Ensure value or reference is valid
public String validate<PropertyName>(<property type> param)
Complements a get/set method for a value or reference property.
This validate method is used to check the value/reference that is to be used to set a property.
Using such a mechanism the programmer can guarantee that a field never becomes invalid. If a
String object is returned the value/reference is consider to be invalid and the field will not be
set. The string itself will then be made visible to user to inform them why it is invalid. If the
method returns null then property will be set, that is, its set or modify method will be called
with the same parameter.