CISC 213- Practice Test 2

Transcription

1 CISC 213- Practice Test 2 Part A- The following questions are multiple choice or short answer: 1. Say we define a class called ABC and we don't define any constructor. Then which of the following is true: a) No constructor is defined b) The constructor ABC() is defined c) The constructor ABC(Object e) is defined d) The constructor ABC(int i) is defined e) The program will not compile 2. Say we want members of a class to be assessable by any member of any class except for non-sublcasses in a different package. Then which access specifier must we use? a) public b) private c) protected d) static e) Don't use any access modifier 3. Say we make the declarations Object o = new Object(); Component c; //Can't make instance of an abstract class Applet a = new Applet(); //Applet is a subclass of Component Which of the following assignment statements is ok? Note that 2 answers may be correct. a) a = o b) a = c c) o = a d) c = a e) None of these 4. How many classes can a class extend? 5. How many interfaces can a class implement? 6. How many constructors must a class have? 7. If abc is an interface and x is a reference variable for this interface, then what can x reference? 8. If we want the applet itself to be a listener for a button, then what keyword can be used? 9. How many functions must we define when we implement the ActionListener interface? a) 1 b) 2 c) 3 d) 4 e) We don't have to define any functions 10. If you tried to read a parameter value and a parameter with that name was not defined between the <applet> and </applet> tags then

2 a) The runtime would throw an exception b) The parameter's value would be null c) The parameter's value would be an empty string d) The program will not compile e) None of these 11. What are the 5 Java keywords used in exception handling? 12. What does a class have to do when it implements an interface? 13. In class, we learned 2 different ways we can define a thread in a Java application. What are they? 14. Consider the following code fragment: try { int i = d/0; catch (Exception e) { System.out.println("Hortense"); Will Hortense be printed? 15. Java does not have multiple inheritance. Explain how Java overcomes this limitation. 16. What is the difference between the following 2 declarations? Integer i = new Integer(6) and int i = 6? What is the advantage of using the 1st declaration? 17. What do we do when we catch an exception? 18. Say we define the class A as follows: class A { void tostr () { System.out.println("Test"); If a is reference for an instance of this class A, is the following statement ok? a.tostring() 19. When defining the function actionperformed(actionevent e), give an example of when we need to use the reference e. Explain how we can avoid using the reference e, and explain what price we have to pay. 20. Give one reason why function overriding is so important.

3 21. Can you write code where a function uses a class or function which is not yet defined? 22. How many functions does the Runnable interface contain? a) 1 b) 2 c) 3 d) 4 e) It doesn't contain any 23. When we override a function, when does Java decide which function to execute? 24. Explain the difference between function overloading and function overriding. Say we make the declaration int[] [] i = new int[5][ ] 25. What is i? 26. What is i[3] and what is its value? 27. Can we then make the declaration i[3] = new int[6]? If the answer is yes, what is i[3]? Part B 1. What is the output of the following program? class A { int x; int y; A() { x = ; y = ; A(int a, int b) { x = a*2; y = b*2; class B extends A { int z; B(int a, int b, int c) { super(a, b); z = c;

4 B(int b, int c) { x = 100; y = 3*b; z = 3*c; class test2c { public static void main(string[] args) { B b; b = new B(8, 9, 10); System.out.println(b.x); System.out.println(b.y); System.out.println(b.z); b = new B(1000, 2000); System.out.println(b.x); System.out.println(b.y); System.out.println(b.z); 2. What is the output of the following program? import java.io.*; class NewThread implements Runnable { Thread t; NewThread() { //This is a constructor. t = new Thread(this, "the Phantom"); //thread name is the Phantom System.out.println("This is " + t + ". "); t.start(); public void run() { try { for(int j = 0; j<3; j++) { System.out.println(j + ": the Phantom knows."); Thread.sleep(400); catch(interruptedexception e) { //tostring() function outputs //Thread[the Phantom, 5, main] System.out.println("Phantom is interrupted.");

5 System.out.println("The Phantom has gone."); class Class1 { public static void main(string[] args) { new NewThread(); try { for (int k = 3; k >=0; k--) { System.out.println(k + ": Main, main, main!"); Thread.sleep(1000); catch (InterruptedException e) { System.out.println("The main is interrupted."); System.out.println("The main is gone."); 3. Consider the following program: import java.awt.*; import java.awt.event.*; import javax.swing.*; public class B3 extends JFrame { JButton b, exit; JTextField t1, t2; static int i = 0; String s = "Pluto"; public B3() { //Constructor i++; Container c = getcontentpane(); c.setlayout(new FlowLayout()); Handler h = new Handler(); String s = "Goofy"; b = new JButton("Press"); exit = new JButton("Exit"); t1 = new JTextField(15); t2 = new JTextField(15); c.add(t1); c.add(t2); c.add(b);

