java11-lambda-patterns

Functional programming patterns in java.

Reference: https://www.youtube.com/watch?v=YnzisJh-ZNI
Reference: https://www.youtube.com/watch?v=e4MT_OguDKg&index=146
Reference: https://www.amazon.com/Modern-Java-Action-functional-programming/dp/1617293563

introduction

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

project description

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

   @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

   @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();
       }
   }
   

   and we want to:
   • 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)
                 );
     }