The pages of this web site are not formatted to conserve paper, but my new book (ISBN: 0-9745862-0-X) is formatted to save paper, save your printer toner, and save money. If you prefer to work my exams from printed pages, then you can save a tree, save your printer toner, and save money if you buy my new book instead of attempting to print the pages of my web site.
Today, you can find my book on the retail web site of the company that prints it and distributes it.
| No. | Answer | Remark | |
|---|---|---|---|
| 1 | d | If an interface is named in the implements clause of a class, then the class must implement all of the methods declared within the interface. | This question asks which answer option is not true. Some true statements are as follows. An interface can be declared within an enclosing class or interface. The members of an interface can be constants, abstract method declarations, class declarations or interface declarations. If an interface is named in the implements clause of a class, then the class must implement all of the methods declared within the interface or the class must be declared abstract. The untrue answer option did not mention that an abstract class is not required to implement any of the methods declared in an interface that is named in the implements clause of the class declaration. |
| 2 | a d e | Encapsulation is a form of data hiding. Encapsulation helps to protect data from corruption. Encapsulation allows for changes to the internal design of a class while the public interface remains unchanged. | A tightly encapsulated class does not allow direct public access to the internal data model. Instead, access is permitted only through accessor (i.e. get) and mutator (i.e. set) methods. The additional time required to work through the accessor and mutator methods typically slows execution speed. Encapsulation is a form of data hiding. A tightly encapsulated class does not allow public access to any data member that can be changed in any way; so encapsulation helps to protect internal data from the possibility of corruption from external influences. The mutator methods can impose contraints on the argument values. If an argument falls outside of the acceptable range, then a mutator method could throw an IllegalArgumentException. The internal design of a tightly encapsulated class can change while the public interface remains unchanged. An immutable class is always tightly encapsulated, but not every tightly encapsulation class is immutable. |
| 3 | a d | A constructor can invoke another constructor of the same class using the alternate constructor invocation, "this(argumentListopt);". A constructor can invoke the constructor of the direct superclass using the superclass constructor invocation, "super(argumentListopt);". | If an alternate constructor invocation appears in the body of the constructor, then it must be the first statement. The same is true for a superclass constructor invocation. A compile-time error is generated if a constructor attempts to invoke itself either directly or indirectly. |
| 4 | a e | An interface declaration can be a member of an interface. An abstract method declaration can be a member of an interface. | An interface can be declared within an enclosing class or interface. The members of an interface can be constants, abstract method declarations, class declarations, or interface declarations. The body of a method declared within an interface is a semicolon. An interface can extend another interface, but can not implement an interface. An abstract class that has an interface, I1, in its implements clause is not required to implement any of the methods declared within I1. |
| 5 | b d f | Encapsulation enhances the maintainability of the code. A tightly encapsulated class allows access to data only through accessor and mutator methods. A tightly encapsulated class might have mutator methods that validate data before it is loaded into the internal data model. | The data members of a tightly encapsulated class are declared private; so changes to the data model are less likely to impact external code. Access to internal data can be provided by public accessor (i.e. get) and mutator (i.e. set) methods. The mutator methods can be used to validate the data before it is loaded into the internal data model. The use of accessor and mutator methods is likely to increase the size of the code and slow execution speed. |
| 6 | b d e f | Instance variables declared in this class or any superclass. Instance methods declared in this class or any superclass. The keyword this. The keyword super. | If the superclass constructor invocation, "super(argumentListopt);", appears explicitly or implicitly, then it must be the first statement in the body of the constructor. Until the superclass constructor invocation runs to completion, no other statements are processed within the body of the constructor. The same is true of the constructors of any superclass. (Note: The primordial class, Object, does not have a superclass, so the constructors do not include a superclass constructor invocation statement.) Suppose class B is a subclass of A. The process of creating and initializing an instance of B includes the creation and initialization of an instance of the superclass A and an instance of the superclass Object. The superclass constructor invocation statement appearing in a constructor of B is invoked before the completion of the constructors of the superclasses A and Object. A superclass constructor invocation statement appearing in B can not refer to the non-static members of the superclasses, because the process of initializing those non-static superclass members is not complete when the superclass constructor invocation occurs in B. The same is true of the non-static members of B. |
