EventDescriptionValidity.java

package com.company;


import java.util.Optional;
import java.util.function.Function;

class Validity { }

class Valid extends Validity { }

class Invalid extends Validity { }

class EventDescription<TValid extends Validity> {

    private final String data;

    private EventDescription(String data) {
        this.data = data;
    }

    public String get() {
        return data;
    }

    public EventDescription<Invalid> map(Function<String, String> mapper) {
        return new EventDescription<>(mapper.apply(data));
    }

    public Optional<EventDescription<Valid>> validate() {
        if (this.data.contains("<script>")) {
            return Optional.empty();
        } else {
            return Optional.of(new EventDescription<>(this.data));
        }
    }

    public static EventDescription<Invalid> create(String data) {
        return new EventDescription<>(data);
    }
}

class EventAPI {
    public static void createEvent(EventDescription<Valid> description) {
        // value is guaranteed to be valid
        String value = description.get();
    }
}

public class Main {

    public static void main(String[] args) {
        EventDescription<Invalid> description = EventDescription.create("hello");

        description.validate().ifPresent(EventAPI::createEvent);
    }
}