Stream的中间操作：筛选与切片、映射、排序

/*
 测试Stream的中间操作
 */
public class StreamAPITest1 {
    //1-筛选与切片
    @Test
    public void test1(){
        List<Employee> list = EmployeeData.getEmployees();
        //filter(Predicate p)-接收 Lambda，从流中排除某些元素
        Stream<Employee> stream = list.stream();
        //查询员工表中薪资大于7000的员工信息
        stream.filter(e -> e.getSalary() > 7000).forEach(System.out::println);

        System.out.println("*******************");

        //limit(n)- 截断流，使其元素不超过给定数量
        //截取前三条数据
        list.stream().limit(3).forEach(System.out::println);
        System.out.println("*******************");

        //skip(n) - 跳过元素，返回一个扔掉了前n个元素的流 若流中元素不足n个，则返回一个空流 与limit(n)互补
        list.stream().skip(3).forEach(System.out::println);
        System.out.println("*******************");

        //distinct() - 筛选，通过流所生成元素的hashCode()和equals()去除重复元素
        list.add(new Employee(1010,"刘强东",40,8000));
        list.add(new Employee(1010,"刘强东",40,8000));
        list.add(new Employee(1010,"刘强东",40,8000));
        list.add(new Employee(1010,"刘强东",40,8000));
        list.add(new Employee(1010,"刘强东",40,8000));

        System.out.println(list);
        System.out.println();

        list.stream().distinct().forEach(System.out::println);
    }

    //2-映射
    @Test
    public void test2(){
        //map(Function f)-接收一个函数作为参数，将元素转换成其他形式或提取信息，该函数会被应用到每个元素上，并将其映射成一个新的元素
        List<String> list = Arrays.asList("aa", "bb", "cc", "dd");
        list.stream().map(str -> str.toUpperCase()).forEach(System.out::println);

        //练习1：获取员工姓名长度大于3的员工的姓名
        List<Employee> employees = EmployeeData.getEmployees();
        Stream<String> namesStream = employees.stream().map(Employee::getName);
        namesStream.filter(name -> name.length() > 3).forEach(System.out::println);

        //练习2：
        Stream<Stream<Character>> streamStream = list.stream().map(StreamAPITest1::fromStringToStream);
        streamStream.forEach(s -> {
            s.forEach(System.out::println);
        });

        System.out.println();
        //flatMap(Function f)-接收一个函数作为参数，将流中的每个值都换成另一个流，然后把所有流连接成一个流
        Stream<Character> characterStream = list.stream().flatMap(StreamAPITest1::fromStringToStream);
        characterStream.forEach(System.out::println);
    }

    //将字符串中的多个字符构成的集合转换为对应的Stream的实例
    public static Stream<Character> fromStringToStream(String str){
        ArrayList<Character> list = new ArrayList<>();
        for (Character c : str.toCharArray()){
            list.add(c);
        }
        return list.stream();
    }

    @Test
    public void test3(){
        ArrayList list1 = new ArrayList();
        list1.add(1);
        list1.add(2);
        list1.add(3);

        ArrayList list2 = new ArrayList();
        list2.add(4);
        list2.add(5);
        list2.add(6);

//        list1.add(list2);
        list1.addAll(list2);
        System.out.println(list1);
    }

    //3-排序
    @Test
    public void test4(){
        //sorted()-自然排序
        List<Integer> list = Arrays.asList(12, 43, 65, 34, 87, 0, -98, 7);
        list.stream().sorted().forEach(System.out::println);

        //抛异常，原因：Employee没有实现Comparable接口
//        List<Employee> employees = EmployeeData.getEmployees();
//        employees.stream().sorted().forEach(System.out::println);

        //sorted(Comparator com)-定制排序
        List<Employee> employees = EmployeeData.getEmployees();
        employees.stream().sorted((e1,e2) -> Integer.compare(e1.getAge(), e2.getAge()))
                .forEach(System.out::println);


    }

}