| 7 | a f | abstract public | All interfaces are implicitly abstract. The explicit application of the abstract modifier to an interface declaration is redundant and is strongly discouraged. The declaration of an interface within the body of an enclosing class or interface is called a member type declaration. Every member type declaration appearing within the body of a directly enclosing interface is implicitly static and public. Use of the access modifiers, private or protected, is contradictory and results in a compile-time error. In contrast, the modifiers, private and protected, are applicable to a member type declaration appearing within the body of a directly enclosing class. The modifier, final, is never applicable to an interface. The keyword, implements, is not a modifier. |
| 8 | e | None of the above | If a class A has a method that returns a reference to an internal, mutable object; then external code can use the reference to modify the internal state of class A. Therefore, class A can not be considered tightly encapsulated. However, the methods of a tightly encapsulated class may return a reference to an immutable object or a reference to a copy or clone of an internal object. |
| 9 | a d e f | abstract private protected public | All interfaces are implicitly abstract. The explicit application of the modifier, abstract, to an interface is redundant and is strongly discouraged. The declaration of an interface within the body of an enclosing class or interface is called a member type declaration. The private, protected and static modifiers are applicable to a member type declaration that appears in the body of a directly enclosing class. In contrast, the modifiers, private and protected, are not applicable to a member type declaration appearing within the body of a directly enclosing interface. The modifier, final, is never applicable to an interface. The keyword, extends, is not a modifier. |
| 10 | f | None of the above | One answer option reads as follows: "Accessor methods are used to prevent fields from being set with invalid data." The answer would be correct if the word "Accessor" were replaced by the word "Mutator". Accessor methods are used to read data members; mutator methods are used to set data members. The mutator methods can validate the parameter values before the values are used to change the state of the internal data model. |
| 11 | a | static | A member interface is always implicitly static. The modifier, static, can not be applied to an interface that is not a member interface. The modifier, synchronized, is applicable to a concrete implementation of a method, but is not applicable to any interface. The modifiers, volatile and transient, are only applicable to variables that are members of a class. The keyword, implements, is not a modifier. |
| 12 | a d | The data members can not be directly manipulated by external code. The superclass is tightly encapsulated. | If a class A is not tightly encapsulated, then no subclass of A is tightly encapsulated. |
| 13 | b | Prints: B.s1 A.s2 A.s1 A.s2 | The variables of a subclass can hide the variables of a superclass or interface. The variable that is accessed is determined at compile-time based on the type of the reference--not the run-time type of the object. The two references x and y refer to the same instance of type B. The name x.s1 uses a reference of type B; so it refers to the variable s1 declared in class B. The name y.s1 uses a reference of type A; so it refers to the variable s1 declared in class A. |
| 14 | b | Prints: ABC | The method invocation expression d1.m1(a1) uses reference d1 of type D to invoke method m1. Since the reference d1 is of type D, the class D is searched for an applicable implementation of m1. The methods inherited from the superclasses, C, B and A, are included in the search. The argument, a1, is a variable declared with the type A; so method A.m1(A a) is invoked. |
| 15 | f | Compile-time error | Suppose a superclass method is not private and is accessible to code in a subclass. If the superclass method is declared static, then any subclass method sharing the same signature must also be declared static. Similarly, if the superclass method is declared non-static, then any subclass method sharing the same signature must also be declared non-static. The attempted declaration of the non-static method D.printS2 generates a compile-time error; because the superclass method, C.printS2, is static. |
| 16 | b | Prints: ABC | Three methods overload the method name m1. Each has a single parameter of type A or B or C. For any method invocation expression of the form m1(referenceArgument), the method is selected based on the declared type of the variable referenceArgument--not the run-time type of the referenced object. The method invocation expression d1.m1(c1) uses reference d1 of type D to invoke method m1 on an instance of type D. The argument, c1, is a reference of type A and the run-time type of the referenced object is C. The argument type is determined by the declared type of the reference variable c1--not the run-time type of the object referenced by c1. The declared type of c1 is type A; so the method m1(A a) is selected. The declared type of c2 is type B; so the method invocation expression d1.m1(c2) invokes method m1(B b). The declared type of c3 is type C; so the method invocation expression d1.m1(c3) invokes method m1(C c). |
