Java typeclasses

Type classes example in Java. Inspired by Scala example (slides), Typeclasses

• In Haskell, typeclass is the language feature.
• In Scala, typeclass is a pattern.
• In Java, typeclass is a experiment.

A type class is a type system construct that supports ad hoc polymorphism. This is achieved by adding constraints to type variables in parametrically polymorphic types. Such a constraint typically involves a type class T and a type variable a, and means that a can only be instantiated to a type whose members support the overloaded operations associated with T. Wikipedia

In this code Scala implicits are replaced by Spring DI. Case classes are created with Lombok @Value and matched by javaslang-match.

Requirements

• Calculate final checkout
  • OrderEvaluation::evaluate
• Calculate average orders price
  • Orders::average through Stats::mean

Goals

• DRY principle
• Separation of concerns
  • Evaluation is delegated to OrderEvaluation and ProductEvaluation
• Decouple functionality form types
• Simple and clean domain model
  • Immutable case classes * Basic | Discounted | OutOfStock implements Product * GeneralOrder | ComplexOrder | CanceledOrder implements Order
• [Single responsibility] (https://en.wikipedia.org/wiki/Single_responsibility_principle) * additional domain features are delegated to specialized classes (Stats and Evaluable implementations)

Run

• ./gradlew test