Fall 1999 - 492/1 (JAVA) - Midterm # 2 - Sample Solution Page 1 of 6 • Question 1. (6 points) Take a look at the following code: 1 2 3 4 5 6 7 8 9 10 11 class B { void f() { ... } void g() { ... } } class E extends B { void g() { ... } } class test { static void p(B x) { x.g() ; } static void main(String a[]) { E e = new E() ; p(e) ; e.f() ; } 12 13 14 15 16 17 } Explain where in the code above, each of the following basic concepts of object-oriented programming is illustrated. Refer to the line numbering in your answer. 1. An object of a derived type is also an object of the base class. On line 14, object e which is of type E is passed as an argument to p() which is a method that expects an argument of type B. But that is legal since class E is derived from B. 2. A derived class inherits all the methods of the base class. On line 15, we called method f() on object e which is of type E. Although no f() method was defined on class E, we were able to do so since E inherits all methods from its base class B which defined method f(). 3. JAVA supports dynamic binding of methods. On line 10, resolving which g() to call depends on the type of object x. For example, when p(e) is executed on line 14, the overriding g() of class E is executed rather than the base g(). Fall 1999 - 492/1 (JAVA) - Midterm # 2 - Sample Solution Page 2 of 6 • Question 2. (6 points) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 class B { int i ; B(int _i) { int g() { int h() { } i = _i ; } return h() ; } return -1 ; } class E extends B { int i ; E(int _i) { super(10) ; i = _i ; int h() { return 2 * i ; } } } B b = new B(1) ; E e = new E(3) ; B be = e ; Fill in the result of evaluating each of the following expressions: b.i 1 e.i 3 be.i 10 b.g() -1 e.g() 6 be.g() 6 • Question 3. (8 points) 4 class class class class 5 X x = new X() ; 1 2 3 B C X Y { ... } extends B { ... } extends C { ... } extends C { ... } Indicate if each of the following expressions would generate a compiletime error, a run-time error, or if it would evaluate to TRUE or FALSE. Compile Error Runtime Error True/ False true return (x instanceof B) ; true B b = x ; return (b instanceof C) ; X B b = x ; Y y = (Y) b ; return (y instanceof B) ; false return (x instanceof Y) ; Y y = x ; return (y instanceof B) ; X Fall 1999 - 492/1 (JAVA) - Midterm # 2 - Sample Solution Page 3 of 6 • Question 4. (8 points) Consider the following class definition. class Person { private String name ; int age ; static int N ; static Person[] Ps = ... ; public String getName() { return name ; } public Person(String n, int a) { name = n ; age = a; Ps[N++] = this ; } } • Write a complete definition of class Student. Every Student is a Person. In addition, each Student has a GPA field which is of type double and is initially set to 0.0 class Student extends Person { double GPA = 0.0 ; Student(String name, int age) { super(name,age) ; } } • Override the getName method in Student. The new method should return the name of the student concatenated to the string "(student)". public String getName() { super.getName() + " (student)" ; } • Complete the following method which is part of the Student class and that utilizes the fields Ps and N to print the names of all the Students only. public static void printAll() { for (int i = 0 ; i < N ; i++) if (Ps[i] instanceof Student) System.out.println(Ps[i].getName()) ; } Fall 1999 - 492/1 (JAVA) - Midterm # 2 - Sample Solution Page 4 of 6 • Question 5. (10 points) DO YOUR WORK ON THE NEXT PAGE You’re given the following code: class Figure extends Canvas { private Color myColor ; public void setColor(Color c) { myColor = c ; public void paint(Graphics g) { g.setColor(myColor) ; ... } ... } } interface Colorable { void changeColor(Color c)) ; } Objects that are Colorable change their color to c whenever the changeColor(c) method is called. • Define CFigure which is an extension of Figure such that it implements the Colorable interface. • Define class CButton which is an extension of Button and has the following constructor: CButton(String s, Color c, Colorable o) ; CButton should display the String s as its caption, and whenever the user clicks on it, the Colorable o should automatically change its color to c. • Use CButton and CFigure to implement the applet you see on the right. The applet displays CFigure on the top portion. Whenever the user clicks any of the three buttons, the figure should change to the respective color. Fall 1999 - 492/1 (JAVA) - Midterm # 2 - Sample Solution Solution page for question 5. class CFigure extends Figure implements Colorable { void changeColor(Color c) { setColor(c) ; repaint() ; } } class CButton extends Button implements ActionListener { Color c ; Colorable o ; CBurron(String s, Color _c, Colorable _o) { super(s) ; c = _c ; o = _o ; addActionListener(this) ; } public void actionPerformed(ActionEvent e) { o.changeColor(c) ; } } class Q extends Applet { void init() { CFigure cf = new CFigure() ; add(cf) ; add(new CButton("red",Color.red,cf)) ; add(new CButton("green",Color.green,cf)) ; add(new CButton("blue",Color.blue,cf)) ; } } Page 5 of 6 Fall 1999 - 492/1 (JAVA) - Midterm # 2 - Sample Solution Page 6 of 6 • Question 6. (8 points) Contrast each pair of items listed below. Do not define the two terms. Emphasize on the difference between them. • this() v.s. super(). this() is used within a construcor to call other constructors of the same class. super() is used within constructors to call constructors of the parent class. • overloading v.s. overriding. Overloading is having two different methods with the same name but with different signatures. Overriding is having two different methods with the same signature. Overriding can only happen across derived classes. • Event-listeners v.s. event-adapters. Event-listeners are interfaces. An event-adapter is a class implementing an event-listener with all methods being implemented as empty procedures. • Abstract v.s. concrete classes. You can’t instantiate an abstract class • Question 7. (4 points) List three differences between abstract classes and interfaces. 1. An abstract class may contain field declarations and method definitions. Interfaces contains only constants and method declarations. 2. interfaces support multiple inheritance while classes support single inheritance only. Similarly, a class may implement multiple interfaces but can only extend a single abstract class. 3. All methods of an interface are public. MATH 492/1 (Java) Second Midterm December 11, 1999 Fall 1999 • This is a closed book, closed notes exam. • Please do all your work in this booklet. The space provided should be enough for your work. You may use the backside for scratch work. • Before you start, write your name, student ID, and class serial number on this cover. • Check that you have 6 different pages. • You have 50 minutes to finish your work. • Relax! Scores: 1. (6) 2. (6) Name: Sample Solution 3. (8) 4. (8) Student ID: 5. (10) 6. (8) 7. (4) total (50) Serial Number:
© Copyright 2024