EventSourcingV3.java

package com.example.demo;

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

import lombok.AllArgsConstructor;
import lombok.Value;

/**
 * This solution is another implementation of EventSourcing. In this implementation each {@link Event} knows how to apply itself on a state. It results in code simplification to apply event on state.
 *
 * <ol>
 *     <li>7 types for events</li>
 * </ol>
 *
 * The main advantages is that it is still safe from a compilation POV. There is less code be Visitor are required for other use cases.
 * The drawback is that if we use event is a client libs, domain code leaks.
 */
public class EventSourcingV3 {

    interface Event {
        String getLoanApplicationReference();

        Data apply(Data s);

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

    }

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

    @Value
    static class CreatedV1 implements EventV1 {

        private String loanApplicationReference;

        @Override
        public Data apply(Data s) {
            return s;
        }
    }

    @Value
    static class GrantedV1 implements EventV1 {

        private String loanApplicationReference;

        @Override
        public Data apply(Data s) {
            return new Data(true);
        }
    }

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

    @Value
    static class CreatedV2 implements EventV2 {

        private String loanApplicationReference;

        @Override
        public Data apply(Data s) {
            return s;
        }

    }

    @Value
    static class GrantedV2 implements EventV2 {

        private String loanApplicationReference;

        @Override
        public Data apply(Data s) {
            new Data(true);
            return s;
        }
    }

    static Data apply(List<Event> es, Data s) {
        return es.stream()
            .reduce(s, (state, event) -> event.apply(s), (s1, s2) -> s2);
    }

    @Value
    static class Data {

        boolean isGranted;

    }

    public static void main(String[] args) {
        String loanApplicationReference = UUID.randomUUID().toString();
        Data initialState = new Data(false);

        Data endState = apply(Arrays.asList(
            new CreatedV1(loanApplicationReference),
            new GrantedV1(loanApplicationReference)
        ), initialState);

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

}