Java8 stream collect list to map in order 2017-09-04 10:20
We can convert list to map easily in Java8 by using stream. The sample code is like following.
List<User> users = new ArrayList<>();
users.add(new User("henry", 28));
users.add(new User("justin", 27));
users.add(new User("Mathew", 26));
Map<String, Integer> nameAgeMap = users.stream().collect(Collectors.toMap(User::getName, User::getAge));
System.out.println(nameAgeMap);
The output is here.
{henry=28, Mathew=26, justin=27}
The order is different from List<User>, If you want to get the map order same with List<User> you have to implement the toMap method of Collectors. What you need to do is using LinkedHashMap replace HashMap in toMap method.
List<User> users = new ArrayList<>();
users.add(new User("henry", 28));
users.add(new User("justin", 27));
users.add(new User("Mathew", 26));
LinkedHashMap<String, Integer> nameAgeOrderMap = users.stream().collect(Collectors.toMap(User::getName, User::getAge, (u, v) -> {
throw new IllegalStateException(String.format("Duplicate key %s", u));
}, LinkedHashMap::new));
System.out.println(nameAgeOrderMap);
The output is here.
{henry=28, justin=27, Mathew=26}
The same order with List<User>.
EOF