Generic doubt

[raja kanak]

Code is from javabeat

    
import java.util.*;
class jb1 {
    public static void main(String... args) {
        HashSet<String> hs = new HashSet<String>();
        hs.add("scjp");
        hs.add("exam");
        HashSet<Object> s = new HashSet<Object>();
        s.add("check");
        s.add(new Integer(5));
//      s=hs;     //1
    }
}

In the above code, if i uncomment //1 then it is compilier error. My doubt is, we can add anything into s (set of Objects), then why can't hs? (HashSet is subclass of Object)

[Srinivasan thoyyeti]

Hi,
Generics conventions are different.
Object o = new String(); // Polymorphic assignment is Ok.
List<Object> lo = new ArrayList<String>() // Polymorphic generic assignments are not considered .

You can use,
List<? extends Object> lo = new ArrayList<String>();

For more information of on generics usage,
1) refer to K&B book
2) Sun Core java book, Generics chapter

[Remko Strating]

Because the generic types don't match. Polymorphism doesn't work for generic types.

If you created a new HashMap<Object> refererence then the assignment should work. See the code example below

 
 import java.util.*;
    class Test {
        public static void main(String... args) {
            HashSet<String> hs = new HashSet<String>();
            HashSet<Object> hso = new HashSet<Object>();
            
            hs.add("scjp");
            hs.add("exam");
            HashSet<Object> s = new HashSet<Object>();
            s.add("check");
            s.add(new Integer(5));
        //  s=hs;     //1
          s = hso;   
        }
    }

[Chris Stann]

import java.util.*;
class jb1 {
    public static void main(String... args) {
        HashSet<String> hs = new HashSet<String>();
        hs.add("scjp");
        hs.add("exam");
        HashSet<Object> s = new HashSet<Object>();
        s.add("check");
        s.add(new Integer(5));
//      s=hs;     //1
    }
}

I thought //1 does not work because HashSet "s" is designed to take only Object, but you are trying to point it to a HashSet which only takes String?


[HENRY: Formatted Code]
[ March 18, 2007: Message edited by: Henry Wong ]

[sadasiva kumar]

import java.util.*;class jb1
{ public static void main(String... args) {
HashSet<String> hs = new HashSet<String>();
hs.add("scjp");
hs.add("exam");
HashSet<Object> s = new HashSet<Object>();
s.add("check");
s.add(new Integer(5));
// s=hs; //1

java.util.HashSet<java.lang.String> and java.util.HashSet<java.lang.Object> are incompatible types.Both are completely different.So we cannot assign s=hs;

regards
SADASIVAKUMAR UTTI
SCJP1.4