Answer / ankur

in java we write public static void main(String args[])
instead of just main because:....

public.....The main method must be public because it is
call from outside the class. It is called by jvm.

static....The main method must be static because without
creating an instace jvm can call it. If the main method is
non static then it is must to create an instance of the
class.

void....Main method doesn't return anything therefore it is
void also.

(String args[])....It is used for receiving any arbitirary
number of arguments and save it in the array.

Answer / ravi

static because once the class is loaded the main() will be
invoked first.
void because main()is not returning any value.

Answer / sumalatha

yes in every progrm in jave the main is static.because when
the program will start ,thit means runnig then the main is
also one method it was called first that means without
instans creations the main is called.thet is the reason we
are put as main is static method(static is used for without
instance creat u call it).void means it does not reaturn
any value

Answer / sanjay gupta

public static void main()

1> Main() is the entry point of the program. Rather we can
say it is the main gate to enter inside the apartment. It
should be public because the compiler need to enter inside
the method (The guest need to access the apartment to reach
the security guard).

Public--> Compile whos is from another instance need to
access this method.

Statis --> Since main method is written inside a class and
to access a method of a class we need to create an object
of that class first. Since main() is the method which
compiler nned to call before creating any object of that
class we need to declare main as static. Static methods can
be called directely by using the class name. Thats why the
neame of the file should be same as the name of the class
contain main() method.

Void--> Since the method can return any type of valu or it
might not return anything also so the comiler is designed
in such a way that it should not take any return value so
we declare main as void type.

For further information please reach me at
sanjaygupt1011@gmail.com

Thanks~
Happy Programming...

Answer / anjani kumar jha

Use of Public-----There is always only one main class that
is one file having only one main file. Since that main file
is used anywhere means might be in another package so there
is a public use of keyword that is public keyword is
required for main function

Static:-----Since compiler in java always look the main
function and when compiler always calls the main so main
class is loaded in first time call for that static keyword
is used.
you can use
1)public void main(String args[])
it compiles file but wont run\

2) void main(String args[])
it compiles file but wont run

3)void main(String args[])
it compiles file but wont run

Answer / manikandan

in java , the entry point for execution is Main(),

it is tha first method to be invoked .

so PUBLIC is declared for no duplicates should occur and for
global declaration .

VOID , the main method will not return any value . So void
is declared .

Finally STATIC , in java everything is accssed through
objects . But here main() is the first method to be invoked
. STATIC is defined for this purpose , If a method is
declared as STATIC it can called without any objects
reference .

So the Main() in java is defined as

public static void main(String args[])

Answer / angela

when we run java program, jvm internally calls "main" method.
jvm is built in such a way that it searches for the entire
signature of "main" method i.e public static void
main(String a[])

Example program :
----------------------
class Demo
{
public static void main(String a[])
{ }
}

String a[] -> is used to pass values at runtime.
(c:/>java Demo hai)

public -> providing accessibility for outside code (jvm).

static -> without creating an object calling "main" method
with the class name

c:/>javac Demo.java
c:/>java Demo
(here java internally calls jvm and passes class name
Demo, then jvm loads Demo class and calls Demo.main()
without creating object )

void -> does not return any value because jvm simply calls
main() method and does not have a variable to assign return
value.

Answer / prabhakant prabhakar rasal

We write public because to access main() method outside the
class.
Why static because to invoke main() method before creating
any object, so it simple to compiler that to identify the
main() method.
void, it represents that it does not returns any value.
string args[]
There are two main reasons to write string args[]
1. TO identify the program
2. To get command_line arguments

Answer / sivadasan

1. public - Access Specifier, properties and methods can
access anywhere, any package.

2. main() - The Java environment starts the execution of a
program from main() method.

3. void - The main() method is not return anything. So the
return type of the main() method must be void.

4. static - The jave environment able to call the main()
method without creating an instance of the class. So its
declared as static

5...(String args[]) -- In java, whatever we give, it will
takes as String. So we are giving the command line argumet
as String.


i hope it will help u..

Shivadasan
Coromandel Infotech

Answer / suresh.t.c

public static void main(String arg[])

public for accessing all object

static for to call main method without instance

void main for not returning values for avoid compilation errors

String arg[] is ,in input takes as strings so takes multiple inputs declared String with array of arg[]

What are the two environment variables that must be set in order to run any java programs?

0 Answers  

what are the uses of Class class and what it returns? explain it with the example code.

1 Answers  

What is meant by class loader and how many types are there?

2 Answers   Apple,

What do you mean by synchronized non access modifier?

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Why does java have two ways to create child threads?

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List some oops concepts in java?

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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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Explain creating threads by implementing runnable class?

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Differentiate between overriding and overloading cases?

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What are the types of casting?

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Why do people says “java is robust”?

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why doesn't java run on all platforms?

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