GWT ADT Example

import java.util.*;
import com.google.gwt.json.client.JSONObject;
import JsonBuilder from https://pastebin.com/SvJtCYee;

public class Event {
    public final String token;
    public final EventTypes.EventType eventType;
    public final String payload;
    public final Date createdAt; // java.time (introduced in Java8) is not supported by GWT

    public Event(String token, EventTypes.EventType eventType, String payload, Date createdAt) {
        this.token = token;
        this.eventType = eventType;
        this.payload = payload;
        this.createdAt = createdAt;
    }

    public JSONObject toJson() {
        return new JsonBuilder().put("token", token).put("eventType", eventType).put("payload", payload).put("createdAt", createdAt).build();
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Event event = (Event) o;
        return token.equals(event.token) &&
                eventType.equals(event.eventType) &&
                payload.equals(event.payload) &&
                createdAt.equals(event.createdAt);
    }

    @Override
    public int hashCode() {
        return Objects.hash(token, eventType, payload, createdAt);
    }

    @Override
    public String toString() {
        return "Event{" +
                "token='" + token + '\'' +
                ", eventType=" + eventType +
                ", payload='" + payload + '\'' +
                ", createdAt=" + createdAt +
                '}';
    }

    public static Optional<Event> fromJson(JSONObject json) {
        return EventTypes.unapply(json.get("eventType").isString().stringValue()).map(eventType -> {
            final String token = json.get("token").isString().stringValue();
            final String payload = json.get("payload").isString().stringValue();
            final Date createdAt = new Date(json.get("createdAt").isString().stringValue());
            return new Event(token, eventType, payload, createdAt);
        });
    }
}



// EventType
import java.util.*;

public class EventTypes {
    public static class EventType {
        @Override public boolean equals(Object obj) {return getClass() == obj.getClass();}
        @Override public String toString() {return getClass().getSimpleName();}
    }
    public final static class Event1 extends EventType {}
    public final static class Event2 extends EventType {}
    public final static class Event3 extends EventType {}

    public static Optional<EventType> unapply(String s) {
        return getAll().stream().filter(eventType -> eventType.getClass().getSimpleName().equals(s)).findFirst();
    }

    public static List<EventType> getAll() {
        return Arrays.asList(            // note that "getDeclaredClasses()" is not supported in GWT
            new Event1(),
            new Event2(),
            new Event3()
        );
    }
}