More Typing Problems

[Ali Ebrahimi]

Hi All,
For this to work compiler should transforms code to this form:

DBSet<Customer> t1=allCustomer();

DBSet<String> t2= t1.select(#(c){c.getName()});

DBSet<String> result = t2.sortedByStringAscending(#(str){str});

otherwise
If (compiler) can not do that, then programmer required to do it.


Best regards,

Ali Ebrahimi

On Tue, Sep 7, 2010 at 11:54 AM, Maurizio Cimadamore <
maurizio.cimadamore at oracle.com> wrote:

> On 06/09/10 22:39, Ming-Yee Iu wrote:
> > I'm having more problems with getting my code to type properly. The
> > following piece of code does not compile due to typing problems:
> >
> The problem you are reporting is related to how javac type-inference
> works (in general, not in this lambda extension).
> In your example, you need the target type DBSet<String> in order to
> correctly type the call to select(...) [as the return type of this
> method is 'open', since U cannot be instantiated from the supplied
> actual argument, as the lambda has untyped parameters].
>
> Unfortunately, if you chain multiple method calls, the target type will
> be used only for the last call in the chain, namely the one to
> 'sortedByStringAscending(...)', meaning that the call to 'select(...)'
> will have U inferred to the 'default' type Object, thus causing a
> compilation error.
>
> We could write a similar example in plain Java (with generics) w/o
> lambda expressions.
>
> Maurizio
> >
> > public class Test
> > {
> >     public static class Customer {
> >        public String getName() { return "";}
> >     }
> >     public abstract static class StringSorter<U>  {
> >        abstract String value(U val);
> >     }
> >     public static interface Select<U, V>  {
> >        public V select(U val);
> >     }
> >     public static class DBSet<T>   {
> >        public<U>  DBSet<U>  select(Select<T,U>  x) {return new
> DBSet<U>();}
> >        public DBSet<T>  sortedByStringAscending(StringSorter<T>  x)
> {return this;}
> >     }
> >
> >     public static DBSet<Customer>  allCustomer()
> >     {
> >        return new DBSet<Customer>();
> >     }
> >
> >     public static void main(String[] args)
> >     {
> >        DBSet<String>  result = allCustomer()
> >           .select(#(c){c.getName()})
> > //; result = result    // if you uncomment this line, then it compiles
> fine
> >           .sortedByStringAscending(#(str){str});
> >     }
> > }
> >
> > But if I uncomment the 4th line from the bottom, it then compiles fine.
> >
> > -Ming
> >
> >
>
>
>