Converting a collection from Entity to DTO in java rest API

I have a java rest application, where I want to display some information as json. I have two entity classes in play right now Pet and event. The relationship is one too many, so one Pet has a collection of Events.

previously i have made a list of Entity pets and made a custom static methods to transform the Pet entities into DTO objects as such:

@Path("/allPets")
@GET
@Produces(MediaType.APPLICATION_JSON)
public Response getPetsfromCollection() { 


    List<Pet> petList = new ArrayList<>();
    List<DTOPet> DTOPetList = new ArrayList<>();


    // here i'm looping through my list and then converting them to DTO object, and adding dem to a string called json   
    petList.addAll(facade.returnAllPets());
    for(Pet pet: petList){
        DTOPet dtopet = EntitytoDTO.convertDTOPet(pet);
        DTOPetList.add(dtopet);
    }

     String json = gson.toJson(DTOPetList);
     return Response.ok().entity(json).build();
}

Now I want to do the same, but I want to display the event collection, inside of the PetDTO aswell

Here is my PetDTO class

    /*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package DTO;

import Entities.Event;
import java.util.Collection;
import java.util.Date;

/**
 *
 * @author kristoffer
 */
public class DTOPet {

private Integer id;
private String name;
private Date birth;
private String species;
private Date death;
private Collection<DTOEvent> eventCollection;

public DTOPet(Integer id, String name, Date birth, String species, Date death, Collection<DTOEvent> eventCollection) {
    this.id = id;
    this.name = name;
    this.birth = birth;
    this.species = species;
    this.death = death;
    this.eventCollection = eventCollection;
}

public DTOPet() {
}

public Integer getId() {
    return id;
}

public void setId(Integer id) {
    this.id = id;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public Date getBirth() {
    return birth;
}

public void setBirth(Date birth) {
    this.birth = birth;
}

public String getSpecies() {
    return species;
}

public void setSpecies(String species) {
    this.species = species;
}

public Date getDeath() {
    return death;
}

public void setDeath(Date death) {
    this.death = death;
}

public Collection<DTOEvent> getEventCollection() {
    return eventCollection;
}

public void setEventCollection(Collection<DTOEvent> eventCollection) {
    this.eventCollection = eventCollection;
}

}

How can I do this?

I have tried several methods but nothing has succeeded yet.

Here I what I have got so far:

 @Path("/allPetsE")
@GET
@Produces(MediaType.APPLICATION_JSON)
public Response getEventofPets() { 

    //med denne metode skal vi bruge et DTO(data transfer object til at formatere til Json)


    List<Pet> petList = new ArrayList<>();
    List<DTOPet> DTOPetList = new ArrayList<>();
    List<Event> eventList = new ArrayList<>();
    List<DTOEvent> DTOevents = new ArrayList<>();

    petList.addAll(facade.returnAllPets());
    eventList.addAll(facade.returnAllEvents());

    //dette er ikke en pæn løsning, men lad os prøve at få hver event lavet om til et DTO og tilføje det til de forskellige dyr


    for(Pet pet: petList){
        DTOPet dtopet = EntitytoDTO.convertDTOPet(pet);
        dtopet.setEventCollection(DTOevents);
        DTOPetList.add(dtopet);
        //dette bliver ikke performance optimere i føste omgang, vi skal bare få det til at virke


    }


     String json = gson.toJson(DTOPetList);
     return Response.ok().entity(json).build();
}

the issue with my code is that I have no way of converting an entire collection of Event Entity objects into DTO objects, I could try looping through inside the method, but that would have a horrible impact on the performance, alternatively i could try making a JPQL query, but that would still not solve converting to DTO objects.

Another way this could be solved was to change the returntype my current JPQL query like this:

 public Collection<Event> returnAllEvents (){

    EntityManager  em = emf.createEntityManager();
    //vi laver en typed query for at specificere hvilken datatype, det er vi leder efter, i dette tilfælde er det en Pet
    TypedQuery<Event> query = em.createNamedQuery("Event.findAll", Event.class);

    return query.getResultList();
}

so it would return a Collection of collections instead, but I'm not sure how to do this.