java - Type safe heterogeneous container pattern to store lists of items

Question

I'm trying to implement a type-safe heterogeneous container to store lists of heterogeneous objects.

I have seen several exameples of type-safe heterogeneous container pattern (link) but all of them store a single object of a type.

I have tryed to implement it as follows:

public class EntityOrganizer {  

    private Map<Class<?>, List<Object>> entityMap = new HashMap<Class<?>, List<Object>>();

    public <T> List<T> getEntities(Class<T> clazz) {
        return entityMap.containsKey(clazz) ? entityMap.get(clazz) : Collections.EMPTY_LIST;
    }

    private <T> void addEntity(Class<T> clazz, T entity) {
        List<T> entityList = (List<T>) entityMap.get(clazz);
        if(entityList == null) {
            entityList = new ArrayList<T>();
            entityMap.put(clazz, (List<Object>) entityList);
        }
        entityList.add(entity);
    }   
}

But the problem is this code is full of unchecked casts. Can someone help with a better way of implementing this?

Many thanks

Answer 1

The question is, what is "unchecked cast"?

Sometimes casts are provably safe, unfortunately the proof is beyond javac's capability, which does only limited static analysis enumerated in the spec. But the programmer is smarter than javac.

In this case, I argue that these are "checked casts", and it's very appropriate to suppress the warning.

See 2 other related examples:

Heterogeneous container to store genericly typed objects in Java

Typesafe forName class loading