TwinColumn

@JmixEntity
@Table(name = "UIEX1_EMPLOYEE")
@Entity(name = "uiex1_Employee")
public class Employee {
    @InstanceName
    @Column(name = "NAME")
    private String name;

    @JmixGeneratedValue
    @Column(name = "ID", nullable = false)
    @Id
    private UUID id;
}

<data>
    <collection id="employeesDc"
                class="ui.ex1.entity.Employee">
        <fetchPlan extends="_local"/>
        <loader id="employeesDl">
            <query>
                <![CDATA[select e from uiex1_Employee e]]>
            </query>
        </loader>
    </collection>
</data>
<layout>
    <twinColumn id="twinColumn"
                addAllBtnEnabled="true"
                optionsContainer="employeesDc"/>
</layout>

<button id="button" caption="Show value"/>

@Autowired
private TwinColumn<Employee> twinColumn;
@Autowired
private Notifications notifications;
@Autowired
private MetadataTools metadataTools;

@Subscribe("button")
protected void onButtonClick(Button.ClickEvent event) {
    StringBuilder sb = new StringBuilder();
    Collection<Employee> employees = twinColumn.getValue();
    if (employees != null) {
        notifications.create()
                .withCaption(
                        employees.stream()
                                .map(employee -> metadataTools.getInstanceName(employee))
                                .collect(Collectors.joining("\n"))
                )
                .show();

    }
}