Editing Immutable object (section)

=== Weak vs strong immutability ===
Sometimes, one talks of certain ''fields'' of an object being immutable. This means that there is no way to change those parts of the object state, even though other parts of the object may be changeable (''weakly immutable''). If all fields are immutable, then the object is immutable. If the whole object cannot be extended by another class, the object is called ''strongly immutable''.<ref>{{cite web
 | url    = http://www.javaranch.com/journal/2003/04/immutable.htm
 | title  = Mutable and Immutable Objects: Make sure methods can't be overridden.
 | author = David O'Meara
 | date   = April 2003
 | work   = Java Ranch
 | quote  = The preferred way is to make the class final. This is sometimes referred to as "Strong Immutability". It prevents anyone from extending your class and accidentally or deliberately making it mutable.
 | access-date = 2012-05-14
}}</ref> This might, for example, help to explicitly enforce certain invariants about certain data in the object staying the same through the lifetime of the object. In some languages, this is done with a keyword (e.g. <code>const</code> in [[C++]], <code>final</code> in [[Java (programming language)|Java]]) that designates the field as immutable. Some languages reverse it: in [[OCaml]], fields of an object or record are by default immutable, and must be explicitly marked with <code>mutable</code> to be so.