If a multi threaded Java program has started numerous number
of threads, at any point in time how to know which thread is
currently executing/running ?
what is the purpose of method overriding in java where v r
completely re-defining a inherited method instead y can't v
create a new method and define.If the question is very silly
plz excuse me and do reply.Thank U!
What is the need to implement Serializable interface (with
no methods) for objects which are to be serialized ?
We can write our own functionality which writes objects to
streams then why we need to implement and tell JVM that
which objects can be serialized.
Can we access private data outside of the class directly in
java programming language? Why There is no runtime checking
in java, which leads to access the private data directly
outside of a class?
Question 7 [8]
Consider the following class and answer the questions below
it:
public class StackWithGuard extends Stack {
public StackWithGuard(int size) {
super(size);
}
synchronized public boolean isEmpty() {
return super.isEmpty();
}
synchronized public boolean isFull() {
return super.isFull();
}
synchronized public int getSize() {
return super.getSize();
}
synchronized public void push(Object obj) {
try {
while (isFull()) {
wait();
}
} catch (InterruptedException e) {}
super.push(obj);
COS2144/102
11
notify();
}
synchronized public Object pop() {
try {
while (isEmpty()) {
wait();
}
} catch (InterruptedException e) {}
Object result = super.pop();
notify();
return result;
}
public static void main(String args[]) {
StackWithGuard stack = new StackWithGuard(5);
new Producer(stack, 15).start();
new Consumer(stack, 15).start();
}
}
Note: The Stack class is provided in the Appendix.
Note also: The following questions all refer to the pop()
method of the StackWithGuard
class given above.
7.1 What does the synchronized keyword ensure for this
method? (2)
7.2 Why is a while loop used to test whether the stack is
empty? In other words, why
wouldn't the following if statement be sufficient?
if (isEmpty()) {
wait();
}
(2)
7.3 Why is the result of popping (provided by the inherited
pop() method) stored in a
temporary variable? In other words, why wouldn't the
following statement be
sufficient?
return super.pop();
(2)
7.4 Why is the while loop placed in a try-catch structure?
(2)
Appendix
The LinkedQueue class:
public class LinkedQueue implements Queue {
private Node first, last;
private int count;
public LinkedQueue() {
first = last = null;
count =0;
}
public int size() {
return count;
}
public boolean isEmpty() {
return (count == 0);
12
}
public void enqueue(Object o) {
Node node = new Node();
node.element = o;
node.next = null;
node.prev = last;
if (last != null){
last.next = node;
}
else {
last = first = node;
}
last = node;
count++;
}
public void dequeue() {
if ((first!= null) & (first.next!=null)) {
first = first.next;
first.prev = null;
count--;
}
else {
first = last = null;
count--;
}
}
public Object front() {
return first;
}
}
class Node {
Object element;
Node next, prev;
}
The Stack class:
public class Stack {
protected Object rep[];
protected int top = -1;
protected int size = 0;
protected int count = 0;
public Stack(int size) {
if (size > 0) {
this.size = size;
rep = new Object[size];
}
}
public boolean isFull() {
return (count == size);
}
public boolean isEmpty() {
return (count == 0);
}
public int getSize() {
return size;
}
public void push(Object e) {
if (e != null && !isFull()) {
COS2144/102
13
top++;
rep[top] = e;
count ++;
}
}
public Object pop() {
Object result = null;
if (!isEmpty()) {
result = rep[top];
top--;
count--;
}
return result;
}
}
28
What are the names of Component subclasses that support
painting?
167
what is difference between business delegate and data access
object(dao)? in java?
179
what is meant by multicast?
146
Explain the difference between Unicast and Multicast
objects?
194
Hi friends am new to java. I read jar file means collection
of java files. For executing struts application what are
the necessary jar files. " struts.jar " file contains
what. can u explain
What are advantages of using Java?s layout managers than
windowing systems?
