<Item>, Delivery> oneClickBuy() {
           return buy()
                   .andThen(order())
                   .andThen(deliver());
       }
   }
   

   • other used classes are as simple as they can be:

     @Value
     class Cart {
         ImmutableList<Item> items;
     
         Cart(List<Item> items) {
             this.items = ImmutableList.copyOf(items);
         }
     }
     
     @Value
     class Delivery {
         Order order;
     }
     
     @Value
     class Item {
         int id;
     }
     
     @Value
     class Order {
         Cart cart;
     }
     

2. it's often helpful to use currying (https://github.com/mtumilowicz/groovy-closure-currying) and functional interfaces to design API (package: converter)

   @FunctionalInterface
   interface CurrableDoubleBinaryOperator extends DoubleBinaryOperator {
   
       default DoubleUnaryOperator rate(double u) {
           return t -> applyAsDouble(t, u);
       }
   }
   

   then we can easily implement conversion classes

   class RateConverter implements CurrableDoubleBinaryOperator {
   
       @Override
       public double applyAsDouble(double value, double rate) {
           return value * rate;
       }
   
       static DoubleUnaryOperator milesToKmConverter() {
           return new RateConverter().rate(1.609);
       }
   
       static DoubleUnaryOperator celsiusToFahrenheitConverter() {
           return new RateConverter().rate(1.8).andThen(x -> x + 32);
       }
   }
   

3. use tuples and know the stream API (package: customer)

   @Value
   @Builder
   public class Customer {
       ImmutableList<Order> orders;
       ImmutableList<Expense> expenses;
       
       // ... methods
   }
   
   @Value
   @Builder
   class Expense {
       Year year;
       ImmutableSet<String> tags;
   
       Stream<String> getTagsStream() {
           return SetUtils.emptyIfNull(tags).stream();
       }
   }
   
   @Value
   @Builder
   class Order {
       int id;
       BigDecimal price;
   
       boolean hasPrice() {
           return nonNull(price);
       }
   }
   

   examples:

   • find order with max price

     Optional<Order> findOrderWithMaxPrice() {
         return ListUtils.emptyIfNull(orders).stream()
                 .filter(Order::hasPrice)
                 .max(comparing(Order::getPrice));
     
     }
     

   • find top3 orders by price

     Triple<Order, Order, Order> findTop3OrdersByPrice() {
         return ListUtils.emptyIfNull(orders).stream()
                 .filter(Order::hasPrice)
                 .sorted(comparing(Order::getPrice, reverseOrder()))
                 .limit(3)
                 .collect(collectingAndThen(toList(), ListToTripleConverter::convert));
     }
     

   • construct an immutable map with (key, value) = (year, tags from that year)

     ImmutableMap<Year, Set<String>> yearTagsExpensesMap() {
         return ListUtils.emptyIfNull(expenses).stream()
                 .collect(collectingAndThen(groupingBy(Expense::getYear, flatMapping(Expense::getTagsStream, toSet())),
                         ImmutableMap::copyOf)
                 );
     }
     

4. try to avoid decorator pattern - use function composition instead (package: decorator)

   @Value
   @RequiredArgsConstructor
   class Camera {
       Function<Color, Color> transformColors;
   
       Camera() {
           this.transformColors = Function.identity();
       }
   
       Camera withFilter(Function<Color, Color> transform) {
           return new Camera(transformColors.andThen(transform));
       }
   
       Color snap(Color color) {
           return transformColors.apply(color);
       }
   }
   

   and a library of functions to transform colors

   class ColorTransformers {
       static Color brighten(Color color, int modifier) {
           Preconditions.checkArgument(nonNull(color));
           Preconditions.checkArgument(modifier >= 0);
   
           return new Color(red(color) + modifier,
                   green(color) + modifier,
                   blue(color) + modifier);
       }
   
       static Color negate(Color color) {
           Preconditions.checkArgument(nonNull(color));
   
           return new Color(negate(red(color)), negate(green(color)), negate(blue(color)));
       }
   
       private static int negate(int color) {
           Preconditions.checkArgument(color <= 255);
           Preconditions.checkArgument(color >= 0);
   
           return 255 - color;
       }
   
       private static int red(Color color) {
           return color.getRed();
       }
   
       private static int green(Color color) {
           return color.getGreen();
       }
   
       private static int blue(Color color) {
           return color.getBlue();
       }
   }
   

   examples:

   given:
   def camera = new Camera().withFilter({ ColorTransformers.negate(it) })
           .withFilter({ ColorTransformers.brighten(it, 20) })
   
   expect:
   camera.snap(new Color(100, 100, 100)) == new Color(175, 175, 175)
   

5. create complex DSL with hiding creation inside (package: dsl)

   @Value
   @RequiredArgsConstructor(access = AccessLevel.PRIVATE)
   public class Mailer {
       private static final Mailer EMPTY = new Mailer();
   
       String from;
       String to;
   
       private Mailer() {
           this.from = "";
           this.to = "";
       }
   
       Mailer from(String from) {
           return new Mailer(StringUtils.defaultIfEmpty(from, ""), to);
       }
   
       Mailer to(String to) {
           return new Mailer(from, StringUtils.defaultIfEmpty(to, ""));
       }
   
       static void send(UnaryOperator<Mailer> block) {
           System.out.println(block.apply(EMPTY));
       }
   }
   

   and the example of usage:

