JDK8StreamAPI

喜欢ヅ旅行 2023-10-11 17:06 37阅读 0赞

*  测试数据

private static final List<User> users;
    
        static {
        
            users = new ArrayList<>();
            users.add(new User("1", "zs", "山东", "111", 20, 200));
            users.add(new User("2", "ls", "山东", "222", 30, 700));
            users.add(new User("3", "ww", "河南", "333", 40, 2000));
            users.add(new User("4", "zl", "广州", "444", 50, 2700));
            users.add(new User("5", "lq", "广州", "555", 60, 3500));
            users.add(new User("6", "xb", "山东", "222", 70, 4200));
            users.add(new User("7", "lj", "北京", "666", 80, 1100));
        }

*  StreamApi中间操作 filter distinct skip limit map sorted peed

@Test
        public void test1() throws Exception {
        
            //过滤出工资大于1500的人，排序，去重，跳过第一个，取2条
            users.stream().filter(u -> u.getSalary() > 1500)
                    .map(User::getSalary)
                    .sorted((s1, s2) -> s2 - s1)
                    .distinct()
                    .skip(1)
                    .limit(2)
                    .forEach(System.out::println);
    
        }

*  终止操作 forEach allMatch noneMatch anyMatch findFirst findAny count max min

@Test
        public void test2() throws Exception {
        
            //判断所有人的住址是不是都是山东
            boolean allMatch = users.stream().allMatch(u -> "山东".equals(u.getAddress()));
            System.out.println("allMatch = " + allMatch);
            //判断所有人的工资是不是都小于10000
            boolean noneMatch = users.stream().noneMatch(u -> u.getSalary() > 10000);
            System.out.println("noneMatch = " + noneMatch);
            // 判断是否有一个人的住址是河南
            boolean anyMatch = users.stream().anyMatch(u -> "河南".equals(u.getAddress()));
            System.out.println("anyMatch = " + anyMatch);
    
            //返回工资最高的人员
            User maxSal = users.stream().sorted((u1, u2) -> u2.getSalary() - u1.getSalary()).findFirst().get();
            System.out.println("maxSal = " + maxSal);
    
            //随机返回一个元素
            User userAny = users.stream().findAny().get();
            System.out.println("userAny = " + userAny);
    
            //统计工资超过2000的人个数
            long count = users.stream().filter(u -> u.getSalary() > 2000).count();
            System.out.println("count = " + count);
    
            //统计最高工资
            int maxSalInt = users.stream().mapToInt(User::getSalary).max().getAsInt();
    
            //统计最低年龄
            int minAge = users.stream().mapToInt(User::getAge).min().getAsInt();
            System.out.println("minAge = " + minAge);
    
        }

*  分组与分区 partitioninng groupby

@Test
        public void test3() throws Exception {
        
            //分区：工资3000以上和3000以下的
            Map<Boolean, List<User>> collect = users.stream().collect(Collectors.partitioningBy(u -> u.getSalary() > 3000));
            System.out.println("大于3000的人员：" + collect.get(true));
            System.out.println("小于3000的人员：" + collect.get(false));
    
            //根据人员的地址分组
            Map<String, List<User>> listMap = users.stream().collect(Collectors.groupingBy(User::getAddress));
            listMap.forEach((key, value) -> {
        
                System.out.println(key + value);
            });
    
            //根据人员年龄段分组，并统计各个年龄段人数（0-20]（20-60] (60-~)
            Map<String, List<User>> ageMap = users.stream().collect(Collectors.groupingBy(u -> {
        
                Integer age = u.getAge();
                if ((age > 0) && (age <= 20)) {
        
                    return "0-20";
                } else if ((age > 20) && (age <= 60)) {
        
                    return "20-60";
                } else {
        
                    return "60以上";
                }
            }));
            Map<String, Long> data = new HashMap<>();
            ageMap.forEach((key, value) -> {
        
                long count = value.stream().count();
                data.put(key, count);
            });
    
            data.keySet().stream().forEach(k -> {
        
                System.out.println(k + " : " + data.get(k));
            });
    
        }

*  收集操作 set list map

@Test
        public void test4() throws Exception {
        
            users.stream().collect(Collectors.toList()).forEach(System.out::println);
            users.stream().collect(Collectors.toSet()).forEach(System.out::println);
    
            users.stream().collect(Collectors.toMap( (User::getId),( (u) -> u ) )).forEach(
                    (k,v) -> {
        
                        System.out.println(k + v);
                    }
            );
    
        }