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Case study: constrained genericity in Microsoft Office Paint 3 of 9 in Microsoft Office Case study: constrained genericity

13.6 Case study: constrained genericity generate, create 3 of 9 none with microsoft office projects Microsoft SQL Server A keyed list is used for account and for customer lookup. Main points in this chapter A generic class handles objects of any type, passed as a parameter through the parameter list in the class header; such a process is called genericity. A generic class makes no assumptions about the behaviour of its objects. A generic class may be used as a client, or as a parent.

A programmer can define a new generic class simply by writing a formal parameter in the class header, and then using it in the class code. Constrained genericity allows a generic class to use features of a parameter, by limiting or constraining the type of the parameter. A constraining parent class is defined, inherited by a child class, and listed as the constraint in the generic class.

Only actual parameters of this type can then be passed. Procedural languages support code reuse by routine call. Object-oriented languages support code reuse by routine call, object creation, inheritance, and genricity.

. Exercises 1. 39 barcode for None 2. Genericity gives one way of reusing code.

How does it do this When are parameters bound How many parameters can be passed to a generic class . 3. Why do we need constrained genericity What does the word "constrained" mean What is constrained How is it constrained 4. 5.

a) b) c) d) 6. What are the main steps in constrained genericity Build a constrained generic class: Write code to display an ARRAY of POINTs. Write code to display an ARRAY of PERSONs.

Write a generic class SHARRAY (show array) that displays all items in the array. Write the code that uses class SHARRAY to replace (a) and (b) List the ways that code is reused in Eiffel..

R. S. Rist, 1993.

14: Assertions and inheritance Keywords: in 3 of 9 barcode for None heritance, class invariants, require else, ensure then. Inheritance of assertions guarantees that the behavior of a class is compatible with that of its ancestors. For example, a class should not assume a state that would be invalid for any of its predecessors; therefore it must satisfy all their invariants.

Similarly, a class must be able to perform (at least) all the functions of its ancestors; therefore, the redefinition of a routine may only weaken the precondition and strengthen the post-condition. The pre- and post-conditions for a routine redefinition are designated by the keywords require else and ensure then respectively. The precondition for a redefined routine is equivalent to the new precondition or else the precondition from the original routine, while the post-condition is equivalent to the new post-condition and then the original.

. 14.1 Look and feel Pre- and pos USS Code 39 for None t-conditions and class invariants allow us to describe many properties of software components. We can use these assertions to write software contracts which may be monitored at run-time to ensure compliance. However, we need to realize that these tools are embedded in a language which supports inheritance; therefore, classes can redefine routines inherited from their parents.

To complete our understanding of assertions and their use, we must now examine how the tools presented so far interact with inheritance. The inheritance mechanisms provided by Eiffel are powerful, and if used correctly can lead to smaller, more elegant systems consisting of easily reusable components. However, these mechanisms can also lead to chaos if misused.

If redefinition and dynamic binding are allowed to arbitrarily change the behavior of operations, clients will be unable to depend on stable, predictable outcomes. Assertions can provide an answer to this problem. Briefly, in Eiffel descendants inherit the class invariants of their parents, and any redefinition of a routine must satisfy the original pre- and post-conditions; therefore, if client code relies only on properties specified using assertions, all descendants are guaranteed to perform all functions required of their ancestors.

We can extend our contracting metaphor to view the inheritance of assertions as programming by sub-contract. In the real world, a general contractor may hire any number of others to perform parts of the original job; however, these sub-contractors must perform the work up to the standards required in the overall contract, and must do it cheaply enough so that the general contractor can complete the entire job for the agreed upon price. In Eiffel, descendants can only do more, cheaper than their ancestors.

Specifically, any redefinition of a routine may only change the post-condition by making it stronger (more difficult to satisfy); therefore, all descendants are guaranteed to perform (at least) all the functions of their ancestors. Similarly, any redefinition may only change the precondition by making it weaker (easier to satisfy); therefore, all descendants are guaranteed to perform in (at least) all cases their ancestors would accept..

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