Intro Java8 Object List To Map
這篇介紹Java8 Object List To Map。
Example
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| List data structure:
User (Long Id, String username)
List<User> userList = new ArrayList<>();
userList.add(new User(1L, "aaa"));
userList.add(new User(2L, "bbb"));
userList.add(new User(3L, "ccc"));
userList.add(new User(2L, "ddd"));
userList.add(new User(3L, "eee"));
1.Map<Long, String> map = userList.stream().collect(Collectors.toMap(User::getId, User::getUsername);
Error: java.lang.IllegalStateException: Duplicate key bbb
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解決方法
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| 1.Map<Long, String> map = userList.stream().collect(Collectors.toMap(User::getId, User::getUsername, (v1, v2) -> v1)); //取前面的值
2.Map<Long, String> map = userList.stream().collect(Collectors.toMap(User::getId, User::getUsername, (v1, v2) -> v2)); //取後面的值
3.Map<Long, List<String>> map = userList.stream().collect(Collectors.toMap(User::getId,
e -> new ArrayList<String>(Arrays.asList(e.getUsername())),
(List<String> oldList, List<String> newList) -> {
oldList.addAll(newList);
return oldList;
}));
//相同的key，把不同的value存在同一個List<String>
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