Idea how to implement VT/VO compatibility in JVM

Stéphane Épardaud stef at
Thu Jan 22 13:26:33 UTC 2015

On 01/22/2015 02:19 PM, Vitaly Davidovich wrote:
> Can you expand a bit on the part where you say frameworks can't 
> iterate a Collection<any T> without knowing the instantiation? Do you 
> mean existing methods that take Collection<?> won't work without 
> change or something else?
If you're writing a framework and you want to traverse instances, for 
example to implement JSON serialisation, you're going to have code that 

private void traverseFields(Object instance){
  for(Field f : instance.getClass().getFields())
   traverseField(instance, f);

private void traverseField(Object instance, Field f){
  if(f.getDeclaringClass() == Collection.class){

private void traverseCollection(Collection<? extends Object> collection){
  for(Object val : collection)

You can see that at compile-time I don't have any idea about which value 
types I could encounter, so the only instantiation of `T` I will have is 
`Object`, never `value Date` which may pop up there, and since (unboxed) 
value types don't have any common superclass I can't traverse 
collections of them without knowing at compile-time what type they are. 
Which is not possible for frameworks.

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