EventSourcingV4.java

package com.example.demo;

import java.util.Arrays;
import java.util.List;
import java.util.UUID;

import lombok.Value;

import static io.vavr.API.$;
import static io.vavr.API.Case;
import static io.vavr.API.Match;
import static io.vavr.Predicates.instanceOf;

/**
 * This solution uses Pattern Matching.
 *
 * The main advantages is that it is concise. Pattern matching can be used everywhere.
 * The main disavantages is that the compiler does not help when new events are created (May be mitigated with unit tests).
 */
public class EventSourcingV4 {

    interface Event {
        String getLoanApplicationReference();

        default String getVersion() {
            return "V1";
        }
    }

    interface EventV1 extends Event {
        @Override
        default String getVersion() {
            return "V1";
        }
    }

    @Value
    static class CreatedV1 implements EventV1 {

        private String loanApplicationReference;
    }

    @Value
    static class GrantedV1 implements EventV1 {

        private String loanApplicationReference;

    }

    interface EventV2 extends Event {
        @Override
        default String getVersion() {
            return "V2";
        }
    }

    @Value
    static class CreatedV2 implements EventV2 {

        private String loanApplicationReference;

    }

    @Value
    static class GrantedV2 implements EventV2 {

        private String loanApplicationReference;

    }

    interface State {
        default boolean isEmpty() {
            return false;
        }
        default boolean isCreated() {
            return false;
        }
        default boolean isGranted() {
            return false;
        }
    }

    static class States {
        public static State empty() {
            return new Empty();
        }
    }

    @Value
    static class Empty implements State {
        boolean isEmpty = true;
    }

    @Value
    static class Created implements State {
        String loanApplicationReference;
        boolean isCreated = true;
    }

    @Value
    static class Granted implements State {
        String loanApplicationReference;
        boolean isGranted = true;
    }

    static State apply(Event event, State state) {
        return Match(event).of(
            Case($(instanceOf(CreatedV1.class)), e -> applyEvent(e, state)),
            Case($(instanceOf(GrantedV1.class)), e -> applyEvent(e, state)),

            Case($(instanceOf(CreatedV2.class)), e -> applyEvent(e, state)),
            Case($(instanceOf(GrantedV2.class)), e -> applyEvent(e, state))
        );
    }

    static State apply(List<Event> events, State state) {
        return events.stream()
            .reduce(state, (s , e) -> apply(e, s), (state1, state2) -> state2);
    }

    static State applyEvent(CreatedV1 event, State state) {
        return new Created(event.getLoanApplicationReference());
    }

    static State applyEvent(GrantedV1 event, State state) {
        return new Granted(event.getLoanApplicationReference());
    }

    static State applyEvent(CreatedV2 event, State state) {
        return new Created(event.getLoanApplicationReference());
    }

    static State applyEvent(GrantedV2 event, State state) {
        return new Granted(event.getLoanApplicationReference());
    }

    public static void main(String[] args) {
        String loanApplicationReference = UUID.randomUUID().toString();
        State endState = apply(Arrays.asList(
            new CreatedV1(loanApplicationReference),
            new GrantedV1(loanApplicationReference)
        ), States.empty());

        System.out.println(endState.isEmpty());
        System.out.println(endState.isCreated());
        System.out.println(endState.isGranted());
    }

}