gistfile1.java

import java.util.*;

public class TypeSafety {

    public static void main(String[] args){
        List<Set<?>> listOfWildcardSet = new ArrayList<Set<?>>();
        listOfWildcardSet.add(new HashSet<String>());
        listOfWildcardSet.add(new HashSet<Integer>());
        listOfWildcardSet.set(0, listOfWildcardSet.get(0));
        listOfWildcardSet.set(1, listOfWildcardSet.get(1));
        // ^^^ This is ok because listOfWildcardSet.get(int) returns a Set<?>
        // and the type of listOfWildcardSet.set is void set(int, Set<?>)

        List<Set> listOfRawSet = new ArrayList<Set>();
        // ^^^ This doesn't generate a warning although IMHO it should. but see below
        listOfRawSet.add(new HashSet<String>());
        listOfRawSet.add(new HashSet<Integer>());
        listOfRawSet.set(0, listOfRawSet.get(0));
        listOfRawSet.set(1, listOfRawSet.get(1));
        // ^^^ This is also ok, because listOfRawSet.get returns a Set
        // and listOfRawSet.set has the type of void set(int, Set)

        /**************************************
         * Here's where things start to differ:
         **************************************/

        // Using the wildcard version:
        Set<?> wild0 = listOfWildcardSet.get(0);
        Set<?> wild1 = listOfWildcardSet.get(0);
        /**
           wild0.addAll(wild1);
           ^^^ This would generate an error at compile time:

           Typesafety.java:28: cannot find symbol
           symbol  : method addAll(java.util.Set<capture#501 of ?>)
           location: interface java.util.Set<capture#896 of ?>
           wset0.addAll(wset1);
                ^
           I interpret the error message above by thinking that the two different <?>
           are actually different types altogether. I think that any other version
           would allowing potentially unsafe operations like those below...
        */

        // Using the raw version:
        Set raw0 = listOfRawSet.get(0);
        Set raw1 = listOfRawSet.get(0);
        raw0.addAll(raw1);
        // ^^^ No compile time error, BUT does generate a warning:
        // warning: [unchecked] unchecked call to addAll(java.util.Collection<? extends E>) as a member of the raw type java.util.Set
	}
}