Im trying to bind selected values of SelectManyCheckbox to ArrayList, which is the value of my HashMap.

If my options are inside an ArrayList and selected options are aswell. i have no issue.

BUT when i want to set these selected options as a value of a map. it will populate an array of Object[] no matter what i try.

im using Mojarra 4.0.5.

MyBean

@Component
@ViewScoped
@Getter
@Setter
public class TestView {
    Map<String, ArrayList<Dto>> map;
    ArrayList<String> strings;
    ArrayList<Dto> options;
    @PostConstruct
    public void init() {
        Dto dto1 = new Dto();
        Dto dto2 = new Dto();
        dto1.setAge(1);
        dto1.setName("OPTION1");
        dto2.setAge(2);
        dto2.setName("OPTION2");
        options = new ArrayList<Dto>();
        options.add(dto1);
        options.add(dto2);
        map = new HashMap<String, ArrayList<Dto>>();
        strings = new ArrayList<String>();
        strings.add(new String("TEST1"));
        strings.add(new String("TEST2"));
        
        strings.forEach(s -> map.put(s, new ArrayList<Dto>()));
    }
    public void debug() {
        System.out.println("debug");
        ArrayList<ArrayList<Dto>> testarr = new ArrayList<ArrayList<Dto>>();
        map.forEach((k,v)->testarr.add(v));
    }
    
}

Dto

public class Dto {
    private String name;
    private int age;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
}

Converter

@FacesConverter(value="myConverter")
public class MyConverter implements Converter<Dto> {

    private Map<Dto, String> entities = new HashMap<Dto, String>();

    @Override
    public String getAsString(FacesContext context, UIComponent component, Dto value) {
        synchronized (entities) {
            if (!entities.containsKey(value)) {
                String uuid = UUID.randomUUID().toString();
                entities.put(value, uuid);
                return uuid;
            } else {
                return entities.get(value);
            }
        }
    }

    @Override
    public Dto getAsObject(FacesContext context, UIComponent component, String value) {
        for (Map.Entry<Dto, String> entry : entities.entrySet()) {
            if (entry.getValue().equals(value)) {
                return entry.getKey();
            }
        }
        return null;
    }
}

Xhtml

<ui:repeat var="str" value="#{testView.strings}" varStatus="strStatus">
    <ui:param name="indexStr" value="#{strStatus.index}" />
    <p:selectManyCheckbox value="#{testView.map[str]}"
        converter="myConverter" layout="responsive" columns="6">
        <p:ajax process="@this" />
        <f:selectItems value="#{testView.options}" var="item"
            itemLabel="#{item.name}" itemValue="#{item}" />
    </p:selectManyCheckbox>
</ui:repeat>
<p:commandButton value="debug" icon="pi pi-check" process="@this"
    action="#{testView.debug()}" />

You're victim of type erasure in generics.

Basically, in raw Java source code you have Map<String, ArrayList<Dto>> map.

But in the compiled output of that code it has become Map map and information about String and ArrayList<Dto> is lost.

This is the same problem the EL expressions #{...} are facing. When it tries to inspect the type of value="#{testView.map[str]}" via ValueExpression#getType() it retrieves java.lang.Object not java.lang.ArrayList.

Technically speaking, setting collectionType attribute as follows must work

<p:selectManyCheckbox ... collectionType="java.util.ArrayList">

But to my surprise it indeed doesn't have any effect. It would only work if the java.lang.Object was actually an instance of java.util.Collection. I consider this a bug in Mojarra. I've fixed this via issue 5427.

By the way, your converter is not thread safe and it's not entirely memory efficient: Arguments against a generic JSF object converter with a static WeakHashMap. Take a look at OmniFaces SelectItemsConverter.