3. Can an abstract class include both abstract methods and non-abstract methods? D

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1 Assignment 13 Abstract Classes and Polymorphism CSC 123 Fall 2018 Answer Sheet Short Answers. Multiple Choice 1. What is an abstract class? C A. An abstract class is one without any child classes. B. An abstract class is any parent class with more than one child class. C. An abstract class is class which cannot be instantiated, but can be a base class. D. abstract class is another name for "base class." 2. What is an abstract method? C A. An abstract method is any method in an abstract class. B. An abstract method is a method which cannot be inherited. C. An abstract method is one without a body that is declared with the reserved word abstract. D. An abstract method is a method in the child class that overrides a parent method. 3. Can an abstract class include both abstract methods and non-abstract methods? D A. No it must have all one or the other. B. No it must have all abstract methods. C. Yes but the child classes do not inherit the abstract methods. D. Yes the child classes inherit both. 4. Can an abstract parent class have non-abstract children? D A. No an abstract parent must have only abstract children. B. No an abstract parent must have no children at all. C. Yes all children of an abstract parent must be non-abstract. D. Yes an abstract parent can have both abstract and non-abstract children. 5. Can an abstract method be defined in a non-abstract class? A A. No if a class defines an abstract method the class itself must be abstract. B. No only classes are abstract, not methods. C. Yes a method can be declared abstract in any parent as long as the child classes also declare it abstract. D. Yes there is no restriction on where abstract methods can be defined. 6. Can an object of a child type be assigned to a variable of the parent type? For example, Card crd; BirthDay bd = new BirthDay("Lucinda", 42); 1

2 crd = bd; // is this correct? C A. No there must always be an exact match between the variable and the object types. B. No but a object of parent type can be assigned to a variable of child type. C. Yes an object can be assigned to a reference variable of the parent type. D. Yes any object can be assigned to any reference variable. 7. What is polymorphism in Java? A A. It is when a single variable is used with several different types of related objects at different places in a program. B. It is when a program uses several different types of objects, each with its own variable. C. It is when a single parent class has many child classes. D. It is when a class has several methods with the same name but different parameter types. 8. What determines what method is run in the following: Card crd = new BirthDay("Lucinda", 42); crd.greeting(); The type of the object or the type of the reference variable? A A. The type of the object. B. The type of the reference variable. 9. In order for the following code to be correct, what must be the type of the reference variable card? D card; card = new Valentine( "Joe", 14 ) ; card.greeting(); card = new Holiday( "Bob" ) ; card.greeting(); card = new Birthday( "Emily", 12 ) ; card.greeting(); A. Valentine B. Holiday C. Birthday D. Card 10. What is an advantage of polymorphism? A A. The same program logic can be used with objects of several related types. B. Variables can be re-used in order to save memory. 2

3 C. Constructing new objects from old objects of a similar type saves time. D. Polymorphism is a dangerous aspect of inheritance and should be avoided. 11. What is a method's signature? C A. The signature of a method is the name of the method the type of its return value. B. The signature of a method is the name of the method and the names of its parameters. C. The signature of a method is the name of the method and the data types of its parameters. D. The signature of a method is the name of the method, its parameter list, and its return type. 12. What must a non-abstract child do about an abstract method in its parent class? A A. A child must override an abstract method inherited from its parent by defining a method with the same signature and same return type. B. A child must define an additional method similar to the one inherited from its parent by defining a method with the same signature and different return type. C. A child must not define any method with the same signature as the parent's abstract method. D. A non-abstract child must define an abstract method with the same signature and same return type as the parent's abstract method. 13. Here is an abstract method defined in the parent: public abstract int sumup ( int[] arr ); Which of the following is required in a non-abstract child? B A. public abstract int sumup ( int[] arr ) {... } B. public int sumup ( int[] arr ) {... } C. public double sumup ( int[] arr ) {... } D. public int sumup ( long[] arr ) {... } 14. What must be true if a child of an abstract parent class does not override all of the parent's abstract methods? B A. This is always an error. B. The child class itself must be declared to be abstract. C. Child classes are automatically non-abstract, so this is OK. D. The parent class is in error. 15. Here is a situation: Birthday happy; happy = new AdultBirthday( "Joe", 39); happy.greeting(); Which greeting() method is run: the one defined for Birthday or the one defined for AdultBirthday? B 3

4 A. The one defined for Birthday because that is the type of the variable happy. B. The one defined for AdultBirthday because that is the type of the object referred to by happy. C. The one closest in the source code to the happy.greeting() statement. D. Neither---the assignment statement is in error. 16. Say that class Rodent has a child class Rat and another child class Mouse. Class Mouse has a child class PocketMouse. Examine the following Rodent rod; Rat rat = new Rat(); Mouse mos = new Mouse(); PocketMouse pkt = new PocketMouse(); Which one of the following will cause a compiler error? D A. rod = rat; B. rod = mos; C. pkt = null; D. pkt = rat; 17. Say that the situation is the same as in question 7. Which of the following array declarations is correct for an array that is expected to hold up to 10 objects of types Rat, Mouse, and PocketMouse? C A. Rat[] array = new Rat[10]; B. Rodent[] array = new Rat[10]; C. Rodent[] array = new Rodent[10]; D. Rodent[10] array; 18. What is the name of the class that is the ancestor to every other class in Java? A A. Object B. Class C. Root D. Java 19. Examine the following code: String str = "Hot Java"; boolean switch = str instanceof String; What value is placed in switch? A A. true B. false C. "Hot Java" D. null 4

5 Homework Programs 1. Vehicle Simulation (40 points) Abstract Vehicle class Write an abstract superclass encapsulating a Vehicle. A Vehicle has two attributes = owner (String) and wheels (an int) Write accessor and mutator methods for both attributes. Write methods for a tostring that includes the owner and the number of wheels. Write an equals() method that is true if both attributes are equal when vomparing two vehicles. Bicycle class Write a class encapsulating a Bicycle that extends Vehicle that only has a constructor and no overriding methods. The Bicycle only has an owner and wheels. MotorizedVehicle class Write a class encapsulating a MotorizedVehicle that extends Vehicle. There is one extra attribute for volume displacement. There is one extra accessor and one extra mutator. There is one extra method computing horsepower which is the volume displacement times the number of wheels. You should also override the to String() and equals() method, taking into account the third attribute VehicleClient class This class includes main. It reads in the attributes for two Motorized objects. It uses the tostring() and equals() methods as appropriate. It set the second vehicle s owner equal to the first s and retests. Ir also reads in the attributes of a bicycle. It uses tostring() to print out the attributes. Bonus 10 points. Read in 5 different vehicles, a mix of bikes and motorized vehicles into an ArrayString of type Vehicle. Demonstrate with a modified VehicleClient (call it VehicleClient2) Sample Output For Simple Program: Please enter the number of wheels for motorized vehicle one as an int: 8 Please enter the volume displacement for motorized vehicle one as a double: 89.2 Owner: Ken Number of wheels: 8 volume displacement: has horsepower Please enter the owner name for motorized vehicle two in one word: Jim 5

6 Please enter the number of wheels for motorized vehicle two as an int: 6 Please enter the volume displacement for motorized vehicle two as a double: Owner: Jim Number of wheels: 6 volume displacement: has horsepower mv1 and mv2 are not equal mv1 and mv2 are still not equal Please enter the owner name for a bicycle in one word: len Please enter the number of wheels for a bicycle as an int: 2 b2 is Owner: len Number of wheels: 2 6