4. Object Structures, Aliasing, and Encapsulation
4. Object Structures, Aliasing, and Encapsulation
4. Object Structures, Aliasing, and Encapsulation
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Sufficient criteria for immutability of an object X:• Instance variables of X cannot be modified aftertermination of its constructor.●X is not exposed during construction.• <strong>Object</strong>s referenced by X are immutableaccording to these criteria.• Constructors can only take immutable objects asparameters.• Methods do not depend on global variables, i.e.,- They do not access global variables.- They do not invoke methods that depend onglobal variables.• Methods do not create new objects.Remarks:• The above criteria are still difficult to check:- It is difficult to show that no modifications canoccur.- Inheritance has to be controlled.• Classes like String are (almost) immutable, althoughthey modify the state <strong>and</strong> do not satisfy the abovecriteria.• Restricted forms of immutability can be checkedby tools.13.05.13© A. Poetzsch-Heffter, TU Kaiserslautern18
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