return (Comparator＜T＞ & Serializable)

public static <T> Comparator<T> comparingInt(ToIntFunction<? super T> keyExtractor) {
    Objects.requireNonNull(keyExtractor);
 // 这里return是个什么鬼,一脸懵逼
    return (Comparator<T> & Serializable)
        (c1, c2) -> Integer.compare(keyExtractor.applyAsInt(c1), keyExtractor.applyAsInt(c2));
}
代码示例:

interface SerializableComparator extends Comparator<Integer>, Serializable {}

@Test
public void test2() {
  Comparator<Integer> comparator =
      new SerializableComparator() {
        @Override
        public int compare(Integer o1, Integer o2) {
          return o1 - o2;
        }
      };
  System.out.println(comparator instanceof Comparator); // true
  System.out.println(comparator instanceof Serializable); // true
/*lambda表达式写法*/
  Comparator<Integer> comparator2 = (Comparator<Integer> & Serializable) (o1, o2) -> o1 - o2;
  System.out.println(comparator2 instanceof Comparator); // true
  System.out.println(comparator2 instanceof Serializable); // true
  System.out.println(comparator2.getClass()); // class com.test.cn.Testclass1$$Lambda$1/1879492184
}

结论: 
    1.return (Comparator<T> & Serializable) (c1, c2) -> 
      Integer.compare(keyExtractor.applyAsInt(c1), keyExtractor.applyAsInt(c2))表示 
      lambda表达式生成的匿名类实例分别实现了Comparator接口和Serializable接口,并将该匿名类实例返回
    2. lambda表达式本质是一个匿名类的实例

    3. lambda表达式本质是一个函数式接口的实例