具有多实例管理的单例

Question

I'm trying to develop something with let's say :
A class : User
Several instances of User : "john", "joe", ...
I would like that each name is instanced only one time, so if the user tries to create an User wich name already exists, it returns the existing user instance.
With the singleton, I can only make one instance.
How can I do that?

Answer 1

What you are looking for is the factory pattern.
Let the factory cache the created instances and return them as they exist.

You could implement it in multiple ways : static method, singleton pattern : enum class or classical singleton or still dependency injection.
Note that the implementation should also consider to synchronize the access to the object that caches the users if the factory is accessed concurrently.

Here is an example with the enum way and thread safe.

Interface implemented by the enum (not mandatory but clearer and more extensible) :

public interface IUserFactory {

    User getUser(String user);

}

Enum factory :

public enum UserFactory implements IUserFactory {
    INSTANCE;

    private Map<String, User> map = new ConcurrentHashMap<>();

    @Override
    public User getUser(String username) {

        User user = map.get(username);
        if (user != null) {
            return user;
        }
        synchronized (INSTANCE) {
            final User user = new User(username);
            map.put(username, user);
            return user;
        }

    }

}

Answer 2

Create a UserFactory class, then add a method to it, like User getUser(String name) .

Also, make User 's contructor package-private to make sure that only UserFactory (belonging to the same package) will be able to instantiate new User objects.

For further info on factory pattern, please refer to https://en.wikipedia.org/wiki/Factory_method_pattern .