Chapter 16 - Logic Programming Language

Review Question 1. What are the three primary uses of symbolic logic in formal logic ? - to express propositions, to express the relationships between propositions, and to describe how new propositions can be inferred from other propositions that are assumed to be true. 2. What are the two parts of a compound term ? - functor and and ordered list of of parameters 3. What are the two modes in which a proposition can be stated ? - one in which a proposition is defined to be true and one in which that the proposition is something to be determined. 4. What is general form of a proposition in clausal form ? -B1 U B2 U . . . U Bn C A1 n A2 n . . . n Am 5. What are antecedents ? Consequents ? - Antecedents are right side of a clausal form proposition. Consequent is left side of a clausal form propositions 6. Give general definitions of resolution and unification - Resolution : inference rule that allows inferred propositions to be computed from given propositions, thus providing a method with potential application to automatic theorem proving. Unification : Process of determining useful values for variables. 7. What are the forms of Horn clauses ? - a. Have a single atomic proposition on the left side b. empty left side. 9. What does it mean for a language to be nonprocedural ? - Language in which the programs do not exactly state how a result is to be computed but rather describe the form of the result. Problem Set 1. “All predicate calculus propositions can be algorithmically converted to clausal form”. Is this statement true or false ? Explain. - True. 8. Critically comment on the following statement : “ Logic programs are nonprocedural” - It is true, because logical programs use lots of different processes based on its conditions. If a certain logical requirement is true, then a program will execute the corresponding process, instead of procedurally executing the statements. 9. From a book on Prolog, learn and write a description of a monkey-banana prolem. Why does Prolog allow this problem to exist in its implementation ? - The problem is defined as this : a monkey is in a room. Suspended from the ceiling is a bunch of bananas, beyond the monkey’s reach. However, in the room there are also a chair and a stick. The ceiling is just the right height so that a monkey standing on a chair could knock the bananas down with the stick. The monkey knows how to move around, carry other things around, reach for the bananas, and wave a stick in the air. What is the best sequence of actions for the monkey? It exists to create a variation in output of Prolog. As Prolog is an AI programming language, a variation might be needed in AI output to make them respond relevant to the situation.

Chapter 15 - Functional Programming Language

REVIEW QUESTION 2. What does a lambda expression specify? => The predicate function is often given as a lambda expression, which in ML is defined exactly like a function, except with the fn reserved word, instead of fun, and of course the lambda expression is nameless. 5. Explain why QUOTE is needed for a parameter that is a data list. => To avoid evaluating a parameter, it is first given as a parameter to the primitive function QUOTE, which simply returns it without change. 6. What is a simple list? => A list which membership of a given atom in a given list that does not include sublists. 7. What does the abbreviation REPL stand for? =>REPL stand for read-evaluate-print loop. 11. What are the two forms of DEFINE? => The simplest form of DEFINE is one used to bind a name to the value of an expression. This form is (DEFINE symbol expression) The general form of such a DEFINE is (DEFINE (function_name parameters) (expression) 13. Why are CAR and CDR so named? => The names of the CAR and CDR functions are peculiar at best. The origin of these names lies in the first implementation of LISP, which was on an IBM 704 computer. The 704’s memory words had two fields, named decrement and address, that were used in various operand addressing strategies. Each of these fields could store a machine memory address. The 704 also included two machine instructions, also named CAR (contents of the address part of a register) and CDR (contents of the decrement part of a register), that extracted the associated fields. It was natural to use the two fields to store the two pointers of a list node so that a memory word could neatly store a node. Using these conventions, the CAR and CDR instructions of the 704 provided efficient list selectors. The names carried over into the primitives of all dialects of LISP. 18. What is tail recursion? Why is it important to define functions that use recursion to specify repetition to be tail recursive? => A function is tail recursive if its recursive call is the last operation in the function. This means that the return value of the recursive call is the return value of the nonrecursive call to the function. It is important to specify repetition to be tail recursive because it is more efficient(increase the efficiency). 19. Why were imperative features added to most dialects of LISP? => LISP began as a pure functional language but soon acquired some important imperative features to increased its execution efficiency. 26. What is type inferencing, as used in ML? => Type inference refers to the automatic deduction of the type of an expression in a programming language. If some, but not all, type annotations are already present it is referred to as type reconstruction. 29. What is a curried function? => Curried functions a function which a new functions can be constructed from them by partial evaluation. 30. What does partial evaluation mean? => Partial evaluation means that the function is evaluated with actual parameters for one or more of the leftmost formal parameters. 32. What is the use of the evaluation environment table? => A table called the evaluation environment stores the names of all implicitly and explicitly declared identifiers in a program, along with their types. This is like a run-time symbol table. 33. Explain the process of currying. => The process of currying replaces a function with more than one parameter with a function with one parameter that returns a function that takes the other parameters of the initial function. PROBLEM SET 8. How is the functional operator pipeline ( |> ) used in F#? => The pipeline operator is a binary operator that sends the value of its left operand, which is an expression, to the last parameter of the function call, which is the right operand. It is used to chain together function calls while flowing the data being processed to each call. 9. What does the following Scheme function do? (define ( y s lis) (cond (( null? lis) ‘ () ) ((equal? s (car lis)) lis) (else (y s (cdr lis))) )) => y returns the given list with leading elements removed up to but not including the first occurrence of the first given parameter. 10.What does the following Scheme function do? (define ( x lis) (cond (( null? lis) 0 ) (( not(list? (car lis))) (cond ((eq? (car lis) #f) (x (cdr lis))) (else (+1 (x (cdr lis)))))) (else (+ (x (car lis)) (x (cdr lis)))) => x returns the number of non-#f atoms in the given list

