Ensure an object is in a valid state to be saved
public String validate()
This validate method is used to check that all properties within an object are valid and that
the object can be persisted. If a String object is returned then the object is deemed to be
invalid should not be persisted. The string itself will then be made visible to user to inform them
why the object has not yet been persisted. If the method returns null then the framework will
persist the object.