Question

I have a class A like this:

public class A<T extends Number>
{
 ....
}

In another class I have this method:

public Hashtable<String, A> createAHashtable()
    {
        Hashtable<String, A> hTable = new Hashtable<String, A>();

        return hTable;
    }

There is a warning for parameter A because it is generic class. So should I do this:

public Hashtable<String, A <?>> createAHashtable()
        {
            Hashtable<String, A<?>> hTable = new Hashtable<String, A<?>>();

            return hTable;
        }

or do this:

public Hashtable<String, A <? extends Number>> createAHashtable()
{
Hashtable<String, A<? extends Number> hTable = new Hashtable<String, A<? extends Number>();

return hTable;
}

or ….???

EDIT:

Tried this (as suggested by Dilum)

public <T extends Number> Hashtable<String, A<T>> createAHashtable()
    {
        Hashtable<String, A<T>> hTable = 
                new Hashtable<String, A<T>>();
        A<Float> constraint = new A<Float>();
        hTable.put("test", constraint);

        return hTable;
    }

But it is invalid to “put” my Float A.

Maybe the wildcard is the way to go.

EDIT 2:

Based on Dilum’s suggestion, the following code (cast to A when put a Float A into the Hashtable) has no error but warning it is unsafe cast. Why we need the cast?

public <T extends Number> Hashtable<String, A<T>> createAHashtable()
        {
            Hashtable<String, A<T>> hTable = 
                    new Hashtable<String, A<T>>();
            A<Float> constraint = new A<Float>();
            hTable.put("test", (A<T>)constraint);

            return hTable;
        }

Answer 1

Try this:

public <T extends Number> Hashtable<String, A<T>> createAHashtable() {
  return new Hashtable<String, A<T>>();
}

Say you did want to pre-fill with a key-value pair, try:

public <T extends Number> Hashtable<String, A<T>> createAHashtableWithEntry(String key, T value) {
  Hashtable<String, A<T>> ht = return new Hashtable<String, A<T>>();
  ht.put(key, new A<T>(value));
  return ht;
}