Question

As we all know the Generics about Java Collections that E or a wildcard ? is required to instantiate the allowed contents / objects in a particular collection.

My question is there a way we could know the wildcard or object of a particular collection from the code below?

Object inbound = java.io.ObjectInputStream().readObject();
if(inbound instanceof List<?>) {
   // know `?.getClass()`
}

Answer 1

You can only find the type parameters if inbound is a class that defines its type parameters in a type declaration. For example, suppose you serialized an instance of NodeList that was declared like this:

final class NodeList extends ArrayList<Node> { }

Then when you deserialize it, you can do this:

Object inbound = ois.readObject();
if (inbound instanceof List<?>) {
  Type t = inbound.getClass().getGenericSuperclass();
  if (t instanceof ParameterizedType) {
    ParameterizedType pt = (ParameterizedType) t;
    for (Type p : pt.getActualTypeArguments()) {
      if (p instanceof TypeVariable<?>)
        System.out.println("Unknown");
      else
        System.out.println(p); /* Prints "interface Node" */
    }
  }
}