Java8 stream List to Map

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• We have a UserInfo class

Stream operations through JAVA8 need to be converted into a map with userId as key and name as value.

public class User {
   private Integer userId;
   private String name;
   private String email;

    public User(Integer userId, String name, String email) {
        this.userId = userId;
        this.name = name;
        this.email = email;
    }

    public Integer getUserId() {
        return userId;
    }

    public void setUserId(Integer userId) {
        this.userId = userId;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    @Override
    public String toString() {
        return "User{" +
                "userId=" + userId +
                ", name='" + name + ''' +
                ", email='" + email + ''' +
                '}';
    }
}
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• implementation code

public class ListToMap {
    public static void main(String[] args) {
        List<User> users = new ArrayList<>();
        users.add(new User(1, "user1", "[email protected]"));
        users.add(new User(2, "user2", "[email protected]"));
        users.add(new User(3, "user3", "[email protected]"));
        users.add(new User(4, "user4", "[email protected]"));

        Map<Integer, String> userIdAndName = users.stream()
                .collect(Collectors.toMap(User::getUserId, User::getName));
        System.out.println(userIdAndName);
    }
}
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output result

• userId is key, user object is value

public class ListToMap {
    public static void main(String[] args) {
        List<User> users = new ArrayList<>();
        users.add(new User(1, "user1", "[email protected]"));
        users.add(new User(2, "user2", "[email protected]"));
        users.add(new User(3, "user3", "[email protected]"));
        users.add(new User(4, "user4", "[email protected]"));

        Map<Integer, User> userIdAndUser = users.stream()
                .collect(Collectors.toMap(User::getUserId, user-> user));
        System.out.println(userIdAndUser);
    }
}
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output

• Duplicate key handling

During the conversion process, if the same key occurs, a duplicate key exception will be thrown.

We changed the userId of 3 to 2, which conflicts with the second user

Collectors.toMap supports a third parameter to select duplicate values

public class ListToMap {
    public static void main(String[] args) {
        List<User> users = new ArrayList<>();
        users.add(new User(1, "user1", "[email protected]"));
        users.add(new User(2, "user2", "[email protected]"));
        users.add(new User(2, "user3", "[email protected]"));
        users.add(new User(4, "user4", "[email protected]"));

        Map<Integer, User> userIdAndUser = users.stream()
                .collect(Collectors.toMap(User::getUserId, user-> user, (oldValue, newValue) -> newValue));
        System.out.println(userIdAndUser);
    }
}
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Select the new value to overwrite the old value and get the result

Only user3 will appear, and user2 will be overwritten.

• Mapping IDs and user lists

The duplicate IDs that appeared above are overwritten according to the value, if in some cases it needs to be mapped to a list.

That is: the case of List -> Map<Integer, List>

This requires the use of groupingBy

public class ListToMap {
    public static void main(String[] args) {
        List<User> users = new ArrayList<>();
        users.add(new User(1, "user1", "[email protected]"));
        users.add(new User(2, "user2", "[email protected]"));
        users.add(new User(2, "user3", "[email protected]"));
        users.add(new User(4, "user4", "[email protected]"));

        Map<Integer, List<User>> userMap = users.stream()
                .collect(Collectors.groupingBy(User::getUserId));
        System.out.println(userMap);
    }
}
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There are multiple pieces of data with ID 2.

• Specify the Map type

Collectors.toMap supports the fourth parameter to instantiate the map type

public class ListToMap {
    public static void main(String[] args) {
        List<User> users = new ArrayList<>();
        users.add(new User(1, "user1", "[email protected]"));
        users.add(new User(2, "user2", "[email protected]"));
        users.add(new User(3, "user3", "[email protected]"));
        users.add(new User(4, "user4", "[email protected]"));

        Map<Integer, User> userIdAndUser = users.stream()
                .collect(Collectors.toMap(User::getUserId, user-> user, (oldValue, newValue) -> newValue, ConcurrentHashMap::new));
        System.out.println(userIdAndUser);
        System.out.println(userIdAndUser.getClass());
    }
}
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output

The type of userIdAndUser class is ConcurrentHashMap