Chapter 14 - Exception Handling and Event Handling

Review Question 1. Define exception, exception handler, raising an exception, disabling an exception, continuation, finalization, and built-in exception. An exception is an unusual event that is detectable by either hardware or software and that may require special processing. The special processing that may be required when an exception is detected is called exception handling. The processing is done by a code unit or segment called an exception handler. Program execution can simply terminate. We term this the question of control continuation after handler execution, or simply continuation. The ability to specify such a computation is called finalization. Built-in exceptions have a built-in meaning, it is generally inadvisable to use these to signal program-specific error conditions. Instead we introduce a new exception using an exception declaration, and signal it using a raise expression when a run-time violation occurs. 6 . What is exception propagation in Ada? Exception propagation allows an exception raised in one program unit to be handled in some other unit in its dynamic or static ancestry. This allows a single exception handler to be used for any number of different program units. This reuse can result in significant savings in development cost, program size, and program complexity. 7. Where are unhandled exceptions propagated in Ada if raised in a subprogram? A block? A package body? A task? When an exception is raised in a block, in either its declarations or executable statements, and the block has no handler for it, the exception is propagated to the next larger enclosing static scope, which is the code that “called” it. The point to which the exception is propagated is just after the end of the block in which it occurred, which is its “return” point. When an exception is raised in a package body and the package body has no handler for the exception, the exception is propagated to the declaration section of the unit containing the package declaration. If the package happens to be a library unit (which is separately compiled), the program is terminated. If an exception occurs at the outermost level in a task body (not in a nested block) and the task contains a handler for the exception, that handler is executed and the task is marked as being completed. If the task does not have a handler for the exception, the task is simply marked as being completed; the exception is not propagated. The control mechanism of a task is too complex to lend itself to a reasonable and simple answer to the question of where its unhandled exceptions should be propagated. 9. What is the scope of exception handlers in Ada? Exception handlers can be included in blocks or in the bodies of subprograms, packages, or tasks. 10. What are the four exceptions defined in the Standard package of Ada? The four exception defined in the standard package of Ada are Constraint_Error, Program_Error, Storage_Error, Tasking_Error 11. What is the use of suppress pragma in Ada? An Ada pragma is a directive to the compiler. Certain run-time checks that are parts of the built-in exceptions can be disabled in Ada programs by use of the Suppress pragma, the simple form of which is pragma Suppress(check_name). Where check_name is the name of a particular exception check. The Suppress pragma can appear only in declaration sections. When it appears, the specified check may be suspended in the associated block or program unit of which the declaration section is a part. Explicit raises are not affected by Suppress. Although it is not required, most Ada compilers implement the Suppress pragma. 13. Describe three problems with Ada’s exception handling. There are several problems with Ada’s exception handling. One problem is the propagation model, which allows exceptions to be propagated to an outer scope in which the exception is not visible. Also, it is not always possible to determine the origin of propagated exceptions. Another problem is the inadequacy of exception handling for tasks. For example, a task that raises an exception but does not handle it simply dies. Finally, when support for object-oriented programming was added in Ada 95, its exception handling was not extended to deal with the new constructs. For example, when several objects of a class are created and used in a block and one of them propagates an exception, it is impossible to determine which one raised the exception. 14. What is the name of all C++ exception handlers? Each catch function is an exception handler. A catch function can have only a single formal parameter, which is similar to a formal parameter in a function definition in C++, including the possibility of it being an ellipsis (. . .). A handler with an ellipsis formal parameter is the catch-all handler; it is enacted for any raised exception if no appropriate handler was found. The formal parameter also can be a naked type specifier, such as float, as in a function prototype. In such a case, the only purpose of the formal parameter is to make the handler uniquely identifiable. When information about the exception is to be passed to the handler, the formal parameter includes a variable name that is used for that purpose. Because the class of the parameter can be any user-defined class, the parameter can include as many data members as are necessary. 