Final Exam 90 minutes Eng. Mohammed S. F. Abdual Al

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1 Islamic University of Gaza Faculty of Engineering Computer Engineering Department Computer Programming Lab (ECOM 2124) Final Exam 90 minutes Eng. Mohammed S. F. Abdual Al Student name Student ID Please read all questions carefully before answering and make your answers legible. You are NOT allowed any cheat sheets, calculators, computers, phones, or other assistance tools. I agree to complete this exam without assistance from any person, prohibited tool, or other unauthorized means. I also agree not to discuss this exam or its contents with anyone who has not taken their final yet. This exam should not be very hard if you have a solid grasp of the concepts covered in the class. Best of success to you!

2 I. Short Answer [11 pts] 1. [1 pts] What is the difference between the == operator and an equals method? Why do we generally want to use an equals method when comparing object type variables as opposed to the == operator? 2. [1 pts] What is inheritance and why would we want to use it? 3. [1 pts] what is Multithreading and what is the difference between multithreading and multiprocessing? 4. [1 pts] What is time sharing in multithreading? 5. [3 pts] Consider the following instantiation, assuming Box is some pre-defined class. Box[][] grid = new Box[3][5]; a. How many Box objects can be referred to in this grid array? b. What type of variable is grid[0]? c. What type of variable is grid[1][2]? 1

3 6. [4 pts] Consider the following section from the definition of class OfficeBuilding. public class OfficeBuilding { public int sqrfeet; private String name; protected double cost; public OfficeBuilding() { protected boolean isbuilt() { public void movein() { public double rentspace() { private int renovate(int days) { For the following lettered questions, just because a member (data, constructor, methods) is in the answer to one of the questions, does not mean it cannot be in the answer to one or more of the other questions. a. Which members are visible to (accessible from) OfficeBuilding and its subclasses? b. Which members are visible to (accessible from) only the OfficeBuilding class? c. Which members are visible to (accessible from) any class? d. Which field(s) should we write getter(s) and setter(s) for, so classes outside OfficeBuilding and its inheritance hierarchy can access it(them)? 2

4 II. Multiple Choice [15 pts] (Please circle the letter of your choice) 1. [1 pts] Which of the following is CORRECT terminology about inheritance? A. We extend from a base class and implement an interface B. We implement a base class and extend from an interface C. We extend from a subclass and implement a base class D. We implement from a subclass and extend an interface 2. [1 pts] Which of the following is CORRECT? A. An object is like the blueprint, and we can have many instances of it, which are called classes. B. A class is like the blueprint, and we can have many instances of it called, which are called objects. C. There is no difference between a class and an object. D. The object keyword indicates we re defining an object and the class keyword indicates we re defining a class. 3. [1 pts] If a variable is declared static in a particular class, what does that mean? A. It cannot be changed. B. It can only be changed by the instances of the class in which it is declared. C. There is only 1 copy of it that all instances of the class can access. D. Each instance of the class has its own copy of the variable. 4. [1 pts] If a method is declared static in a particular class, what does that mean? A. The method cannot modify any variables. B. It can only be called from within the class. C. You need an instance of the class to access it, even though it can t access instance variables/methods. D. You do not need an instance of the class to access it, because it can t access instance variables/methods. 3

5 Question 5-9 refer to the following code. 1. public class Desk { private int length; 4. private int width; 5. private int height; public Desk(int length, int width, int h) { 8. length = length; 9. this.width = width; 10. height = this.h; public int getlength() { 15. return this.length; [1 pts] What are the fields defined in this class? A. The three integers passed as parameters in the constructor, defined in line 7 B. The three integers declared in lines 3-5 C. There are none D. Only the ones where the this keyword is used in front of them 6. [1 pts] What is line 8 doing? A. Assigning the value of the parameter length to the field length B. Assigning the value of the field length to the parameter length C. Assigning the value of the parameter length to itself, i.e. doing nothing useful D. Assigning the value of the field length to itself, i.e. doing nothing useful 7. [1 pts] Line 9 is attempting to assign the value of width, passed into the constructor, to the variable width in the Desk class. It was claimed that the this keyword is not needed. Which of the following is TRUE? A. It is needed, but the left and right hand sides of the assignment should be swapped, so that the equation is: width = this.width; B. It is needed, because we need to differentiate between the width parameter and the width field (use this to refer to the field one), and the assignment is correct as written. C. It is needed, because we need to differentiate between the width parameter and the width field (use this to refer to the parameter one), and the assignment is correct as written. D. It is not needed, and should be rewritten as: width = width; 4

