unique numbers API Exception


While importing import com.haulmont.cuba.core.app.UniqueNumbersAPI i am getting the following error
Cannot find symbol
import com.haulmont.cuba.core.app.UniqueNumbersAPI;

Where i need to add the related jar(cuba-core-6.5.3-sources.jar) in cuba & how to compile it?

UniqueNumbersAPI is accessible only from the core module, i.e. on the middle tier. If you need this functionality on the client side, use com.haulmont.cuba.core.app.UniqueNumbersService.