java 8 stream学习整理

Posted 2021-01-31 艾白羊

1、什么是stream：

Stream 使用一种类似用 SQL 语句从数据库查询数据的直观方式来提供一种对 Java 集合运算和表达的高阶抽象。

这种风格将要处理的元素集合看作一种流， 流在管道中传输， 并且可以在管道的节点上进行处理， 比如筛选， 排序，聚合等。

元素流在管道中经过中间操作（intermediate operation）的处理，最后由最终操作(terminal operation)得到前面处理的结果。

 

 

2、常用的流操作：

2.1、map：map 方法用于映射每个元素到对应的结果。

2.11、List对象转List<Long>

@Slf4j
public class Streamdemo {
    
    public static List<User> createList() {

        User user = new User(1L, "by", 18, "音乐");
        User user1 = new User(2L, "gz", 26, "音乐");
        User user2 = new User(3L, "aby", 19, "旅行");
        List<User> userList = Lists.newArrayList(user, user1, user2);
        return userList;

    }

    public static void main(String[] args) {

        List<User> userList = createList();
        // List对象转List<Long>
        List<Long> longList = userList.stream().map(o->o.getId()).collect(Collectors.toList());
        log.info("longList"+longList);
    }

}

 

2.2、filter：filter 方法用于通过设置的条件过滤出元素。

2.21、集合相减

@Slf4j
public class Streamdemo {

    public static List<User> createList() {

        User user = new User(1L, "by", 18, "音乐");
        User user1 = new User(2L, "gz", 26, "音乐");
        User user2 = new User(3L, "aby", 19, "旅行");
        List<User> userList = Lists.newArrayList(user, user1, user2);
        return userList;

    }

    public static List<User> newList() {

        User user = new User(1L, "by", 18, "音乐");
        User user1 = new User(2L, "gz", 26, "音乐");
        List<User> userList = Lists.newArrayList(user, user1);
        return userList;

    }

    public static void main(String[] args) {

        List<User> userList = createList();
        List<User> newList = newList();
        //userList - newList
        List<User> resultList = createList().stream().filter(o->!newList.contains(o)).collect(Collectors.toList());
        log.info("resultList"+resultList);

    }

}

 

2.3 Collectors ： Collectors 类实现了很多归约操作。

2.31、List转Map

@Slf4j
public class Streamdemo {


    public static List<User> createList() {

        User user = new User(1L, "by", 18, "音乐");
        User user1 = new User(2L, "gz", 26, "音乐");
        User user2 = new User(3L, "aby", 19, "旅行");
        List<User> userList = Lists.newArrayList(user, user1, user2);
        return userList;

    }

    public static void main(String[] args) {

        List<User> userList = createList();

        //key = id , value = userName
        Map<Long, String> lsMap = userList.stream().collect(Collectors.toMap(User::getId, User::getUserName));
        log.info("lsMap" + lsMap);

        //key = userName , value = love
        Map<String, String> ssMap = userList.stream().collect(Collectors.toMap(o -> o.getUserName(), o -> o.getLove()));
        log.info("ssMap" + ssMap);

        //key = id , value = User
        Map<Long, User> luMap = userList.stream().collect(Collectors.toMap(o -> o.getId(), o -> o));
        log.info("luMap" + luMap);

        //key = love + userName , value = User
        Map<String,User> map = new HashMap<>(16);
        userList.forEach(o->{
            map.put(o.getLove()+o.getUserName(),o);
        });

    }

                  

参考：https://lw900925.github.io/java/java8-stream-api.html