6 8. [1 pts] Line 10 is attempting to assign the value of h to the variable height. It was claimed that there is an error in line 10. Which of the following is TRUE? A. The error is there s no field h in the Desk class, so can t write this.h. It should be corrected to: this.height = h; B. The error is the same as specified in option A, but it should be corrected to the equation: h = height; C. The error is simply that the right and left hand sides of the assignment are switched. It should be corrected to: this.h = height; D. There is no error. 9. [1 pts] Assume that any compilation (i.e. compile time) errors in the above code are corrected. If an instance of this class is made with the instantiation: Desk d = new Desk(1,2,3); What is the return value of calling d.getlength()? The default value of integer field variables is 0. A. 0 B. 1 C. 2 D [1 pts] Both abstract classes and interfaces allow you to declare method signatures and provide no implementation of such methods. Why then might you use an abstract class as opposed to an interface? A. There s no good reason why, they re basically the same B. If you have an abstract class A you can create an instance of it like so: A var = new A(); but you wouldn t be able to do this with an interface C. Abstract classes can define functionality and non-constant fields that subclasses can use, while requiring subclasses to provide their own implementations for certain operations, whereas interfaces just define method signatures (and return types) and constants D. You wouldn t, because abstract classes are only used in special situations where you want to prevent inheritance, since they cannot be inherited from, but you can inherit from inte rfaces 11. [1 pts] You have two methods named calc in the same class that both return an integer, but one takes 1 double and the other takes 2 doubles. We say method calc is: A. Overridden B. Overloaded C. Instantiated D. Invoked 5

7 12. [1pts]Which of these statements about constructors is FALSE? A. A constructor has no return type. B. Its name must be the same as the class in which it is defined. C. There must be exactly one constructor defined for a class. D. Constructors are almost always declared as public. E. They can appear anywhere in the class where it is legal to declare a method. 13. [1pts]Which of the following expressions is equivalent to the boolean expression A. A > 5 B!= C B. A >= 5 && B == C C.!(A < 5) (B!= C) D. A >= 5 B == C E. A < 5 && B == C!(A < 5 && B!= C) 14. [1pts]What is the output of this program fragment? Read it carefully! String greet = "Hi"; String name = "Smedley"; String nickname = name.substring(0,4); if (nickname == name.substring(0,4)); System.out.println("has real nickname"); else if (greet + name == greet + nickname) System.out.println("no real nickname"); else System.out.println("hmmmchanged names?"); A. has real nickname B. no real nickname C. hmmmchanged names? D. it's one of the three lines given in A, B, and C above, we can't tell which one without running the program E. none, because there is at least one compile-time error 15. [1pts]When you want a program to be multithreading you have to extends: A. Runnable B. Serializable C. Thread D. run 6

8 III. Problem Code [4 pts] There is an interface Packable. Assume Item is some pre-defined class. public interface Packable { public boolean additem(item x); public Item removeitem(item x); The following class is supposed to be a subclass of some pre -defined class Bag, and implement the Packable interface. The in line 17 is not an error, it just indicates some irrelevant code goes there that is not shown. 1. public class Backpack extends Packable implements Bag 2. { 3. int maxitems; 4. int num; public Backpack(int capacity) 7. { 8. this.maxitems = capacity; 9. this.num = 0; public boolean additem(item x, int index) 13. { 14. if (this.num < this.capacity) 15. { 16. // Add the item somehow this.num++; 19. return true; return false; 22. There are four errors in this code (only in Backpack, none in Packable). Identify them and say how they can be fixed. 7

9 IV. JavaFx code Problems [10pts] Correct and modify the following code to make the following stage. a. AddItem button make an action for taking item from TextField and add that item to ComboBox. b. After you added items on combobox,you can select the item from combobox and display on TextField. 1. package testjavafxexam; 2. // all library was imported 3. public class TestJavaFxExam extends Application { ObservableList<Integer> data = FXCollections.observableArrayList(); 6. ComboBox<String> combobox = new ComboBox<>(); public void start(stage stage) { 10. Button btn = new Button("Add Item"); 11. TextField text = new TextField(); 12. text.setmaxwidth(200); 13. btn.setonaction(); 14. HBox hb = new HBox(comboBox,text,btn); 15. hb.setspacing(10); 16. hb.setalignment(pos.center); 17. VBox vb = new VBox(hb); 18. vb.setspacing(10); 19. vb.setalignment(pos.center); 20. Scene scene = new Scene(hb, 300, 300); 21. stage.settitle("add Items in ComboBox"); 22. stage.setscene(); public static void main(string[] args) { 27. launch(args); public void showintextfield() { 30. combobox.getselectionmodel().getselecteditem();