注意：这是从Comparable和Comparator合约衍生出来的，涉及null

该代码可以在Eclipse（20090920-1017）中编译并正常运行

import java.util.*;
public class SortNull {
   static <T extends Comparable<? super T>>
   Comparator<T> nullComparableComparator() {
      return new Comparator<T>() {
         @Override public int compare(T el1, T el2) {
         return
            el1 == null ? -1 :
            el2 == null ? +1 :
            el1.compareTo(el2);
         }
      };
   }
   public static void main(String[] args) {
      List<Integer> numbers = new ArrayList<Integer>(
         Arrays.asList(3, 2, 1, null, null, 0)
      );
      Comparator<Integer> numbersComp = nullComparableComparator();
      Collections.sort(numbers, numbersComp);
      System.out.println(numbers);
      // "[null, null, 0, 1, 2, 3]"

      List<String> names = new ArrayList<String>(
         Arrays.asList("Bob", null, "Alice", "Carol")
      );
      Comparator<String> namesComp = nullComparableComparator();
      Collections.sort(names, namesComp);
      System.out.println(names);
      // "[null, Alice, Bob, Carol]"
   }
}

但是它不能在上编译javac 1.6.0_17。这是错误消息：

SortNull.java:17: incompatible types; no instance(s) of type variable(s) T exist
 so that java.util.Comparator<T> conforms
 to java.util.Comparator<java.lang.Integer>
found   : <T>java.util.Comparator<T>
required: java.util.Comparator<java.lang.Integer>
     Comparator<Integer> numbersComp = nullComparableComparator();

SortNull.java:25: incompatible types; no instance(s) of type variable(s) T exist
 so that java.util.Comparator<T> conforms
 to java.util.Comparator<java.lang.String>
found   : <T>java.util.Comparator<T>
required: java.util.Comparator<java.lang.String>
     Comparator<String> namesComp = nullComparableComparator();

2 errors

有人可以解释为什么差异吗？这是一个错误吗？如果是这样，谁有错误？