| 17 | c | Prints: F.printS1 E.printS2 | A static method is selected based on the compile-time type of the reference--not the run-time type of the object. A non-static method is selected based on the run-time type of the object--not the compile-time type of the reference. Both method invocation expressions, x.printS1() and x.printS2(), use a reference of the superclass type, E, but the object is of the subclass type, F. The first of the two expressions invokes an instance method on an object of the subclass type; so the overriding subclass method is selected. The second invokes a static method using a reference of the superclass type; so the superclass method is selected. |
| 18 | b | Prints: ABC | Three methods overload the method name m1. Each has a single parameter of type A or B or C. For any method invocation expression of the form m1(referenceArgument), the method is selected based on the declared type of the variable referenceArgument--not the run-time type of the referenced object. The method invocation expression c4.m1(c1) uses reference c4 of type C to invoke method m1 on an instance of type C. The argument, c1, is a reference of type A and the run-time type of the referenced object is C. The argument type is determined by the declared type of the reference variable c1--not the run-time type of the object referenced by c1. The declared type of c1 is type A; so the method A.m1(A a) is selected. The declared type of c2 is type B; so the method invocation expression c4.m1(c2) invokes method B.m1(B b). The declared type of c3 is type C; so the method invocation expression c4.m1(c3) invokes method C.m1(C c). |
| 19 | b | Prints: P.printS1 Q.printS2 | Suppose a method m1 is invoked using the method invocation expression m1(). If m1 is a static member of the class where the invocation expression occurs, then that is the implementation of the method that is invoked at run-time regardless of the run-time type of the object. If m1 is non-static, then the selected implementation is determined at run-time based on the run-time type of the object. The program invokes method printS1S2 on an instance of class Q. The body of method printS1S2 contains two method invocation expressions, printS1() and printS2(). Since method printS1 is static, the implementation declared in class P is invoked. Since printS2 is non-static and the run-time type of the object is Q, the invoked method is the one declared in class Q. |
| 20 | a | Prints: A | The reference c1 is of the superclass type, A; so it can be used to invoke only the method m1 declared in class A. The methods that overload the method name m1 in the subclasses, B and C, can not be invoked using the reference c1. A method invocation conversion promotes the argument referenced by c2 from type C to type A, and the method declared in class A is executed. Class A declares only one method, m1. The single parameter is of type A. Class B inherits the method declared in class A and overloads the method name with a new method that has a single parameter of type B. Both methods sharing the overloaded name, m1, can be invoked using a reference of type B; however, a reference of type A can be used to invoke only the method declared in class A. Class C inherits the methods declared in classes A and B and overloads the method name with a new method that has a single parameter of type C. All three methods sharing the overloaded name, m1, can be invoked using a reference of type C; however, a reference of type B can be used to invoke only the method declared in class B and the method declared in the superclass A. The method invocation expression c1.m1(c2) uses reference c1 of type A to invoke method m1. Since the reference c1 is of type A, the search for an applicable implementation of m1 is limited to class A. The subclasses, B and C, will not be searched; so the overloading methods declared in the subclasses can not be invoked using a reference of the superclass type. |
| 21 | b | Prints: R.printS1 S.printS2 | A private method of a superclass is not inherited by a subclass. Even if a subclass method has the same signature as a superclass method, the subclass method does not override the superclass method. Suppose a non-static method m1 is invoked using the method invocation expression m1(). If m1 is a private member of the class T where the invocation expression occurs, then the implementation in class T is selected at run-time regardless of the run-time type of the object. If the non-static method m1 is not private, then the selected implementation is determined at run-time based on the run-time type of the object. The program invokes the non-static method printS1S2 on an instance of class S, so the run-time type is S. The body of method R.printS1S2 contains two method invocation expressions, printS1() and printS2(). Since class R contains a private implementation of the instance method printS1, it is the implementation that is selected regardless of the run-time type of the object. Since printS2 is not private and not static, the selected implementation of printS2 depends on the run-time type of the object. The method printS1S2 is invoked on an instance of class S; so the run-time type of the object is S, and the implementation of printS2 declared in class S is selected. |
| 22 | a | Prints: AAA | The declared type of the reference variables, a1, b1 and c1, is the superclass type, A; so the three reference variables can be used to invoke only the method m1(A a) that is declared in the superclass, A. The methods that overload the method name m1 in the subclasses, B and C, can not be invoked using a reference variable of the superclass type, A. A method invocation conversion promotes the argument referenced by c4 from type C to type A, and the method declared in class A is executed. |