Gnerics Wild Card

[Amirr Rafique]

Hi All
Please explain following lines from Generics Tutorial (Article 9)

A Comparator objcet can be passed to the constructor of TreeSet which will be used to sort elements of the TreeSet according to desired ordering.

TreeSet(Comparator<E> c)

The Comparator interface is essentially :

interface Comparator<T>{
int comapre(T fst, T snd);}

Suppose we want to create a TreeSet<String> and pass in a suitable comparator, We need to pass ut a Comparator that can compare String. This can be done by a Comparator<String>, but a Comparator<Object> will do just well. However, we won't be able to invoke constructor given above on a Comaparator<Object>. We can use a lower bounded wildcard to get flexibility we want:

TreeSet(<Comparator ? super E> c)

Any example will be great

Thanks

[Keith Lynn]

Basically what they are saying is that making E the type accepted by the Comparator limits the Comparator to only accept E.

By using E as the lower bound, then you can send a Comparator that has a parameterized type that is any superclass of E.
[ May 28, 2006: Message edited by: Keith Lynn ]

[Amirr Rafique]

Originally posted by Keith Lynn:
Basically what they are saying is that making E the type accepted by the Comparator limits the Comparator to only accept E.

By using E as the lower bound, then you can send a Comparator that has a parameterized type that is any superclass of E.

[ May 28, 2006: Message edited by: Keith Lynn ]

Comparator<String> will only work with String. and If we say Comparator<Object> it will work only with object i t can't work for String or any other type.

if we say Comparator<? super E> then it can work with any super type of E.
This is what i understand but I am confused in what benefit we will get

[Keith Lynn]

The benefit is that you have one method which can accept either a Comparator<String> or Comparator<Object>