Answer / subrahmanyam

transient
identifies a variable not to be written out when an
instance is serialized (It can'nt be copied to remove
system)

volatile
indicates that the field is used by synchronized threads
and that the compiler should not attempt to perform
optimizations with it.

When more than one thread share a (volatile) data it is
checked every time. Every thread keeps the latest value of
volatile variable

Answer / jagan kumar(zylog systems ltd.

Transient
.........
Only variable can be declared as Transient, When
used as a Modifier in a variable Declaration it suggests
that a variable may not be return out when the class is
serialized

Volatile
.........
The keyword volatile can be used to declare
variables. The use of the keyword volatile in a variable
declaration suggests the compiler that multiple threads may
access the variable. Therefore the value of the variable
may change unexpectedly. A Compile time error will occur
declaring a variable both volatile and final.

Example for Volatile:
.....................

volatile int v = 0;
Thread 1:
v++;
Thread 2:
v--;

The questioners usually want the answer "v can only
be 0 after this code is run", because

volatile int v = 0;
Thread 1:
r1 = v;
r2 = r1 + 1;
v = r2;
Thread 2:
r3 = v;
r4 = r3 - 1;
v = r4;
So, if Threads 1 and 2 both read v and see the value
0, then Thread 1 will write 1 to it and Thread 2 will
write -1 to it. You are not guaranteed to see the value 0!

Answer / sudhadevi

Transient
~~~~~~~~~
identifies a variable not to be written out when
an instance is serialized (It can'nt be copied to remove
system)

Volatile
~~~~~~~~
indicates that the field is used by synchronized
threads and that the compiler should not attempt to perform
optimizations with it.When more than one thread share a
(volatile) data it is checked every time. Every thread
keeps the latest value of volatile variable

Answer / lakshmidontukurthy

Trying to put a non-serializable variable in a
seriealisible class,we can use transient modifier ,so that
the jvm skips the transient variable ,and make that class
as serializable class

Answer / manodeep pandey

Java defines two interesting type modifiers: transient and
volatile. These modifiers are used to handle somewhat
specialized situations.

When an instance variable is declared as transient, then
its value need not persist when an object is stored. For
example:

class T {

transient int a; // will not persist

int b; // will persist

}

Here, if an object of type T is written to a persistent
storage area, the contents of a would not be saved, but the
contents of b would. The volatile modifier tells the
compiler that the variable modified by volatile can be
changed unexpectedly by other parts of your program. One of
these situations involves multithreaded programs. In a
multithreaded program, sometimes, two or more threads share
the same instance variable. For efficiency considerations,
each thread can keep its own, private copy of such a shared
variable. The real (or master) copy of the variable is
updated at various times, such as when a synchronized
method is entered. While this approach works fine, it may
be inefficient at times. In some cases, all that really
matters is that the master copy of a variable always
reflects its current state. To ensure this, simply specify
the variable as volatile, which tells the compiler that it
must always use the master copy of a volatile variable (or,
at least, always keep any private copies up to date with
the master copy, and vice versa). Also, accesses to the
master variable must be executed in the precise order in
which they are executed on any private copy.

Answer / deepak divvela

1.transient variables can not be serialized.
2.volatile variables may change with the affect of other one

Answer / pasindu

Trsnsient:
If you mark an instance variable as transient you're telling
the JVM to skip this variable when you attempt to serialize
the object containing it.

Volatile:
The volatile modifier tells the JVM that a thread accessing
the variable must always reconcile its own private copy of
the variable with the master copy in memory.
It can be apply only to instance variables.

Answer / soumay m cyriac

we cant write the value of a transient variable into
datastream.volatile means its just like temporary variable

Answer / ravikiran

Transient variable is used to make the variable out of
synchronization.
Volatile variable is used when the variable is changing
it's state continuously

Answer / loganathan s

both are used for declaring a vaiables.we can say it is a
modifiers.

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; } }

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