java stream 流操作一些示例

2022-05-08 06:49| 来源: 网络整理| 查看: 265

List userList = Lists.newArrayList( new User(1L), new User(2L), new User(3L), new User(null), new User(3L) ); { System.out.println("---遍历元素，然后过滤出某个字段的集合---"); { List userIds = new ArrayList(); for (User user : userList) { userIds.add(user.getUserId()); } System.out.println(userIds); } { List userIds = userList.stream() .map(User::getUserId) .collect(Collectors.toList()); System.out.println(userIds); } } { System.out.println("---筛选元素---"); { List userIds = new ArrayList(); for (User user : userList) { if (user.getUserId() != null) { userIds.add(user.getUserId()); } } System.out.println(userIds); } { List userIds = userList.stream() .filter(item -> item.getUserId() != null) .map(User::getUserId) .collect(Collectors.toList()); System.out.println(userIds); } } { System.out.println("---去重---"); { Set userIds = userList.stream() .filter(item -> item.getUserId() != null) .map(User::getUserId) .collect(Collectors.toSet()); System.out.println(userIds); } { List userIds = userList.stream() .filter(item -> item.getUserId() != null) .map(User::getUserId) .distinct() .collect(Collectors.toList()); System.out.println(userIds); } } { System.out.println("---数据类型转换---"); { List userIdStrs = userList.stream() .map(x -> x.getUserId() != null ? x.getUserId().toString() : null) .collect(Collectors.toList()); System.out.println(userIdStrs); } } { System.out.println("---数组转集合---"); { //创建一个字符串数组 String[] strArray = new String[]{"a", "b", "c"}; //转换后的List 属于 java.util.ArrayList 能进行正常的增删查操作 List strList = Stream.of(strArray).collect(Collectors.toList()); System.out.println(strList); } } { System.out.println("---集合转 map（不分组）---"); { Map userMap = new HashMap(); for (User user : userList) { userMap.put(user.getUserId(), user); } System.out.println(userMap); } { Map userMap = userList.stream().collect(Collectors.toMap(User::getUserId, v -> v, (k1, k2) -> k1)); System.out.println(userMap); } } { System.out.println("---集合转map（分组）---"); { Map userListMap = new HashMap(); for (User user : userList) { if (userListMap.containsKey(user.getUserId())) { userListMap.get(user.getUserId()).add(user); } else { List users = new ArrayList(); users.add(user); userListMap.put(user.getUserId(), users); } } System.out.println(userListMap); } { Map userMap = userList.stream() .filter(item -> item.getUserId() != null) .collect(Collectors.groupingBy(User::getUserId)); System.out.println(userMap); } } { System.out.println("---分页操作---"); { List numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5, 10, 6, 20, 30, 40, 50, 60, 100); List dataList = numbers.stream() .sorted(Integer::compareTo) .skip(0).limit(10) .collect(Collectors.toList()); System.out.println(dataList.toString()); } { List dataList = userList.stream() .filter(item -> item.getUserId() != null) .sorted(Comparator.comparing(User::getUserId)) .skip(0).limit(10) .collect(Collectors.toList()); System.out.println(dataList.toString()); } } { System.out.println("---查找与匹配操作---"); { System.out.println("---allMatch（检查是否匹配所有元素）---"); List list = Arrays.asList(10, 5, 7, 3); boolean allMatch = list.stream().allMatch(x -> x > 2);//是否全部元素都大于2 System.out.println(allMatch); } { System.out.println("---findFirst（返回第一个元素）---"); List list = Arrays.asList(10, 5, 7, 3); Optional first = list.stream().findFirst(); Integer val = first.get(); System.out.println(val);//输出10 } { System.out.println("---reduce（可以将流中元素反复结合起来，得到一个值）---"); List list = Arrays.asList(10, 5, 7, 3); Integer result = list.stream().reduce(2, Integer::sum); System.out.println(result);//输出27，其实相当于2+10+5+7+3，就是一个累加 } } { System.out.println("---并行操作---"); { List strings = Arrays.asList("abc", "", "bc", "efg", "abcd", "", "jkl"); // 采用并行计算方法，获取空字符串的数量 long count = strings.parallelStream().filter(String::isEmpty).count(); System.out.println(count); } }

集合转 map（不分组）

打开Collectors.toMap方法源码，一起来看看到底是啥。

public static Collector toMap(Function

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java stream 流操作 一些示例

java stream 流操作一些示例