Understanding Object Visibility in Object-Oriented Systems

1. Chapter 12: Visibility Chapter 18 in Applying UML and Patterns Book.

2. What is Visibility ? • The visibility between objects. • Four Kinds of Visibility: • Attribute • Pattern • Local • Global Overview

3. What is Visibility? Visibility is the ability of one object to see or have reference to another. • When is Visibility necessary? To send a message from one object to another, the receiver object must be visible to the sender, so the sender has to have a pointer or reference to the receiver. • Example: • If A sends messages to B, which must be visible to which? B is visible to A means A can send a message to B. • Some say that "B is an acquaintance of A". What is Visibility?

4. Visibility between Objects

5. Visibility is related to the scope: • Is one resource (such as an instance) within the scope of another? • The motivation to consider visibility: • For an object A to send a message to an object B, B must be visible to A. Visibility

6. How Visibility can be achieved from Object A to Object B: • Attribute visibility - B is an attribute of A. • Parameter visibility - B is a parameter of a method of A. • Local visibility - B is a local object in a method of A. • Global visibility - B is in some way globally visible. Four Kinds of Visibility

7. Attribute visibility from A to B exists when B is an attribute of A. • Relatively permanent visibility because it persists as long as A and B exist. • Common form of visibility. • Example: public class Register { … private ProductCatalog Catalog; … } 1. Attribute Visibility

8. 1. Attribute Visibility A Register instance may have attribute visibility to a ProductCatalog.

9. 1. Attribute Visibility

10. Parameter visibility from A to B exists when B is passed as a parameter to a method of A. • Relatively temporary visibility because it persists only within the scope of the method. • The 2nd most common form of visibility in the OO systems. 2. Parameter Visibility

11. Within the scope of the makeLineItem method, the sale has parameter visibility to ProductSpecification. 2. Parameter Visibility

12. It is common to transform parameter visibility into attribute visibility. 2. Parameter Visibility

13. 2. Parameter Visibility

14. Local visibility from A to B exists when B is declared as a local object within a method of A. • Relatively temporary visibility since it persists only within the scope of the method. 3. Local Visibility

15. There are two common means by which local visibility is achieved: • Create a new local instance and assign it to a local variable. • Assign the returning object from a method invocation to a local variable. A variation of this method does not explicitly declare a variable, but one implicitly exists as the result of a returning object from a method invocation. • Example: anObject.getAnotherObject.doSomething(); 3. Local Visibility

16. Global visibility from A to B exists when B is global to A. • Relatively permanent visibility since it persists as long as A and B exist. • The least common form of visibility in OO Systems. 4. Global Visibility

17. Example

18. Adding Navigability and Dependency Relationships

19. Public: • Any outside classifier with visibility to the given classifier can use the feature; specified by pre-pending the symbol “+”. • Protected: • Any descendant of the classifier can use the feature; specified by pre-pending the symbol “#”. • Private: • Only the classifier itself can use the feature; specified by pre-pending the symbol “-”. Visibility in the UML