How to use in or to select multiple items?

Your best bet is to bind the <h:selectBooleanCheckbox> value with a Map<Item, Boolean> property where Item represents the object behind the corresponding row.

<h:dataTable value="#{bean.items}" var="item">
    <h:column>
        <h:selectBooleanCheckbox value="#{bean.checked[item]}" />
    </h:column>
    ...
</h:dataTable>
<h:commandButton value="submit" action="#{bean.submit}" />

public class Bean {
    private Map<Item, Boolean> checked = new HashMap<Item, Boolean>();
    private List<Item> items;

    public void submit() {
        List<Item> selectedItems = checked.entrySet().stream()
            .filter(Entry::getValue)
            .map(Entry::getKey)
            .collect(Collectors.toList());

        checked.clear(); // If necessary.

        // Now do your thing with selectedItems.
    }

    // ...
}

You see, the map is automatically filled with all table items as key and the checkbox value is automatically set as map value associated with the item as key.

This only requires that the Item#equals() and Item#hashCode() is properly implemented as per their contracts.

If you can’t guarantee that, then you’d better use a Map<RowId, Boolean> instead where RowId represents the type of the row identifier. Let’s take an example that you’ve a Item object whose identifier property id is a Long:

public class Item {
    private Long id;
    // ...
}

<h:dataTable value="#{bean.items}" var="item">
    <h:column>
        <h:selectBooleanCheckbox value="#{bean.checked[item.id]}" />
    </h:column>
    ...
</h:dataTable>
<h:commandButton value="submit" action="#{bean.submit}" />

public class Bean {
    private Map<Long, Boolean> checked = new HashMap<Long, Boolean>();
    private List<Item> items;

    public void submit() {
        List<Item> selectedItems = items.stream()
            .filter(item -> checked.get(item.getId()))
            .collect(Collectors.toList());

        checked.clear(); // If necessary.

        // Now do your thing with selectedItems.
    }

    // ...
}