   Mailer.send(
       mailer -> mailer.from("mtumilowicz01@gmail.com")
                       .to("abc@o2.pl")
   )
   

   note that at any point we don't have direct access to the object, we cannot create object manually and we cannot reuse it (there is NO Mailer object)

6. know the comparator API (package: person)

   suppose we want to compare person by name, then by surname (if surname is null goes first)

   @Value
   @Builder
   class Person {
       static final Comparator<Person> NAME_SURNAME_COMPARATOR = comparing(Person::getName)
               .thenComparing(Person::getSurname, nullsFirst(naturalOrder()));
   
       String name;
       String surname;
   }
   

   and tests:

   given:
   def B_B = Person.builder().name("B").surname("B_B").build()
   def C_A = Person.builder().name("C").surname("C_A").build()
   def A = Person.builder().name("A").surname("A").build()
   def B_A = Person.builder().name("B").surname("B_A").build()
   def C_null = Person.builder().name("C").surname(null).build()
   def C_null2 = Person.builder().name("C").surname(null).build()
   
   when:
   def list = List.of(B_B, C_A, A, B_A, C_null, C_null2)
           .stream()
           .sorted(Person.NAME_SURNAME_COMPARATOR)
           .collect(toList())
   
   then:
   list == [A, B_A, B_B, C_null, C_null2, C_A]
   

7. compose behaviours instead of accumulating objects in lists (package: salary)

   suppose we want to calculate salary according to some salary rules

   public enum SalaryRules {
       TAX(new RateConverter().rate(0.81)),
       BONUS(new RateConverter().rate(1.2)),
       ADDITION(salary -> salary + 100);
   
       public final DoubleUnaryOperator operator;
   
       SalaryRules(DoubleUnaryOperator operator) {
           this.operator = operator;
       }
   }
   

   • naive approach

     class NaiveSalaryCalculator {
         final List<SalaryRules> operators = new LinkedList<>();
     
         NaiveSalaryCalculator with(SalaryRules rule) {
             operators.add(rule);
     
             return this;
         }
     
         double calculate(double salary) {
             return operators.stream()
                     .map(rule -> rule.operator)
                     .reduce(DoubleUnaryOperator.identity(), DoubleUnaryOperator::andThen)
                     .applyAsDouble(salary);
     
         }
     }
     

   • better approach - composing functions

     class SalaryCalculator {
         private final DoubleUnaryOperator operator;
     
         SalaryCalculator() {
             this(DoubleUnaryOperator.identity());
         }
     
         private SalaryCalculator(DoubleUnaryOperator operator) {
             this.operator = operator;
         }
     
         SalaryCalculator with(SalaryRules rule) {
             return new SalaryCalculator(operator.andThen(rule.operator));
         }
     
         double calculate(double salary) {
             return operator.applyAsDouble(salary);
     
         }
     }
     

   • tests

     given:
     def calculator = new SalaryCalculator().with(SalaryRules.BONUS)
             .with(SalaryRules.ADDITION)
             .with(SalaryRules.TAX)
     
     expect:
     calculator.calculate(1000) == 1053
     

8. strategy pattern (library of functions) (package: strategy)

   we have PriceProvider to get the current stock price (Stock class is as simple as possible)

   @Value
   class PriceProvider {
       @Getter(AccessLevel.NONE)
       IntUnaryOperator priceSource;
   
       int getPrice(int id) {
           return priceSource.applyAsInt(id);
       }
   }
   
   @Value
   class Stock {
       int id;
   }
   

   example: suppose we want to calculate prices for a given stream of stocks (with some custom filtering)

   @Value
   class Calculator {
       PriceProvider priceProvider;
   
       int totalValues(List<Stock> integers, IntPredicate take) {
           return integers.stream()
                   .map(Stock::getId)
                   .mapToInt(priceProvider::getPrice)
                   .filter(take)
                   .sum();
       }
       
       // library of functions
       static IntPredicate priceLessThan(int limit) {
           return it -> it < limit;
       }
   
       static IntPredicate priceEquals(int limit) {
           return it -> it == limit;
       }
   }
   

   suppose we want to sum stocks with prices < 3 or prices == 5

   given:
   def stocks = [new Stock(1),
                 new Stock(2),
                 new Stock(3),
                 new Stock(4),
                 new Stock(5),
                 new Stock(6),
                 new Stock(7)]
   
   def calculator = new Calculator(new PriceProvider(IntUnaryOperator.identity()))
   
   when:
   def sum = calculator.sumPrices(stocks, Calculator.priceLessThan(3) | Calculator.priceEquals(5))
   
   then:
   sum == 8
   

9. template method (variation of DSL example) (package: template)

   suppose we have AutoCloseable resource

   @Value
   class Resource implements AutoCloseable {
       private Resource(String param) {
           System.out.println("create");
       }
   
       void op1() {
           System.out.println("op1");
       }
   
       void op2() {
           System.out.println("op2");
       }
   
       static void use(String param, Consumer<Resource> block) {
           try (final var resource = new Resource(param)) {
               block.accept(resource);
           }
       }
   
       @Override
       public void close() {
           System.out.println("close");
       }
   }
   

   we publish only template method (use), we don't have direct access to the object - so we guarantee that if someone will use resource it will be closed in the end

   example:

   Resource.use("param", resource -> {resource.op1(); resource.op2();})
   

   produces output:

   create
   op1
   op2
   close