import java.util.Hashtable;

public class Network {

    private static Network theInstance;

    private Hashtable<String, Phone> phones;

    static Network instance() {
        if (Network.theInstance == null) {
            Network.theInstance = new Network();
        }
        return Network.theInstance;
    }

    private Network() {
        phones = new Hashtable<String, Phone>();
    }

    void register(Phone p) throws NumberInUseException {
        String number = p.getNumber();
        if (this.phones.containsKey(number)) {
            throw new NumberInUseException();
        } else {
            this.phones.put(number, p);
        }
    }

    void deliverText(String number, String message) throws
        MailboxFullException, NonWorkingNumberException {
        if (!this.phones.containsKey(number)) {
            throw new NonWorkingNumberException();
        } else {
            Phone p = this.phones.get(number);
            p.acceptTextFromNetwork(message);
        }
    }
}