比如 Comparator 接口中 thenComparing() 方法：
/*** Returns a lexicographic-order comparator with another comparator.* If this {@code Comparator} considers two elements equal, i.e.* {@code compare(a, b) == 0}, {@code other} is used to determine the order.** <p>The returned comparator is serializable if the specified comparator* is also serializable.** @apiNote* For example, to sort a collection of {@code String} based on the length* and then case-insensitive natural ordering, the comparator can be* composed using following code,** <pre>{@code* Comparator<String> cmp = Comparator.comparingInt(String::length)* .thenComparing(String.CASE_INSENSITIVE_ORDER);* }</pre>** @param other the other comparator to be used when this comparator* compares two objects that are equal.* @return a lexicographic-order comparator composed of this and then the* other comparator* @throws NullPointerException if the argument is null.* @since 1.8*/default Comparator<T> thenComparing(Comparator<? super T> other) {Objects.requireNonNull(other);return (Comparator<T> & Serializable) (c1, c2) -> {int res = compare(c1, c2);return (res != 0) ? res : other.compare(c1, c2);};}
(Comparator & Serializable)，& 是 Java8 的新语法，表示同时满足两个约束（约束这个词不知道用的恰不恰当）。相当于：
default Comparator<T> thenComparing(Comparator<? super T> other) {Objects.requireNonNull(other);return (Comparator<T>)(Serializable) (c1, c2) -> {int res = compare(c1, c2);return (res != 0) ? res : other.compare(c1, c2);};}
这意味着结果值将被转换为 Comparator 和 Serializable（即可序列化的比较器）。
请注意，在进行该类转换时，您只能指定一个类（以及无限量的接口），因为 Java 不支持类继承多个类。