Deck 10: Thinking in Objects

Question 1

When you implement a method that is defined in a superclass, you __________ the original method.

A) overload
B) override
C) copy
D) call

Answer 1

B

Question 2

Design and create GUI applications
Write a Java applet to add two numbers from text fields, and displays the result in a non-editable text field. Enable your applet to run standalone with a main method. A sample run of the applet is shown in the following figure.

Answer 2

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Test extends JApplet {
// Text fields for Number 1, Number 2, and Result
private JTextField jtfNum1, jtfNum2, jtfResult;
// Buttons "Add"
private JButton jbtAdd;
// Main method
public static void main(String[] args) {
// Create a frame
JFrame frame = new JFrame("Test");
// Create an instance of the applet
Test applet = new Test();
// Add the applet instance to the frame
frame.add(applet, BorderLayout.CENTER);
// Invoke init() and start()
applet.init();
applet.start();
// Display the frame
frame.pack();
frame.setLocationRelativeTo(null); // Center the frame
frame.setVisible(true);
}
// Default Constructor
public Test() {
// Panel p1 to hold text fields and labels
JPanel p1 = new JPanel();
p1.setLayout(new FlowLayout());
p1.add(new JLabel("Number 1"));
p1.add(jtfNum1 = new JTextField(3));
p1.add(new JLabel("Number 2"));
p1.add(jtfNum2 = new JTextField(3));
p1.add(new JLabel("Result"));
p1.add(jtfResult = new JTextField(8));
jtfResult.setEditable(false);
jtfResult.setHorizontalAlignment(SwingConstants.RIGHT);
// Panel p2 to hold buttons
JPanel p2 = new JPanel();
p2.setLayout(new FlowLayout());
p2.add(jbtAdd = new JButton("Add"));
// Set mnemonic keys
jbtAdd.setMnemonic('A');
// Add panels to the frame
setLayout(new BorderLayout());
add(p1, BorderLayout.CENTER);
add(p2, BorderLayout.SOUTH);
// Register listeners
jbtAdd.addActionListener(new Listener());
}
class Listener implements ActionListener {
// Handle ActionEvent from buttons
public void actionPerformed(ActionEvent e) {
// Obtain Number 1 and Number 2
int num1 = Integer.parseInt(jtfNum1.getText().trim());
int num2 = Integer.parseInt(jtfNum2.getText().trim());
int result = num1 + num2;
// Set result in jtfResult
jtfResult.setText(String.valueOf(result));
}
}
}

Question 3

Show the output of running the class Test in the following code lines: interface A {
Void print();
}
Class C {}
Class B extends C implements A {
Public void print() { }
}
Class Test {
Public static void main(String[] args) {
B b = new B();
If (b instanceof A)
System.out.println("b is an instance of A");
If (b instanceof C)
System.out.println("b is an instance of C");
}
}

A) Nothing.
B) b is an instance of A.
C) b is an instance of C.
D) b is an instance of A followed by b is an instance of C.

Answer 3

D

Question 4

_________ describes the state of an object.

A) data fields
B) methods
C) constructors
D) none of the above

Question 5

Design and use interfaces
Write a class named Octagon that extends GeometricObject and implements the Comparable and Cloneable interfaces. Assume that all eight sides of the octagon are of equal size. The area can be computed using the following formula: Draw the UML diagram that involves Octagon, GeometricObject, Comparable, and Cloneable.

Question 6

Suppose s is a string with the value "java". What will be assigned to x if you execute the following code? char x = s.charAt(4);

A) 'a'
B) 'v'
C) Nothing will be assigned to x, because the execution causes the runtime error StringIndexOutofBoundsException.
D) None of the above.

Question 7

How can you get the word "abc" in the main method from the following call? java Test "+" 3 "abc" 2

A) args[0]
B) args[1]
C) args[2]
D) args[3]

Question 8

If a class named Student has no constructors defined explicitly, the following constructor is implicitly provided.

A) public Student()
B) protected Student()
C) private Student()
D) Student()

Question 9

What would be the result of attempting to compile and run the following code? public class Test {
Static int x;
Public static void main(String[] args){
System.out.println("Value is " + x);
}
}

A) The output "Value is 0" is printed.
B) An "illegal array declaration syntax" compiler error occurs.
C) A "possible reference before assignment" compiler error occurs.
D) A runtime error occurs, because x is not initialized.

Question 10

Suppose the xMethod() is invoked in the following constructor in a class, xMethod() is _________ in the class. public MyClass() {
XMethod();
}

A) a static method
B) an instance method
C) a static method or an instance method

Question 11

What is the printout for the following code? class Test {
Public static void main(String[] args) {
Int[] x = new int[3];
System.out.println("x[0] is "+x[0]);
}
}

A) The program has a syntax error because the size of the array wasn't specified when declaring the array.
B) The program has a runtime error because the array elements are not initialized.
C) The program runs fine and displays x[0] is 0.
D) None of the above.

