匿名类在排序方法中的使用

为什么如果我在List的sort方法中放置一个带有Comparator的匿名类,编译器会向我显示一个错误?

public class Example2 {

public static void main(String[] args) {
   List<String> l = Arrays.asList("a","b","c","d");
   l.sort(Comparator<String> c= new Comparator<>() {   //compiler error
        public int compare(String a, String b) {
            return b.compareTo(a);
        }
   });

}

sort方法接受一个比较器。如果我编写此代码,它将编译:

public class Example2 {

public static void main(String[] args) {
   List<String> l = Arrays.asList("a","b","c","d");
   l.sort(new Comparator<String>() {                  //it's ok
        public int compare(String a, String b) {
            return b.compareTo(a);
        }
   });

}

public class Example2 {

public static void main(String[] args) {
   List<String> l = Arrays.asList("a","b","c","d");
   Comparator <String> c = new Comparator<String>() {     
        public int compare(String a, String b) {
            return b.compareTo(a);
        }
   };
   l.sort(c);                               //it's ok

}

为什么会这样?