6 c.add(exit); t1.addactionlistener(h); b.addactionlistener(h); exit.addactionlistener(h); public static void main(string[] agrs) { i++; B3 window = new B3(); window.setsize(400,400); window.show(); class Handler implements ActionListener { if(e.getsource()==b) t2.settext(s); else if(e.getsource()==t1) t2.settext(t1.gettext()+ i); else if(e.getsource()==exit) System.exit(0); a) What will appear in the 2nd text field if the user types in abc in the 1st text field and presses the return key? b) What will appear in the 2nd text field if the user types in abc in the 1st text field, doesn't press the return key, and then clicks on the "Press" button? c) What will appear in the 2nd text field if the user types in abc in the 1st text field, presses the tab key, and then clicks on the "Press" button. 4. The following applet will present the user with 2 buttons labeled A and B. Describe what will happen if the user clicks on the A button 5 times and then clicks on the B button 5 times. import java.awt.*; import javax.swing.*; import java.awt.event.*; public class B3 extends JApplet { String msg1 = "Bob"; String msg2 = "Carol"; JTextField t1, t2;

7 public void init() { Container c = getcontentpane(); c.setlayout(new FlowLayout()); JButton a = new JButton("A"); JButton b = new JButton("B"); t1 = new JTextField(20); t2 = new JTextField(20); c.add(a); c.add(b); c.add(t1); c.add(t2); t1.settext(msg1); t2.settext(msg2); a.addactionlistener(new abuttonlistener()); b.addactionlistener(new bbuttonlistener()); class abuttonlistener implements ActionListener { msg1 = " " + msg1; t1.settext(msg1); class bbuttonlistener implements ActionListener { msg2 = " " + msg2; t2.settext(msg2); 5. Consider the following program and answer the 4 questions below: Assume we highlight items two, three, and four from the list and then press the execute button. import javax.swing.*; import java.awt.*; import java.awt.event.*; public class ListPt2 extends JApplet implements ActionListener{ String msg = "Hortense"; JList a; String[] items = {"One", "Two", "Three", "Four"; JButton execute;

8 public void init() { Container c = getcontentpane(); c.setlayout(new FlowLayout()); a= new JList(items); c.add(a); execute = new JButton("Press"); c.add(execute); execute.addactionlistener(this); repaint(); public void paint(graphics g) { super.paint(g); msg = "Numbers: "; int[] i; i = a.getselectedindices(); for (int k = i.length-1; k>=0; k--) msg += items[i[k]] + " " ; g.drawstring(msg, 10, 140); a) What is the output from the g.drawstring statement? b) What is the value of i.length? c) What is the value of i[0]? d) What is the value of i[1]? Part C Consider the following program and answer the following questions: import java.awt.*; import java.applet.applet; import java.awt.event.*; public class Applet1 extends Applet { TextField number; Button click; String s = " "; //don't want s to be null when using g.drawstring() function int i = 0;

9 public void init() { int i = 10; number = new TextField(20); add(number); number.addactionlistener(new Responder1()); click = new Button("Click on me"); add(click); click.addactionlistener(new Responder2()); class Responder1 implements ActionListener { s = number.gettext(); i++; repaint(); class Responder2 implements ActionListener { s = number.gettext(); i++; repaint(); public void paint(graphics g) { int i = 5; g.drawstring(s, 10, 80); g.drawstring("i = " + this.i, 10, 100); g.drawstring("i = " + i, 10, 120); 1. Say we type in the name "Hortense" in the text field and hit the tab key. What is the output from the g.drawstring() statements? 2. Say we type in the name "Hortense" in the text field and click on the button. What is the output from the g.drawstring() statements after we do this? 3. Say we type in the name "Hortense" in the text field, press the return key, and then click on the button. What is the output from the g.drawstring() statements after we do this?

10 Part D What is the output from the following program? class Problem extends Exception { double x; Problem(double y) { System.out.println("Goofy"); x = y; public String tostring(){ return x + " is too high"; public class Class1 { static void function(double z) throws Problem { System.out.println(z); if (z < 6) throw new Problem(z); System.out.println(z*z); public static void main(string[] args) { try { for (int i = 7; i >=0; i--) { function(i); System.out.println(i + " Yea"); catch (Problem e) { System.out.println(e); System.out.println("The end");