Generic not compiling

[Karsten Wutzke]

Why doesn't

NavigableSet<? super String> set = new TreeSet<String>();
set.add(new Object());

compile?

What would be the logical explanation?

Thanks
Karsten

[Ankit Garg]

the compiler doesn't know the exact type of a collection when you use <? super Type> syntax. What the compiler knows is that you can add elements of Type to that collection but not its super classes. In your case you can add elements of type String to the set but not super classes of String...

[Karsten Wutzke]

Ok this makes sense: the only safe operation would be to add a String, because the collection could be anything of a higher type. Since an Object isn't a String, the operation is considered to be unsafe -> error.

Thanks for helping!
Karsten

[Arjun Srivastava]

Ankit Garg wrote:the compiler doesn't know the exact type of a collection when you use <? super Type> syntax. What the compiler knows is that you can add elements of Type to that collection but not its super classes. In your case you can add elements of type String to the set but not super classes of String...

import java.util.*;
public class Main 
{
  public static void main(String[] args)
    { 
  NavigableSet<? super Main> set = new TreeSet<Main>();  
  set.add(new Main()); 
 //set.add(new Object()); line 1 
 NavigableSet<Main> set1 = new TreeSet<Main>();
 NavigableSet<Object> set2 = new TreeSet<Object>();
 go(set2);   
 }
 static void go(NavigableSet<? super Main> s) { }
}

line 1 if uncommented gives error.
<? super Type> says you can add elements of Type to that collection but not its super classes,also please accept any collection with generic type or a it's super type.
here you can pass super type to the method demanding <? super Type> but can't add super type to collection directly.