Question 12

Text I/O
Write a program that will count the number of characters (excluding control characters '\r' and '\n'), words, and lines, in a file. Words are separated by spaces, tabs, carriage return, or line-feed characters. The file name should be passed as a command-line argument, as shown in the following sample run.

Question 13

An attribute that is shared by all objects of the class is coded using ________.

A) an instance variable
B) a static variable
C) an instance method
D) a static method

Question 14

If a class named Student has a constructor Student(String name) defined explicitly, the following constructor is implicitly provided.

A) public Student()
B) protected Student()
C) private Student()
D) Student()
E) None

Question 15

Analyze the following code: public class Test {
Private int t;
Public static void main(String[] args) {
Test test = new Test();
System.out.println(test.t);
}
}

A) The variable t is not initialized and therefore causes errors.
B) The variable t is private and therefore cannot be accessed in the main method.
C) Since t is an instance variable, it cannot appear in the static main method.
D) The program compiles and runs fine.

Question 16

Suppose the xMethod() is invoked from a main method in a class as follows, xMethod() is _________ in the class. public static void main(String[] args) {
XMethod();
}

A) a static method
B) an instance method
C) a static or an instance method

Question 17

Write a program that converts US dollars into Canadian dollars, as shown in the following figure. The program let the user enter an amount in US dollars and display it equivalent value in Canadian dollars when clicking the Convert button. One dollar is 1.5 Canadian dollars.

Question 18

Design and implement classes.
Design a class named Person and its two subclasses named Student and Employee. Make Faculty and Staff subclasses of Employee. A person has a name, address, phone number, and email address. A student has a class status (freshman, sophomore, junior, or senior). Define the status as a constant. An employee has an office, salary, and date hired. Define a class named MyDate that contains the fields year, month, and day. A faculty member has office hours and a rank. A staff member has a title. Override the toString method in each class to display the class name and the person's name.
Draw the UML diagram for the classes. Write the code for the Student class only.

Question 19

What modifier should you use on a variable so that it can only be referenced inside its defining class.

A) public
B) private
C) protected
D) Use the default modifier.

Question 20

Which code fragment would correctly identify the number of arguments passed via the command line to a Java application, excluding the name of the class that is being invoked?

A) int count = args.length;
B) int count = args.length - 1;
C) int count = 0; while (args[count] != null) count ++;
D) int count=0; while (!(args[count].equals(""))) count ++;

Question 21

Analyze the following program. class Test {
Public static void main(String[] args) {
Try {
String s = "5.6";
Integer.parseInt(s); // Cause a NumberFormatException
Int i = 0;
Int y = 2 / i;
System.out.println("Welcome to Java");
}
Catch (Exception ex) {
System.out.println(ex);
}
}
}

A) An exception is raised due to Integer.parseInt(s);
B) An exception is raised due to 2 / i;
C) The program has a compilation error.
D) The program compiles and runs without exceptions.

Question 22

To append data to an existing file, use _____________ to construct a FileOutputStream for file out.dat.

A) new FileOutputStream("out.dat")
B) new FileOutputStream("out.dat", false)
C) new FileOutputStream("out.dat", true)
D) new FileOutputStream(true, "out.dat")

Question 23

What is displayed on the console when running the following program? class Test {
Public static void main(String[] args) {
Try {
System.out.println("Welcome to Java");
Int i = 0;
Int y = 2/i;
System.out.println("Welcome to Java");
}
Catch (RuntimeException ex) {
System.out.println("Welcome to Java");
}
Finally {
System.out.println("End of the block");
}
}
}

A) The program displays Welcome to Java three times followed by End of the block.
B) The program displays Welcome to Java two times followed by End of the block.
C) The program displays Welcome to Java three times.
D) The program displays Welcome to Java two times.

Question 24

After the following program is finished, how many bytes are written to the file t.dat? import java.io.*;
Public class Test {
Public static void main(String[] args) throws IOException {
DataOutputStream output = new DataOutputStream(
New FileOutputStream("t.dat"));
Output.writeShort(1234);
Output.writeShort(5678);
Output.close();
}
}

A) 2 bytes.
B) 4 bytes.
C) 8 bytes.
D) 16 bytes.

Question 25

What is the output of running class C?
class A {
public A() {
System.out.println(
"The default constructor of A is invoked");
}
}
class B extends A {
public B() {
System.out.println(
"The default constructor of B is invoked");
}
}
public class C {
public static void main(String[] args) {
B b = new B();
}
}
a. none
b. "The default constructor of B is invoked"
c. "The default constructor of A is invoked" followed by "The default constructor of B is invoked"
d. "The default constructor of A is invoked"