Please avoid abbreviations like "sth" for reasons explained here
The way to prevent private reference instance fields from being modified is to return a copy of the field. In the case of the method with a List return type above, try this method
Note the returned List will reflect any changes made in the original class.
For other mutable objects, a copy of the original field is useful; such classes should provide a copy constructor or implement a clone() method.
For things like String
which are not mutable, or for primitives, there is no need for any special precautions.