Java instantiating child object from parent object

Question

I have a following class that shares the same "state" and performs CRUD actions. This operation is not repeatable/idempotent.

public class Client {
    private static String state;
    public Client(args){
        this.state = buildStateFromArgs();  // this value will not change
    }

    public void createUser();
    public User getUser();
    public User updateUser();
    public void deleteUser();

    public void createJob();
    public Job getJob();
    public Job updateJob();
    public void deleteJob();

    // other CRUD functions ....
}

I am thinking of refactoring it to something like

public class Client {
    public Client(args){
        this.state = buildStateFromArgs();
    }
    private static String state;
}

public class UserClient extends Client{
    public void createUser();
    public User getUser();
    public User updateUser();
    public void deleteUser();
}

But I am not sure what is the best approach to instatiate the child class. Suppose this is the current implementation,

Client client = new Client(args);
client.createUser();
client.createJob();

Should I simply just downcast?

Client client = new Client(args);
UserClient userClient = (UserClient) client;
userClient.createUser();
JobClient jobClient = (JobClient) client;
jobClient.createJob();

Or should I construct the child from parent?

Client client = new Client(args);
UserClient userClient = new UserClient(client);
userClient.createUser();
JobClient jobClient = new JobClient(client);
jobClient.createJob();

Or is there a better design pattern suited for this kind of problem?

Why don't you call super(args) from your UserClient constructor? — Scary Wombat
– Scary Wombat, Commented Oct 30, 2017 at 2:42
Composition is always preferred over Inheritance in cases like this. — user177800
– user177800, Commented Oct 30, 2017 at 2:45
@ScaryWombat, I forgot to mention in my question that I cannot create the "state" parameter twice (it will be different every time buildStateFromArgs is called) and this value has to be shared across child instances — Wai Yan
– Wai Yan, Commented Oct 30, 2017 at 2:46

dave · Accepted Answer · 2017-10-30 03:24:32Z

1

Given that UserClient is-a Client, I would take the following approach:

UserClient client = new UserClient();
client.createUser();

My reason for taking this approach is that user-specific functions should only exist on a UserClient and can therefore only be called from such.

Given the new information that the state must be shared across all clients, I'd probably use a static initializer:

class Client {
    private static String state;
    static {
        state = buildStateFromArgs(args);  // this value will not change
    }
}

This assumes that the args are available this point. If they are not, then you need to make buildStateFromArgs() a public, static function and call it before any sub-classes are instantiated:

Client.buildStateFromArgs(args);
UserClient client = new UserClient();
...

edited Oct 30, 2017 at 3:24

answered Oct 30, 2017 at 2:43

dave

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4 Comments

Wai Yan Over a year ago

I forgot to mention in my question that I cannot create the "state" parameter twice (it will be different every time buildStateFromArgs is called) and this value has to be shared across child instances.

dave Over a year ago

@WaiYan OK, I've updated my answer to reflect that state is static, ie. shared.

Wai Yan Over a year ago

one more question if you don't mind, is this approach thread-safe?

dave Over a year ago

I would think so. The only caveat is you need to ensure the state is set before any thread creates a client (and potentially uses that state).

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