Expected behaviour of Java interfaces

Question

I'm very surprise by a non expected behaviour of a class I'm building (kind of Composite pattern).

Here's the point and the classes involved :

• IGroupable is an interface with 2 methods

  public interface IGroupable {

  BigDecimal getTotal();
  Boolean isManuallySettlable();
  

  }

• Event class implements IGroupable

  @Entity @Table(name="V_ALL_BUSINESS_EVENTS") public class EventView implements IGroupable, Serializable {

  @Id
  @Column(name="UNIQUE_ID")
  private long eventId;
  ...
  
  @Override
  public BigDecimal getTotal() {
      return this.eventGrossAmount;
  }
  
  @Override
  public Boolean isManuallySettlable() {
      return (this.eventIsManuallySettlable == null || this.eventIsManuallySettlable == false) ? false : true; 
  }
  

• AbstractGroup abstract class implements IGroupable :

  public abstract class AbstractGroup implements IGroupable, Serializable {

  private static final long serialVersionUID = 1L;
  
  protected int nbRows;
  protected transient List<IGroupable> groups;
  ...
  

• CurrencyGroup, BuyerPlatformGroup and CounterpartExecDateGroup all extend AbstractGroup, so each has a List<IGroupable> groups property

  public class CurrencyGroup extends AbstractGroup {

  protected String currency;
  
  // --------------------------------------------------------
  
  public CurrencyGroup(String currency) {
      this.currency = currency;
      super.groups = new ArrayList<>();
  }
  ...
  
  @Override
  public BigDecimal getTotal() {
      BigDecimal total = new BigDecimal(0);
      total = this.groups.stream()
          .map(groupable -> groupable.getTotal())
          .reduce(total, BigDecimal::add);
      return total;
  }
  
  @Override
  public Boolean isManuallySettlable() {
      boolean isSettlable = true;
      for(IGroupable g : groups) {
          if(!g.isManuallySettlable()) {
              isSettlable = false;
          }
      }
      return isSettlable;
  }
  

  public class BuyerPlatformGroup extends AbstractGroup {

  protected String buyerName;
  protected String buyerRTS;
  protected String platform;
  
  // --------------------------------------------------------
  
  public BuyerPlatformGroup(String buyerName, String buyerRTS, String platform) {
      this.buyerName = buyerName;
      this.buyerRTS = buyerRTS;
      this.platform = platform;
      super.groups = new ArrayList<>();
  }
  
  ...
  
  @Override
  public BigDecimal getTotal() {
      BigDecimal total = new BigDecimal(0);
      total = this.groups.stream()
          .map(groupable -> groupable.getTotal())
          .reduce(total, BigDecimal::add);
      return total;
  }
  
  @Override
  public Boolean isManuallySettlable() {
      boolean isSettlable = true;
      for(IGroupable g : groups) {
          if(!g.isManuallySettlable()) {
              isSettlable = false;
          }
      }
      return isSettlable;
  }
  

  }

  public class CounterpartExecDateGroup extends AbstractGroup {

  protected String counterpart;
  protected Date execDate;
  protected BigDecimal amount;
  protected String step;
  protected String operationType;
  
  // --------------------------------------------------------
  
  public CounterpartExecDateGroup() {
      super.groups = new ArrayList<>();
  }
  ...
  
  @Override
  public BigDecimal getTotal() {
      BigDecimal total = new BigDecimal(0);
      total = groups.stream()
          .map(groupable -> groupable.getTotal())
          .reduce(total, BigDecimal::add);
      return total;
  }
  
  @Override
  public Boolean isManuallySettlable() {
      boolean isSettlable = true;
      for(IGroupable g : groups) {
          if(!g.isManuallySettlable()) {
              isSettlable = false;
          }
      }
      return isSettlable;
  }
  

  }

-> they all inherit from 'AbstractGroup' and also all are 'IGroupable' objects

@Test
    public void allIGroupableTest() {

        List<EventView> events = eventDao.findAll();
        EventView event = (EventView) events.get(0);

        CurrencyGroup ccyGrp = new CurrencyGroup("EUR");
        BuyerPlatformGroup buyerGrp = new BuyerPlatformGroup(event.getEventBuyerName(), event.getEventRTSCode(), event.getEventPlatform());
        CounterpartExecDateGroup counterpartGrp = new CounterpartExecDateGroup();

        ccyGrp.addGroup(buyerGrp); // OK
        buyerGrp.addGroup(counterpartGrp); // OK
        counterpartGrp.setGroups(events); **// ERROR**

But I've got an error, telling me : The method setGroups(List<IGroupable>) in the type AbstractGroup is not applicable for the arguments (List<EventView>).

Why does it behave this way ? As the Event class implements IGroupable interface, a list of IGroupable objects should be able to accept a list of Events, isn't it ?