When defining Vector<?> is useful?

Question

When following container will be useful

        Vector<?> wilcardVector;

if only I can do with this is to create new container

    wilcardVector = new Vector<String>();

or to add null value

    wilcardVector.add(null);

Trying to add some other types or pass with different generic method fails when compiling. So what is it for?

Answer 1

One valid use case for unbounded wildcard is to use it as a method parameter, when you only use those Vector's methods independent of type parameter, such as size(), isEmpty(), etc. :

something like :

void sampleMethod(Vector<?> vector) {

    if (vector.isEmpty()) {
        // your logic
    }
}

and compiler will allow to pass any Vector to sampleMethod above.

Answer 2

Vector<?> wilcardVector;

means

Vector<?> wilcardVector=null;

and calling wilcardVector.add(null); will lead to exception as wilcardVector=null

So, you ahve to use

wilcardVector = new Vector<String>();

wilcardVector = new Vector<String>(); should be sued when you are adding "only" Strings and nothing else.