Java 8 新特性

1、Stream 流

        List<String> strings = Arrays.asList("a", "a", "", "b", "c","", "d");
        List<String> filtered = strings.stream() //获取流
                .filter(string -> !string.isEmpty()) //过滤为空的字符串
                .distinct() //去重
                .collect(Collectors.toList()); //流转集合
        filtered.stream().forEach(System.out::println); // 遍历输出结果：a  b  c  d

        filtered.stream().map(str -> str.toUpperCase()) //转大写字母（map用于映射每个元素到对应的结果）
                .limit(3) //限制只有3个数据
                .forEach(System.out::println); //输出结果：A  B  C

        String strResult = filtered.stream().collect(Collectors.joining(", ")); //拼接集合字符串

class Student {
    String name;
    int age;
}

List<Student> studentList = Arrays.asList(new Student("周一", 11), new Student("周五", 55), 
    new Student("周二", 22), new Student("周三", 33), new Student("周四", 44));
    List<Student> studentList2 = studentList .stream()
        .map(s -> new Student(s.getName(), s.getAge() * 2))  //映射对象年龄乘以2
        .collect(Collectors.toList()); //返回学生对象数组
    studentList.stream().map(s -> s.getAge() * 2)  //映射结果为年龄乘以2
        .forEach(System.out::println); //输出结果：22  110   44  66  88
    
        studentList.stream().filter(s -> s.getAge() > 20)
                .sorted((s1, s2) -> s1.getAge() - s2.getAge())
                .forEach(System.out::println); /**输出结果：Student{name='周二', age=22}
                                                            Student{name='周三', age=33}
                                                            Student{name='周四', age=44}
                                                            Student{name='周五', age=55} */
    Student student = studentList.stream().findFirst().orElse(null); //获取集合中第一个元素
     int sumAge = studentList.stream().mapToInt(s -> s.getAge()).sum(); //映射整形累加结果：165