GSM

package PhoneCall;

class GSM {

    private String model;        // model of the phone;
    private boolean hasSimCard;  // does the phone have SimCard;
    private String simMobileNumber; // the phone number IF it has SimCard;
    private int outgoingCallsDuration; // the sum of all outgoing calls (in minutes);
    private Call lastIncomingCall;
    private Call lastOutgoingCall;

    GSM(String model) {
        this.model = model;
    }

    public String getModel() {
        return this.model;
    }

    public String getSimMobileNumber() {
        return this.simMobileNumber;
    }

    public Call getLastIncomingCall() {
        return this.lastIncomingCall;
    }

    public Call getLastOutgoingCall() {
        return this.lastOutgoingCall;
    }

    public boolean hasSimCard () {
        return this.hasSimCard;
    }

    public int getOutgoingCallsDuration() {
        return outgoingCallsDuration;
    }

    public double getSumForCall() {
        return (this.outgoingCallsDuration * Call.getPriceForAMinute());
    }

    void insertSimCard(String mobileNumber) {

        if (!this.hasSimCard) {

            if (mobileNumber.length() != 10) {
                System.out.println("Invalid number. Try again:");
                return;
            } else if ((mobileNumber.charAt(0) != '0') || (mobileNumber.charAt(1) != '8')) {
                System.out.println("Invalid number. Try again:");
                return;
            }

            for (int i = 2; i < mobileNumber.length(); i++) {
                if (mobileNumber.charAt(i) < '0' || mobileNumber.charAt(i) > '9') {
                    System.out.println("Invalid number. Try again:");
                     return;
                }
            }

            this.hasSimCard = true;
            this.simMobileNumber = mobileNumber;
        } else {
            System.out.println("Sim Card has been already inserted. You have to remove it first.");
        }
    }

    void removeSimCard() {

        this.hasSimCard = false;
        this.simMobileNumber = null;
    }

    void call(GSM mobile, int duration) {

        if (this.hasSimCard) {
            if (mobile.simMobileNumber.equals(this.simMobileNumber)) {
                System.out.println("Can not call to the same number.");
                return;
            }
            if (duration <= 0) {
                System.out.println("Invalid duration");
                return;
            }
            if (!mobile.hasSimCard) {
                System.out.println("Unidentified number.");
                return;
            }

            this.lastOutgoingCall = new Call(this.simMobileNumber, mobile.simMobileNumber, duration); ;
            mobile.lastIncomingCall = new Call(mobile.simMobileNumber, this.simMobileNumber, duration);
            this.outgoingCallsDuration += duration;
        } else {
            System.out.println("No SIM Card inserted.");
            return;
        }
    }

}