How to create an array of generics?

Question

I have the following snippet of code using array of vectors:
Vector[] tmpArr = new Vector[n];
for(int j=0; j<n; j++)
tmpArr[j] = new Vector();
String str = ...
if (!tmpArr[j].contains(str))
tmpArr[j].add(str);
And I want to convert to generics:
Vector<String>[] tmpArr = new Vector<String>[n];
for(int j=0; j<n; j++)
tmpArr[j] = new Vector<String>();
String str = ....
if (!tmpArr[j].contains(str))
tmpArr[j].add(str);
But the first row gives me an error:
-->Generic array creation.
If I change it in
Vector<String>[] tmpArr = new Vector<String>[n];
(as I've seen in a pdf by G.Bracha talking aout collections)
it gives me the error:
-->cannot find symbol
-->method add(String)
in the
tmpArr[j].add(str);
row.
The only way it seems to work is
Vector<String>[] tmpArr = new Vector[n];
but it gives me the unchecked conversion warning.
How can I create an array of generics?
Thank you!
Matteo

Answer 1

You can't
Actually, that depends on the exact definition of a generic array. If by generic array someone means "an array comprised of elements defined by a type parameter", there is a solution.
import java.lang.reflect.Array;
public class Something<T> {
    private final Class<T> type;
    public Something(Class<T> type) {
        this.type = type;
    public String getTypeName() {
        return this.type.getName();
    public T[] newArray(final int length) {
        return (T[]) Array.newInstance(this.type, length);
}The constructor introduces the type information (T) to the runtime environment, which means an instance of Something will know its type. Method newArray therefore does not cause a warning on unchecked type conversion, so this approach is typesafe.
// Type parameter class is demanded by constructor...
Something<String> stringThing = new Something<String>(String.class);
// Cheating won't work, because the compiler catches the error...
Something<Vector> vectorThing = new Something<Vector>(Integer.class);This approach, however, doesn't enable you to create an array of typed elements (like Vector<String>[]). If that is the definition of a generic array, then it is indeed not possible to create one.
After all, while Vector[].class exists and can be resolved at runtime, there's no such thing as Vector<String>[].class, so there's no way you could provide the class definition of the component type to the constructor of Something.
This may be a surprise to mdt_java, because if I remember correctly, templates in C++ cause actually different classes to be created for 'generic' types. This is not the case with Java.