Re: generics + new = problem

CD1 <>
20 Apr 2007 15:08:10 -0700
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.


On Apr 20, 3:39 pm, Mauro <> wrote:

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.

public interface IOCElement extends Serializable, ChangeEmitter {
        public String getName();


public class GenericListModel<T extends IOCElement> extends
                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();
                        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

Thanks in Advance

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