15. Which standard libraries define and throw the exception out_of_range in C++? The exception out_of_range in C++ thrown by library container classes 16. Which standard libraries define and throw the exception overflow_error in C++? The exception overflow_error in C++ thrown by math library functions 19. State the similarity between the exception handling mechanism in C++ and Ada In some ways, the C++ exception-handling mechanism is similar to that of Ada. For example, un-handled exceptions in functions are propagated to the function’s caller. 20. State the differences between the exception handling mechanism in C++ and Ada There are no predefined hardware-detectable exceptions that can be handled by the user, and exceptions are not named. Exceptions are connected to handlers through a parameter type in which the formal parameter may be omitted. The type of the formal parameter of a handler determines the condition under which it is called but may have nothing whatsoever to do with the nature of the raised exception. 30. In which version were assertions added to Java? Assertions were added to Java in version 1.4. 31. What is the use of the assert statement? The assert statement is used for defensive programming. A program may be written with many assert statements, which ensure that the program’s computation is on track to produce correct results. 32. What is event-driven programming? Event-driven programming is a programming where parts of the program are executed at completely unpredictable times, often triggered by user interactions with the executing program. 33. What is the purpose of a Java JFrame? The JFrame class defines the data and methods that are needed for frames. So, a class that uses a frame can be a subclass of JFrame. A JFrame has several layers, called panes. 34. What are the different forms of assert statement? There are two possible forms of the assert statement: assert condition; assert condition : expression; Problem Set 1 . What mechanism did early programming languages provide to detect or attempt to deal with errors? Early programming languages were designed and implemented in such a way that the user program could neither detect nor attempt to deal with such errors. In these languages, the occurrence of such an error simply causes the program to be terminated and control to be transferred to the operating system. The typical operating system reaction to a run-time error is to display a diagnostic message, which may be meaningful and therefore useful, or highly cryptic. After displaying the message, the program is terminated. 2. Describe the approach for the detection of subscript range errors used in C and Java. Java compilers usually generate code to check the correctness of every subscript expression (they do not generate such code when it can be determined at compile time that a subscript expression cannot have an out-of-range value, for example, if the subscript is a literal). In C, subscript ranges are not checked because the cost of such checking was (and still is) not believed to be worth the benefit of detecting such errors. In some compilers for some languages, subscript range checking can be selected (if not turned on by default) or turned off (if it is on by default) as desired in the program or in the command that executes the compiler. 5. From a textbook on FORTRAN, determine how exception handling is done in FORTRAN programs. For example, a Fortran “Read” statement can intercept inputerrors and end-of-file conditions, both of which are detected by the input device hardware. In both cases, the Read statement can specify the label of some statement in the user program that deals with the condition. In the case of the end-of-file, it is clear that the condition is not always considered an error. In most cases, it is nothing more than a signal that one kind of processing is completed and another kind must begin. In spite of the obvious difference between end-of-file and events that are always errors, such as a failed input process, Fortran handles both situations with the same mechanism. 6. In languages without exception-handling facilities, it is common to have most subprograms include an “error” parameter, which can be set to some value representing “OK” or some other value representing “error in procedure”. What advantage does a linguistic exception-handling facility like that of Ada have over this method? There are several advantages of a linguistic mechanism for handling exceptions, such as that found in Ada, over simply using a flag error parameter in all subprograms. One advantage is that the code to test the flag after every call is eliminated. Such testing makes programs longer and harder to read. Another advantage is that exceptions can be propagated farther than one level of control in a uniform and implicit way. Finally, there is the advantage that all programs use a uniform method for dealing with unusual circumstances, leading to enhanced readability. 7. In a language without exception handling facilities, we could send an error-handling procedure as a parameter to each procedure that can detect errors that must be handled. What disadvantages are there to this method? There are several disadvantages of sending error handling subprograms to other subprograms. One is that it may be necessary to send several error handlers to some subprograms, greatly complicating both the writing and execution of calls. Another is that there is no method of propagating exceptions, meaning that they must all be handled locally. This complicates exception handling, because it requires more attention to handling in more places.

