private void checkInitialization() {
        if (sslContext == null)
            throw new IllegalStateException(
                    "SSL Certificates and attribute are not initialized\n"
                    + "Use .withCert() methods.");
        if (gatewayHost == null || gatewayPort == -1)
            throw new IllegalStateException(
                    "The Destination APNS server is not stated\n"
                    + "Use .withDestination(), withSandboxDestination(), "
                    + "or withProductionDestination().");
    }
Returns a fully initialized instance of {@link ApnsService},
according to the requested settings.

@return  a new instance of ApnsService
src/main/java/com/notnoop/apns/ApnsServiceBuilder.java