227
The following program reads data (details of students) from
a file named students.txt and
converts it into e-mail addresses. The results are written
to a file named studentemail.txt.
students.txt consists of a number of lines, each containing
the data of a student in colon
delimited format:
Last Name:First Name:Student Number
Each input record is converted to an e-mail address and
written to studentemail.txt in the
following format:
the first character of the last name + the first character
of the first name + the last four digits of
the student number + “@myunisa.ac.za”
import java.io.*;
public class EmailConverter {
public static void main(String[] args) throws IOException {
BufferedReader input = new BufferedReader(new FileReader
("students.txt"));
PrintWriter output = new PrintWriter(new FileWriter
("studentemail.txt"));
String line = input.readLine();
while (line != null) {
// Extract the information for each student
String[] items = line.split(":");
// Generate the email address
String email = "" + items[0].charAt(0) + items[1].charAt(0)
+
items[2].substring(4,8) + "@myunisa.ac.za";
email = email.toLowerCase();
// Output
output.println(email);
line = input.readLine();
}
input.close();
output.close();
}
}
Rewrite the class so that it handles possible errors that
may occur. In particular, it should do the
following:
• It should catch at least three appropriate exceptions
that might occur, and display suitable
messages.
• At this stage, the program will not run correctly if
there is an empty line in the input file.
Change the program so that if an empty line is encountered,
an exception is thrown and the
empty line is ignored. This exception should be handled
with the display of a suitable error
message.
• Before the e-mail address is added to the output file,
check if the student number has 8
digits. If not, throw an InvalidFormatException (which the
program should not handle itself)
21
Question 1 [8]
Draw a UML class diagram for the code fragment given below:
public class StringApplet extends Applet {
private Label sampleString;
private Button showTheString;
private ButtonHandler bHandler;
private FlowLayout layout;
public StringApplet() {
sampleString = new Label(" ");
showTheString = new Button (" Show the String");
bHandler = new ButtonHandler();
layout = new FlowLayout();
showTheString.addActionListener(bHandler);
setLayout(layout);
add(sampleString);
add(showTheString);
}
class ButtonHandler implements ActionListener {
public void actionPerformed(ActionEvent e) {
samplestring.setText("Good Morning");
}
}
}
Note: The methods need not be indicated on the diagram.
6
Question 2 [10]
The following program reads data (details of students) from
a file named students.txt and
converts it into e-mail addresses. The results are written
to a file named studentemail.txt.
students.txt consists of a number of lines, each containing
the data of a student in colon
delimited format:
Last Name:First Name:Student Number
Each input record is converted to an e-mail address and
written to studentemail.txt in the
following format:
the first character of the last name + the first character
of the first name + the last four digits of
the student number + “@myunisa.ac.za”
import java.io.*;
public class EmailConverter {
public static void main(String[] args) throws IOException {
BufferedReader input = new BufferedReader(new FileReader
("students.txt"));
PrintWriter output = new PrintWriter(new FileWriter
("studentemail.txt"));
String line = input.readLine();
while (line != null) {
// Extract the information for each student
String[] items = line.split(":");
// Generate the email address
String email = "" + items[0].charAt(0) + items[1].charAt(0)
+
items[2].substring(4,8) + "@myunisa.ac.za";
email = email.toLowerCase();
// Output
output.println(email);
line = input.readLine();
}
input.close();
output.close();
}
}
Rewrite the class so that it handles possible errors that
may occur. In particular, it should do the
following:
• It should catch at least three appropriate exceptions
that might occur, and display suitable
messages.
• At this stage, the program will not run correctly if
there is an empty line in the input file.
Change the program so that if an empty line is encountered,
an exception is thrown and the
empty line is ignored. This exception should be handled
with the display of a suitable error
message.
• Before the e-mail address is added to the output file,
check if the student number has 8
digits. If not, throw an InvalidFormatException (which the
program should not handle itself).
COS2144/102
7
Question 3 [12]
3.1 Say you want to store the information about a number of
pets in an array. Typical
information that you could store for each pet (where
relevant) would be
• Breed of animal
• Animal's name
• Its birth date
• Its sex
• Whether it has been sterilised or not
• When it is due for its next inoculation
• When it last had its wings clipped
For each type of pet (eg. dog, cat or bird) you would
typically define a class to hold
the relevant data.
Note: You do not need to implement these classes. Just
answer the following
questions.
3.1.1 What would be the advantage of creating a superclass
(eg. Pet) and
declaring an array of Pet objects over simply using an
array of Objects,
storing each of the instances of the different pet classes
(eg. Dog, Cat or
Bird) in it?