Chapter 13 - Concurrency

REVIEW QUESTION 1. What are the three possible levels of concurrency in programs? => – Instruction level (executing two or more machine instructions simultaneously) – Statement level (executing two or more high-level language statements simultaneously) – Unit level (executing two or more subprogram units simultaneously) 7. What is the difference between physical and logical concurrency? => Physical concurrency is several program units from the same program that literally execute simultaneously. Logical concurrency is multiple processors providing actual concurrency, when in fact the actual execution of programs is taking place in interleaved fashion on a single processor. 8. What is the work of a scheduler? => Scheduler manages the sharing of processors among the tasks. 12. What is a heavyweight task? What is a lightweight task? => Heavyweight task executes in its own address space. Lightweight task all run in the same address space. 16. What is a task descriptor? => Task descriptor is a data structure that stores all of the relevant information about the execution state of a task. 18. What is the purpose of a task-ready queue? => The purpose of a task-ready queue is to be storage of tasks that are ready to run. 21. What is a binary semaphore? What is a counting semaphore? => Binary semaphore is a semaphore that requires only a binary-valued counter, like the one used to provide competition synchronization. A counting semaphore is a synchronization object that can have an arbitrarily large number of states. 30. What is purpose of an Ada terminate clause? => The purpose of an Ada terminate clause is to mark that the task is finished with its job but is not yet terminated. 34. What does the Java sleep method do? => Sleep method blocks the the thread. 35. What does the Java yield method do? => Yield method surrenders the processor voluntarily as a request from the running thread. 36. What does the Java join method do? => Java forces a method to delay its execution until the run method of another thread has completed its execution. 37. What does the Java interrupt method do? => Interrupt becomes one way to communicate to a thread that it should stop. 55. What is Concurrent ML? => Concurrent ML is an extension to ML that includes a fform of threads and a form of synchronous message passing to support concurrency. 56. What is the use of the spawn primitive of CML? => The use of Spawn primitive of CML is to create a thread. 57. What is the use of subprograms BeginInvoke and EndInvoke in F#? => The use of subprograms BeginInvoke and Endinvoke in F# is to call threads asynchronously. 58. What is the use of the DISTRIBUTE and ALIGN specification of HPC? => The use of DISTRIBUTE and ALIGN specification of HPC is to provide information to the compiler on machines that do not share memory, that is, each processor has its own memory. PROBLEM SET 1. Explain clearly why a race condition can create problems for a system. => Because two or more tasks are racing to use the shared resource and the behavior of the program depends on which task arrives first (and wins the race). The importance of competition synchronization should now be clear. 2. What are the different ways to handle deadlock? => – Ignoring deadlock – Detection – Prevention – Avoidance 3. Busy waiting is a method whereby a task waits for a given event by continuously checking for that event to occur. What is the main problem with this approach? => Busy-waiting or spinning is a technique in which a process repeatedly checks to see if a condition is true, such as whether keyboard input or a lock is available. Spinning can also be used to generate an arbitrary time delay, a technique that was necessary on systems that lacked a method of waiting a specific length of time. Processor speeds vary greatly from computer to computer, especially as some processors are designed to dynamically adjust speed based on external factors, such as the load on the operating system. Busy waiting may loop forever and it may cause a computer freezing.

