Java8 convert list to map 2017-06-04 10:43
Before Java8 if you need convert list to map you have to iterate over the list. Java8 provide stream to help you iterate over the list. You can convert list to map by using stream with only one line of code.
Old way
List<User> userList = new ArrayList<>();
userList.add(new User(1, "Henry"));
userList.add(new User(2, "Justin"));
Map<Integer, User> map = new HashMap<>();
for (User user : userList) {
map.put(user.getId(), user);
}
System.out.println(map);
Use stream
List<User> userList = new ArrayList<>();
userList.add(new User(1, "Henry"));
userList.add(new User(2, "Justin"));
Map<Integer, User> mapByStream = userList.stream().collect(Collectors.toMap(new Function<User, Integer>() {
@Override
public Integer apply(User t) {
return t.getId();
}
}, new Function<User, User>() {
@Override
public User apply(User user) {
return user;
}
}));
System.out.println(mapByStream);
The stream method help you iterate the list. collect method need a Collector to convert list to map. Collectors.toMap() means this collector need two parameters: key and value. For every user in stream, the key is its id and the value is itself.
Use lambda simplify code
The code of above can be more simplified with lambda.
List<User> userList = new ArrayList<>();
userList.add(new User(1, "Henry"));
userList.add(new User(2, "Justin"));
Map<Integer, User> mapByStream = userList.stream().collect(Collectors.toMap(User::getId, user -> user));
System.out.println(mapByStream);
EOF