3.1.2 Would you define Pet as a class or as an interface?
Why?
(2)
(2)
3.2 Consider the following class:
public class Point {
protected int x, y;
public Point(int xx, int yy) {
x = xx;
y = yy;
}
public Point() {
this(0, 0);
}
public int getx() {
return x;
}
public int gety() {
return y;
}
public String toString() {
return "("+x+", "+y+")";
}
}
Say you wanted to define a rectangle class that stored its
top left corner and its
height and width as fields.
3.2.1 Why would it be wrong to make Rectangle inherit from
Point (where in fact it
would inherit the x and y coordinates for its top left
corner and you could just
add the height and width as additional fields)?
(1)
8
Now consider the following skeleton of the Rectangle class:
public class Rectangle {
private Point topLeft;
private int height, width;
public Rectangle(Point tl, int h, int w) {
topLeft = tl;
height = h;
width = w;
}
public Rectangle() {
this(new Point(), 0, 0);
}
// methods come here
}
3.2.2 Explain the no-argument constructor of the Rectangle
class given above.
3.2.3 Write methods for the Rectangle class to do the
following:
• a toString() method that returns a string of the format
"top left = (x, y); height = h; width = w "
where x, y, h and w are the appropriate integer values.
• an above() method that tests whether one rectangle is
completely
above another (i.e. all y values of the one rectangle are
greater than
all y values of the other).
For example, with the following declarations
Rectangle r1 = new Rectangle();
Rectangle r2 = new Rectangle(new Point(2,2), 1, 4);
the expression r2.above(r1) should give true, and r2.above
(r2)
should give false. (You can assume that the height of a
rectangle is
never negative.)
(2)
(5)
Question 4 [8]
4.1 Supply contracts (in the form of comments specifying
pre- and post conditions)
for the enqueue() method of the LinkedQueue class given in
the Appendix.
(2)
4.2 Let Thing be a class which is capable of cloning
objects, and consider the code
fragment:
Thing thing1 = new Thing(); //(1)
Thing thing2 = thing1; //(2)
Thing thing3 = (Thing) thing1.clone(); //(3)
Explain how the objects thing2 and thing3 differ from each
other after
execution of the statements.
(4)
COS2144/102
9
Question 5 [15]
Consider the following classes, illustrating the Strategy
design pattern:
import java.awt.*;
abstract class Text {
protected TextApplet tA;
protected Text(TextApplet tApplet) {
tA = tApplet;
}
abstract public void draw(Graphics g);
}
class PlainText extends Text {
protected PlainText(TextApplet tApplet) {
super(tApplet);
}
public void draw(Graphics g) {
g.setColor(tA.getColor());
g.setFont(new Font("Sans-serif", Font.PLAIN, 12));
g.drawString(tA.getText(), 20, 20);
}
}
class CodeText extends Text {
protected CodeText(TextApplet tApplet) {
super(tApplet);
}
public void draw(Graphics g) {
g.setColor(tA.getColor());
g.setFont(new Font("Monospaced", Font.PLAIN, 12));
g.drawString(tA.getText(), 20, 20);
}
}
public class TextApplet extends java.applet.Applet {
protected Text text;
protected String textVal;
protected Color color;
public String getText() {
return textVal;
}
public Color getColor() {
return color;
}
public void init() {
textVal = getParameter("text");
String textStyle = getParameter("style");
String textColor = getParameter("color");
if (textStyle == "code")
text = new CodeText(this);
else
text = new PlainText(this);
if (textColor == "red")
color = Color.RED;
else if (textColor == "blue")
color = Color.BLUE;
else
color = Color.BLACK;
}
public void paint(Graphics g) {
text.draw(g);
10
}
}
The Text class is more complicated than it should be (there
is too much coupling between the
Text and TextApplet classes). By getting rid of the
reference to a TextApplet object in the
Text class and setting the colour in the paint() method,
one could turn the Text class into an
interface and simplify the strategy classes considerably.
