java8新特性：利用Lambda處理List集合

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Java 8新增的Lambda運算式，我們可以用簡潔高效的代碼來處理List。

1、遍曆

 public static void main(String[] args) {        List<User> userList = Lists.newArrayList();        User user1 = new User(1L, "張三", 24);        User user2 = new User(2L, "李四", 27);        User user3 = new User(3L, "王五", 21);        userList.add(user1);        userList.add(user2);        userList.add(user3);        userList.stream().forEach(user ->{            System.out.println(user.getName());        });    }

運行結果：

2、list轉為Map

 public static void main(String[] args) {        List<User> userList = Lists.newArrayList();//存放apple對象集合        User user1 = new User(1L, "張三", 24);        User user2 = new User(2L, "李四", 27);        User user3 = new User(3L, "王五", 21);        userList.add(user1);        userList.add(user2);        userList.add(user3);        //ID為key，轉為Map        Map<Long,User> userMap = userList.stream().collect(Collectors.toMap(User::getId, a -> a,(k1, k2)->k1));        System.out.println(userMap);    }

運行結果：

3、將List分組：List裡面的對象元素，以某個屬性來分組

  public static void main(String[] args) {        List<User> userList = Lists.newArrayList();//存放apple對象集合        User user1 = new User(1L, "張三", 24);        User user2 = new User(2L, "李四", 27);        User user3 = new User(3L, "王五", 21);        User user4 = new User(4L, "張三", 22);        User user5 = new User(5L, "李四", 20);        User user6 = new User(6L, "王五", 28);        userList.add(user1);        userList.add(user2);        userList.add(user3);        userList.add(user4);        userList.add(user5);        userList.add(user6);        //根據name來將userList分組        Map<String, List<User>> groupBy = userList.stream().collect(Collectors.groupingBy(User::getName));        System.out.println(groupBy);    }

運行結果：

4、過濾：從集合中過濾出來合格元素

 public static void main(String[] args) {        List<User> userList = Lists.newArrayList();//存放apple對象集合        User user1 = new User(1L, "張三", 24);        User user2 = new User(2L, "李四", 27);        User user3 = new User(3L, "王五", 21);        User user4 = new User(4L, "張三", 22);        User user5 = new User(5L, "李四", 20);        User user6 = new User(6L, "王五", 28);        userList.add(user1);        userList.add(user2);        userList.add(user3);        userList.add(user4);        userList.add(user5);        userList.add(user6);        //取出名字為張三的使用者        List<User> filterList = userList.stream().filter(user -> user.getName().equals("張三")).collect(Collectors.toList());        filterList.stream().forEach(user ->{            System.out.println(user.getName());        });    }

運行結果：

5、求和：將集合中的資料按照某個屬性求和

 public static void main(String[] args) {        List<User> userList = Lists.newArrayList();//存放apple對象集合        User user1 = new User(1L, "張三", 24);        User user2 = new User(2L, "李四", 27);        User user3 = new User(3L, "王五", 21);        User user4 = new User(4L, "張三", 22);        User user5 = new User(5L, "李四", 20);        User user6 = new User(6L, "王五", 28);        userList.add(user1);        userList.add(user2);        userList.add(user3);        userList.add(user4);        userList.add(user5);        userList.add(user6);        //取出名字為張三的使用者        int totalAge = userList.stream().mapToInt(User::getAge).sum();        System.out.println("和：" + totalAge);    }

運行結果：

 

java8新特性：利用Lambda處理List集合