Get generic class at compile time

Question

While I know you cannot actually get the type of a generic at runtime because of type erasure, I was wondering if it is possible to get it at compile time.

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Answer 1

Since generic types are subjected to erasure, you will need to specify the java.lang.Class somewhere in the code. One way is to pass it to a generic method:

ObjType obj = /*...*/;
handleObj(obj, SubObjType.class);

// ...

private <T extends ObjType> void handleObj(ObjType obj,
                                           ObjectHandle<T> handle,
                                           Class<T> handleableObjClass) {
    if (handleableObjClass.isInstance(obj)) {
        handle.setObj(handleableObjClass.cast(obj));
    }
}

If you don't know what subclasses of ObjType you're looking for, you will need to add a reifiable Class property to ObjectHandle, similar to how java.util.EnumSet and java.util.EnumMap do it:

class ObjectHandle<T extends ObjType> {

    T obj;

    private final Class<T> objectClass;

    ObjectHandle(Class<T> cls) {
        objectClass = Objects.requireNonNull(cls);
    }

    Class<T> getObjectClass() {
        return objectClass;
    }

    void setObj(T o) {
        obj = o;
    }
}

// ...
ObjectHandle<SubObjType> handle = new ObjectHandle<SubObjType>();
// ...

ObjectType obj = /*...*/;
if (handle.getObjectClass().isInstance(obj)) {
    handle.setObj(handle.getObjectClass().cast(obj));
}