Chapter 12 - Support for Object-Oriented Programming

Review Questions 4. What is message protocol? Message protocol is the entire collection of methods of an object. 5. What is an overriding method? Overriding method is method that overrides the inherited method. 7. What is dynamic dispatch? Dynamic dispatch is the third characteristic (after abstract data types and inheritance) of object-oriented programming language which is a kind of polymorhphism provided by the dynamic binding of messages to method definitions. 12. From where are Smalltalk objects allocated? Smalltalk objects are allocated from the heap and are referenced through reference variables, which are implicitly dereferenced. 15. What kind of inheritance, single or multiple, does Smalltalk support? Smalltalk supports single inheritance; it does not allow multiple inheritance. 19. How are C++ heap-allocated objects deallocated? C++ heap-allocated objects are deallocated using destructor. 29. Does Objective-C support multiple inheritance? No Objective-C doesn’t support it. (It supports only single inheritance). 33. What is the purpose of an Objective-C category? The purpose of an Objective-C category is to add certain functionalities to different classes and also to provide some of the benefits of multiple inheritance, without the naming collisions that could occur if modules did not require module names on their functions. 38. What is boxing? Boxing is primitive values in Java 5.0+ which is implicitly coerced when they are put in object context. This coercion converts the primitive value to an object of the wrapper class of the primitive value’s type. 39. How are Java objects deallocated? By implicitly calling a finalizemethod when the garbage collector is about to reclaim the storage occupied by the object. Problem Set 3. Compare the inheritance of C++ and Java. - In Java, all classes inherit from the Object class directly or indirectly. Therefore, there is always a single inheritance tree of classes in Java, and Object class is root of the tree. In Java, if we create a class that doesn’t inherit from any class then it automatically inherits from Object Class. In C++, there is forest of classes; when we create a class that doesn’t inherit from anything, we create a new tree in forest. - In Java, members of the grandparent class are not directly accessible. - The meaning of protected member access specifier is somewhat different in Java. In Java, protected members of a class “A” are accessible in other class “B” of same package, even if B doesn’t inherit from A (they both have to be in the same package) - Java uses extends keyword for inheritence. Unlike C++, Java doesn’t provide an inheritance specifier like public, protected or private. Therefore, we cannot change the protection level of members of base class in Java, if some data member is public or protected in base class then it remains public or protected in derived class. Like C++, private members of base class are not accessible in derived class. Unlike C++, in Java, we don’t have to remember those rules of inheritance which are combination of base class access specifier and inheritance specifier. - In Java, methods are virtual by default. In C++, we explicitly use virtual keyword. - Java uses a separate keyword interface for interfaces, and abstract keyword for abstract classes and abstract functions. - Unlike C++, Java doesn’t support multiple inheritance. A class cannot inherit from more than one class. A class can implement multiple interfaces though. – In C++, default constructor of parent class is automatically called, but if we want to call parametrized constructor of a parent class, we must use Initalizer list. Like C++, default constructor of the parent class is automatically called in Java, but if we want to call parametrized constructor then we must use super to call the parent constructor 5. Compare abstract class and interface in Java. - First and major difference between abstract class and interface is that, abstract class is a class while interface is a interface, means by extending abstract class you can not extend another class becauseJava does not support multiple inheritance but you can implement multiple inheritance in Java. - Second difference between interface and abstract class in Java is that you can not create non abstract method in interface, every method in interface is by default abstract, but you can create non abstract method in abstract class. Even a class which doesn’t contain any abstract method can be abstract by using abstract keyword. - Third difference between abstract class and interface in Java is that abstract class are slightly faster than interface because interface involves a search before calling any overridden method in Java. This is not a significant difference in most of cases but if you are writing a time critical application than you may not want to leave any stone unturned. - Fourth difference between abstract class vs interface in Java is that, interface are better suited for Type declaration and abstract class is more suited for code reuse and evolution perspective. - Another notable difference between interface and abstract class is that when you add a new method in existing interface it breaks all its implementation and you need to provide an implementation in all clients which is not good. By using abstract class you can provide default implementation in super class. 7. What is one programming situation where multiple inheritance has a significant disadvantage over interfaces? A situation when there are two classes derived from a common parent and those two derived class has one child. 9. Give an example of inheritance in C++, where a subclass overrides the superclass methods. class plan{ public: void fly(){ cout << “fly” << endl; } }; class jet:public class plan{ public: void fly() { cout << “Fly! Rocket jet activated!” << endl; } } 10. Explain one advantage of inheritance. Inheritance offers a solution to both the modification problem posed by abstract data type reuse and the program organization problem. If a new abstract data type can inherit the data and functionality of some existing type, and is also allowed to modify some of those entities and add new entities, reuse and is also allowed to modify some of those entities and add new entities, reuse is greatly facilitated without requiring change to the reused abstract data type. Programmers can begin with an existing abstract data type and design a modified descendant of it to fit a new problem requirement. Furthermore, inheritance provides a framework for the definition of hierarchies of related classes that can reflect the descendant relationship in the problem space. 12. Compare inheritance and nested classes in C++. Which of these supports an is-a relationship? Inheritance is where one class (child class) inherits the members of another class (parent class).Nested class is a class declared entirely within the body of another class or interface. Inheritance does. 17. What are the different options for object destruction in Java? There is no explicit deallocation operator. A finalize method is implicitly called when the garbage collector is about to reclaim the storage occupied by the object.

