Chapter 11 - Abstract Data Types and Encapsulation Constructs

Review question 2. Define abstract data type. Answer : data type that satisfies the following conditions: -The representation of objects of the type is hidden from the program units that use the type, so the only direct operations possible on those objects are those provided in the type’s definition. -The declarations of the type and the protocols of the operations on objects of the type, which provide the type’s interface, are contained in a single syntactic unit. The type’s interface does not depend on the representation of the objects or the implementation of the operations. Also, other program units are allowed to create variables of the defined type. 8. What is the difference between private and limited private types in Ada? Answer : Limited private is more restricted form and objects of a type that is declared limited private have no built-in operations. 10. What is the use of the Ada with clause? Answer : With clause makes the names defined in external packages visible; in this case Ada. Text_IO, which provides functions for input of text. 11. What is the use of the Ada use clause? Answer : The with clause makes the names defined in external packages Visible. 12. What is the fundamental difference between a C++ class and an Ada package? Answer : Ada packages are more generalize encapsulations that can define any number of types. 15. What is the purpose of a C++ destructor? Answer : The purpose of a C++ desctructor is as a debugging aid, in which case they simply display or print the values of some or all of the object’s data members before those members are deallocated. 16. What are the legal return types of a desctructor? Answer : Destructor has no return types and doesn’t use return statements. 20. What is the use of limited private types? Answer : An alternative to private types is a more restricted form: limited private types. Nonpointer limited private types are described in the private section of a package specification, as are nonpointer private types. The only syntactic difference is that limited private types are declared to be limited private in the visible part of the package specification. The semantic difference is that objects of a type that is declared limited private have no built-in operations. Such a type is useful when the usual predefined operations of assignment and comparison are not meaningful or useful. For example, assignment and comparison are rarely used for stacks. 21. What are initializers in Objective-C? Answer : The initializers in Objective-C are constructors. 22. What is the use of @private and @public directives? Answer : The use is to specify the access levels of the instance variables in a class definition. 27. Where are all Java methods defined? Answer : All Java methods are defined in a class. 30. What is a friend function? What is a friend class? Answer : a “friend” of a given class is allowed access to public, private, or protected data in that class. Normally, function that is defined outside of a class cannot access such information. Class that can access the private and protected members of the class in which it is declared as a friend. On declaration of friend class all member function of the friend class become friends of the class in which the friend class was declared. Problem set 4. What are the advantages of the nonpointer concept in Java? Any task that would require arrays, structures, and pointers in C can be more easily and reliably performed by declaring objects and arrays of objects. Instead of complex pointer manipulation on array pointers, you access arrays by their arithmetic indices. The Java run-time system checks all array indexing to ensure indices are within the bounds of the array. You no longer have dangling pointers and trashing of memory because of incorrect pointers, because there are no pointers in Java. 10. Which two conditions make data type “abstract”? The representation of objects of the type is hidden from the program units that use the type, so the only direct operations possible on those objects are those provided in the type’s definition. The declarations of the type and the protocols of the operations on objects of the type, which provide the type’s interface, are contained in a single syntactic unit. The type’s interface does not depend on the representation of the objects or the implementation of the operations. Also, other program units are allowed to create variables of the defined type. 12. How are classes in Ruby made dynamic? Classes in Ruby are dynamic in the sense that members can be added at any time. This is done by simply including additional class definitions that specify the new members. Moreover, even predefined classes of the language, such as String, can be extended. 13. Compare and contrast the data abstraction of Java and C++. Java support for abstract data types is similar to that of C++. There are, however, a few important differences. All objects are allocated from the heap and accessed through reference variables. Methods in Java must be defined completely in a class. A method body must appear with its corresponding method header. Therefore, a Java abstract data type is both declared and defined in a single syntactic unit. A Java compiler can inline any method that is not overridden. Definitions are hidden from clients by declaring them to be private. Rather than having private and public clauses in its class definitions, in Java access modifiers can be attached to method and variable definitions. If an instance variable or method does not have an access modifier, it has package access. 19. Compare Java’s packages with Ruby’s modules. In Ruby, the require statement is used to import a package or a module. For example, the extensions package/module is imported as follows. require ‘extensions’ External files may be included in a Ruby application by using load or require. For example, to include the external file catalog.rb, add the following require statement. require “catalog.rb” The difference between load and require is that load includes the specified Ruby file every time the method is executed and require includes the Ruby file only once. In Java, the import statement is used to load a package. For example, a Java package java.sql is loaded as follows. import java.sql.*;