java8 stream flatmap

Student s1 = new Student();        s1.setName("jack");        s1.addBook("java");        s1.addBook("c++");        s1.addBook("php");        Student s2 = new Student();        s2.setName("kobe");        s2.addBook("python");        s2.addBook(".net");        List<Student> list = new ArrayList<>();        list.add(s1);        list.add(s2);        List<String> collect =                list.stream()                        .map(x -> x.getBook())                              .flatMap(x -> x.stream())                          .distinct()                        .collect(Collectors.toList());        collect.forEach(x -> System.out.println(x));

输出：java c++php python .net