4. Object Structures, Aliasing, and Encapsulation

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Balloon invariant:If B is an object of a ballon type then:- B is referenced by at most one instance variable.- If such a stored reference exists it is from anobject external to B.- No object internal to B is referenced by any objectexternal to B.Remark:• The invariant allows that objects referenced onlyby dynamic aliases reference internal objects.Such objects are internal to the balloon.That is why a balloon B can contain more objectsthan just the set of all objects reachable from B.• To guarantee the balloon invariant, Almeida usesthe following techniques:- Assignment of references to instance variablesof balloon objects is not always allowed.- Data flow analysis13.05.13© A. Poetzsch-Heffter, TU Kaiserslautern42
Ownership:Idea:• Introduce an ownership relation between objects:object X owns object Y.• Define encapsulation based on ownership:Only the owner may directly access his objects.• Declare the ownership relation as extended typeinformation.Example:(Ownership annotations)class Engine {void start() { ... }void stop() { ... }}class Driver { ... }class Car {rep Engine engine; // belongs to representationDriver driver;Car() {engine = new rep Engine();driver = null;}// not part of representation13.05.13© A. Poetzsch-Heffter, TU Kaiserslautern43
Page 1 and 2: 4. Object Structures, Aliasing,and
Page 3 and 4: Examples:(Object structures)1. A ve
Page 5 and 6: Problems:• Structural integrity (
Page 7 and 8: Undesirable Aspects of Aliasing:1.
Page 9 and 10: Forms of Alias Creation:Capturing:
Page 11 and 12: General approaches to solve alias p
Page 13 and 14: Examples:1. Immutable class:(Immuta
Page 15 and 16: class NotImmutableList extends Immu
Page 17 and 18: 4. Immutable objects with varying I
Page 19 and 20: 4.3 Alias Control and Encapsulation
Page 21 and 22: Roles in Object Structures:• Inte
Page 23 and 24: 2. Representations (rep mode):• E
Page 25 and 26: Meaning of alias modes:• Alias mo
Page 27 and 28: Programming Discipline (simplified)
Page 29 and 30: Example:(Representation exposure)-
Page 31 and 32: 4.3.2 Type-based EncapsulationType
Page 33 and 34: Connection with type systems:• Ea
Page 35 and 36: Scenarios in which references are i
Page 37 and 38: Remark:• The behavior of anonymou
Page 39 and 40: Further Structuring Techniques:Diff
Page 41: Cluster:Let G be the undirected gra
Page 45: The ownership relation is a binary

4. Object Structures, Aliasing, and Encapsulation

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