HI All,
This problem is not new, I read a lot, but I still do not understand
if there's a solution or not (shame on me).
The problem is: I want to build a generic class that constructs new
instances of the type variable.
i.e.:
public interface IOCElement extends Serializable, ChangeEmitter {
public String getName();
}
public class GenericListModel<T extends IOCElement> extends
AbstractListModel
implements ComboBoxModel, ChangeListener {
static final long serialVersionUID = 1l;
private ArrayList<T> tList = null;
public GenericListModel() {
tList = new ArrayList<T>();
}
// AbstractListModel implementation
public Object getElementAt(int index) { return tList.get(index); }
public int getSize() { return tList.size(); }
// needed for FreeMarker
public ArrayList<T> getTasks() { return tList; }
// ComboBoxModel implementation
transient private int selectedItem = -1;
public Object getSelectedItem() { return (selectedItem>=0)?
get(selectedItem): null; }
public void setSelectedItem(Object anItem) { if (anItem instanceof
IOCElement) { selectedItem = pos(((IOCElement)anItem).getName()); } }
// API
public T get(int index) { return tList.get(index); }
public int add(String name) {
int index = 0;
if (tList.add(new T(name))) {
index = tList.size();
tList.get(index-1).addChangeListener(this);
selectedItem = -1;
fireIntervalAdded(this, index, index);
}
return index-1;
}
.... etc. etc.
Everything is ok BUT the line that reads:
if (tList.add(new T(name))) {
I understand that erasures make this difficult for the compiler, but:
Is there a way around this (possibly using newInstance(),
constructor() or whatever)?
Can someone tell me exactly what I should do? (if anything can be
done).
Thanks in Advance
Mauro
Tom Hawtin - 20 Apr 2007 20:00 GMT
> The problem is: I want to build a generic class that constructs new
> instances of the type variable.
> if (tList.add(new T(name))) {
Even if T was, say, an interface you would still have a problem,
generics or not.
What you need is a factory.
interface NamedObjectFactory<T> {
T create(String name);
}
...
interface IOCElementSubtype extends IOCElement {
...
}
...
class ConcreteIOCElement implements IOCElementSubtype {
...
}
...
new GenericListModel<IOCElementSubtype>(
new NamedObjectFactory<IOCElementSubtype>{
public IOCElementSubtype create(String name) {
return new ConcreteIOCElement(name);
}
}
)
...
if (data.add(factory.create(name))) {
Tom Hawtin
CD1 - 20 Apr 2007 23:08 GMT
Hi Mauro,
How can you tell the type T has a constructor with a String parameter?
If you know you'll only use a type (or certain types) in this method,
try not using generics.
Cya!
> HI All,
> This problem is not new, I read a lot, but I still do not understand
[quoted text clipped - 57 lines]
> Thanks in Advance
> Mauro
Adam Maass - 21 Apr 2007 06:22 GMT
> This problem is not new, I read a lot, but I still do not understand
> if there's a solution or not (shame on me).
>
> The problem is: I want to build a generic class that constructs new
> instances of the type variable.
> i.e.:
The classical answer is to require a parameter of type Class<T>.
Then you can keep the Class variable and reflect on it using
...newInstance().
Of course, there's no guarantee that the Class will actually have a
constructor with the parameters you expect.
Tom Hawtin - 21 Apr 2007 13:56 GMT
> The classical answer is to require a parameter of type Class<T>.
That's the classical answer to a different question...
> Then you can keep the Class variable and reflect on it using
> ...newInstance().
Noooo, not reflection! Class.newInstance is the most evil of them all.
Tom Hawtin