First two blocks compile and run, third block wouldn't if it was compiled

gistfile1.java

package test;

import com.google.common.collect.Lists;

import java.util.HashSet;
import java.util.List;
import java.util.Set;


public class TypeSafety {

	public static void main(String[] args){
		List<Set<?>> listOfWildcardSet = Lists.newArrayList();
		listOfWildcardSet.add(new HashSet<String>());
		listOfWildcardSet.add(new HashSet<Integer>());
		listOfWildcardSet.set(0, listOfWildcardSet.get(0));
		listOfWildcardSet.set(1, listOfWildcardSet.get(1));

		List<Set> listOfRawSet = Lists.newArrayList();  //I thought this would generate a warnig the problem
		listOfRawSet.add(new HashSet<String>());
		listOfRawSet.add(new HashSet<Integer>());
		listOfRawSet.set(0, listOfWildcardSet.get(0));
		listOfRawSet.set(1, listOfWildcardSet.get(1));

		/*
		These alll generate unchecked warnings
		 */
		List listRaw = Lists.newArrayList();
		listRaw.add(new HashSet<String>());
		listRaw.add(new HashSet<Integer>());
		listRaw.set(0, listRaw.get(0));
		listRaw.set(1, listRaw.get(1));



		List<Set<String>> listOfStringSet = Lists.newArrayList();
		listOfStringSet.add(new HashSet<String>());
		/* Doesnt compile, enforcing typesafety
		listOfStringSet.add(new HashSet<Integer>());
		*/

		//It's all not very typesafe
		Set<String> first = (Set<String>)listOfWildcardSet.get(0);
		Set<Integer> second = (Set<Integer>)listOfWildcardSet.get(0);

		Set<String> first2 = (Set<String>)listOfRawSet.get(0);
		Set<Integer> second2 = (Set<Integer>)listOfRawSet.get(0);
	}
}