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()) {
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13
top++;
rep[top] = e;
count ++;
}
}
public Object pop() {
Object result = null;
if (!isEmpty()) {
result = rep[top];
top--;
count--;
}
return result;
}
}
27
how i secure my site with the https protocol.what are the steps?
96
what is meant by JRMP?
191
For ease of programming you can consider the maze as a 2D
array with colors represented by below integer and
characters (in capital letters).
• B - Black
• W -White
• G- Green
• R- Red
R B W B W W W W
W W
B W B B W W W W
W W
W B W B W W W B
W W
W W B B W W W B
W W
W B W W B B B B
W B
W B W W B W W W
B W
W W B B B W W B
W W
W B W W B W B W
W W
B W B W W W W B
B W
W W W B W W W W
W G
Shortest Route Problem:
• Solution that finds the shortest Route between Red
and Green
 White will have 1 Weight.
 Red and Green carry no weights.
 Shortest path is the path with less weight when you
add up the weights in the path.
we use MainFrame and using os390 for operating system with
DB2 data base in IRAN and interest programing with java and
use webspere for world wide,please help me where i should
start?
how to make a index.jsp for running the site in internet
and find an error for connection with weblogic server and
java that give an error invalid object name.and how to
maintain session.
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
21
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)
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