Generic compilation issue

[chirag visavadia]

import java.util.*;
class GenericTest {
	public static void main(String... x){
		NavigableSet<?> set = new TreeSet<Object>();
		set.add(new Object());		
	}
}

Can anyone please help me to understand this code? As per my understanding below code should take Object() as argument but it gives compile error. Thanks

NavigableSet<?> set = new TreeSet<Object>();

GenericTest.java:5: cannot find symbol
symbol : method add(java.lang.Integer)
location: interface java.util.NavigableSet<capture#964 of ?>
set.add(new Integer(2));
^
1 error

[Himai Minh]

The generic set with <?> won't let you add anything except null as a rule of the compiler to make your collection type safe.

During compilation time, the set means it refers to a collection that contains a type of object as specified on the right hand side.
During compilation time, the compiler does not know what set will contain. It can be Object, Animal, Cat, Dog....
For example, you may want to add this to the class in the future:

public static void modify(NavigableSet<?> s){
   s.add(new Animal);    //This won't compile because the compiler does not know what s will contain.

}
public static void main (String... x){
      NavigableSet<?> set = new TreeSet<Object>();
     modify(s);   //Can you add an Animal object to a set of Object? No. You can only have Object in this set.
}

Again during compilation time, set.add( ...) will not compile because the compiler does not know what set will contain. The compiler won't let you put a wrong type of object to set.

[chirag visavadia]

Thank you now I understand...