I'm trying to write this method using java generics :

public void trackUserActivity(String operation,TemplateEntity templateEntity,String user,,String entity, HistoryJpaRepository  historyJpaRepository)
{
            HistoryEntity historyEntity = new HistoryEntity();
            historyEntity.setData(new java.sql.Date(Calendar.getInstance().getTimeInMillis()));
            historyEntity.setAction(operation);
            historyEntity.setEntity("TEMPLATE");
            historyEntity.setEntityJson(null);
            if(!operation.contentEquals("CREATE"))
            {
                ObjectWriter ow = new ObjectMapper().writer().withDefaultPrettyPrinter();
                
                String json = null;
                try {
                    json = ow.writeValueAsString(templateEntity);
                } catch (JsonProcessingException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                historyEntity.setEntityJson(json.getBytes());  
       
          
            } 
            historyEntity.setIdEntity(templateEntity.getId());
            historyEntity.setUser(user);       
             historyJpaRepository.save(historyEntity);
     
}

inside this method I have the getId() method which is common to multiple object types, I tried to create an interface with a default method with a generic type T that extends multiple classes :

public interface HistoryUtils <T extends LavorazioneDto,DizionarioDto,CampoDizionarioDto,FontDto,RegolaCampoDizionarioDto,TemplateEntity >{
    
  
    
    
    
    public default void trackUserActivity(String operation,T t,String user,String entity, HistoryJpaRepository  historyJpaRepository)
    {
        HistoryEntity historyEntity = new HistoryEntity();
                historyEntity.setData(new java.sql.Date(Calendar.getInstance().getTimeInMillis()));
                historyEntity.setAction(operation);
                historyEntity.setEntity(entity);
                historyEntity.setEntityJson(null);
                if(!operation.contentEquals("CREATE"))
                {
                    ObjectWriter ow = new ObjectMapper().writer().withDefaultPrettyPrinter();
                    String json = null;
                    try {
                        json = ow.writeValueAsString(t);
                    } catch (JsonProcessingException e) {
                        e.printStackTrace();
                    }
                    historyEntity.setEntityJson(json.toString().getBytes());
              
                }
                historyEntity.setIdEntity(t.getId());
                historyEntity.setUser(user);       
            historyJpaRepository.save(historyEntity);
        
    }

only that after I autowired my interface in one of the services and I try to call the interface method I get this error: The method trackUserActivity(String, LavorazioneDto, String, String, HistoryJpaRepository) in the type HistoryUtils is not applicable for the arguments (String, TemplateEntity, String, String, HistoryJpaRepository). How can I write an interface that makes use of generics and extends all the classes I need?

CodePudding user response：

The declaration

<T extends LavorazioneDto,DizionarioDto,CampoDizionarioDto,FontDto,RegolaCampoDizionarioDto,TemplateEntity >

requires T to extend LavorazioneDto, and to extend DizionarioDto, and so on. That is, it requires T to extend all these types, rather than any of these types. In type theoretic terms, T is an intersection type, not a union type. Java doesn't support union types in type bounds.

The usual solution would be to have the possible subtypes implement a common interface, and use that interface as a parameter type:

interface Identifiable {
    String getId();
}

class LavorazioneDto implements Identifiable {
    public String getId() {
        return id;
    }
}

interface HistoryUtils {
    public default void trackUserActivity(String operation,Identifiable t,String user,String entity, HistoryJpaRepository  historyJpaRepository) {
        t.getId(); // works just fine
    }
}

That is, I don't see why you are using generics in the first place, if plain old polymorphism does the job just fine.

CodePudding user response：

Instead of 'T t' you can pass reference to method t.getId, object class and obj

 trackUserActivity(obj, ObjA::getId, ObjA.class);
 trackUserActivity(obj, ObjB::getId, ObjB.class);
 trackUserActivity(obj, ObjC::getId, ObjC.class);

and replace invocation inside method with:

clazz.cast(obj);
func.apply(above obj)