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.*;

Chapter 10 - Implementing Subprograms

REVIEW QUESTIONS 1. What is the definition used in this chapter for “simple subprograms”? Subprogram cannot be nested and all local variables are static. 4.What is the task of a linker? The task of a linker is to find the files that contain the translated subprograms referenced in that and load them into memory. 8. What kind of machines often use registers to pass parameters? RISC. 11. What is an EP, and what is its purpose? EP is a point or first address of the activation record instance of the main program. It is required to control the execution of a subprogram. 14. What are two potentialproblems with the static-chain method? - It is difficult for a programmer working on a time-critical program to estimate the costs of nonlocal references, because the cost of each reference depends on the depth of nesting between the reference and the scope of declaration. - Subsequent code modifications may change nesting depths, thereby changing the timing of some references, both in the changed code and possibly in code far from the changes. 17. Describe the shallow-access method of implementing dynamic scoping. Shallow access is an alternative implementation method, not an alternative semantics. The semantics of deep access and shallow access are identical. In the shallow-access method, variables declared in subprograms are not stored in the activation records of those subprograms. PROBLEM SET 6. Although local variables in Java methods are dynamically allocated at the beginning of each activation, under what circumstances could the value of a local variable in a particular activation retain the value of previous activation? If the variable is declared as static. Static modifier is a modifier that makes a variable history – sensitive. 8. Pascal allows gotos with nonlocal targets. How could such statements be handled if static chains were used for nonlocal variable access? Finding the correct activation record instance of a nonlocal variable using static links is relatively straightforward. When a reference is made to nonlocal variable, the activation record instance containing the variable can be found by searching the static chain until a static ancestor activation record instance is found that contains the variable. 9. The static-chain method could be expanded slightly by using two static links in each activation record instance where the second points to the static grandparent activation record instance. How would this approach affect the time required for subprogram linkage and nonlocal references? Including two static links would reduce the access time to nonlocals that are defined in scopes two steps away to be equal to that for nonlocals that are one step away. Overall, because most nonlocal references are relatively close, this could significantly increase the execution efficiency of many programs. 11. If a compiler uses the static chain approach to implementing blocks, which of the entries in the activation records for subprograms are needed in the activation records for blocks? There are two options for implementing blocks as parameterless subprograms: One way is to use the same activation record as a subprogram that has no parameters. This is the most simple way, because accesses to block variables will be exactly like accesses to local variables. Of course, the space for the static and dynamic links and the return address will be wasted. The alternative is to leave out the static and dynamic links and the return address, which saves space but makes accesses to block variables different from subprogram locals.