5.1 Rewrite the Text and PlainText classes to do what is
described above. (6)
5.2 Explain the consequent changes that are necessary to
the TextApplet class. (4)
5.3 Write an additional strategy class called FancyText (to
go with your simplified
strategy classes) to allow fancy text to be displayed for
the value "fancy" provided
for the style parameter. It should use the font Font
("Serif", Font.ITALIC, 12).
(3)
5.4 Explain what changes are necessary to the TextApplet
class for this. (2)
Question 6 [9]
6.1 In what situations (in general) would you use a
TreeMap? (3)
6.2 In what situations (in general) would you use a HashSet
to store a collection of
values?
(3)
6.3 Name three software design patterns (besides the
Strategy pattern) that are
covered in the syllabus of COS2144.
(3)
Question 7 [8]
Consider the following class and answer the questions below
it:
public class StackWithGuard extends Stack {
public StackWithGuard(int size) {
super(size);
}
synchronized public boolean isEmpty() {
return super.isEmpty();
}
synchronized public boolean isFull() {
return super.isFull();
}
synchronized public int getSize() {
return super.getSize();
}
synchronized public void push(Object obj) {
try {
while (isFull()) {
wait();
}
} catch (InterruptedException e) {}
super.push(obj);
COS2144/102
11
notify();
}
synchronized public Object pop() {
try {
while (isEmpty()) {
wait();
}
} catch (InterruptedException e) {}
Object result = super.pop();
notify();
return result;
}
public static void main(String args[]) {
StackWithGuard stack = new StackWithGuard(5);
new Producer(stack, 15).start();
new Consumer(stack, 15).start();
}
}
Note: The Stack class is provided in the Appendix.
Note also: The following questions all refer to the pop()
method of the StackWithGuard
class given above.
7.1 What does the synchronized keyword ensure for this
method? (2)
7.2 Why is a while loop used to test whether the stack is
empty? In other words, why
wouldn't the following if statement be sufficient?
if (isEmpty()) {
wait();
}
(2)
7.3 Why is the result of popping (provided by the inherited
pop() method) stored in a
temporary variable? In other words, why wouldn't the
following statement be
sufficient?
return super.pop();
(2)
7.4 Why is the while loop placed in a try-catch structure?
(2)
Appendix
The LinkedQueue class:
public class LinkedQueue implements Queue {
private Node first, last;
private int count;
public LinkedQueue() {
first = last = null;
count =0;
}
public int size() {
return count;
}
public boolean isEmpty() {
return (count == 0);
12
}
public void enqueue(Object o) {
Node node = new Node();
node.element = o;
node.next = null;
node.prev = last;
if (last != null){
last.next = node;
}
else {
last = first = node;
}
last = node;
count++;
}
public void dequeue() {
if ((first!= null) & (first.next!=null)) {
first = first.next;
first.prev = null;
count--;
}
else {
first = last = null;
count--;
}
}
public Object front() {
return first;
}
}
class Node {
Object element;
Node next, prev;
}
The Stack class:
public class Stack {
protected Object rep[];
protected int top = -1;
protected int size = 0;
protected int count = 0;
public Stack(int size) {
if (size > 0) {
this.size = size;
rep = new Object[size];
}
}
public boolean isFull() {
return (count == size);
}
public boolean isEmpty() {
return (count == 0);
}
public int getSize() {
return size;
}
public void push(Object e) {
if (e != null && !isFull()) {
COS2144/102
13
top++;
rep[top] = e;
count ++;
}
}
public Object pop() {
Object result = null;
if (!isEmpty()) {
result = rep[top];
top--;
count--;
}
return result;
}
}
110
What is clipping and repainting and what is the relation
between them?
430
what is difference betweem home interface and remote interface?
can any one tell me what is advantage of encapsulation
115
What is Session reduplication and how its done?
70
Hai friends I am MCA 2006 passout. Spend time in 1 yr for
teacing and 5 months in small s/w firm. worked in 4 or 5
months in few places. Presently I am writing bank exams. I
want to get back to IT sector. Are there any possibilities.
any growth prospects?. I am having some knowledge in core
and advanced java. I am so desperate. Any suggestions thanks
in advance. u can write to my mail mahidestiny@gmail.com
40
How to know the host from which Applet is originated?
156
What is inner class?what is the use of inner class?where we
create the object for inner class? and inner class can
extend any class or inner class can implement any interface?
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