IT Assignments paper for IT Students and readers

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OOPS with Java
BT0074 Part 2

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1.What is bytecode? Explain.
Java is both interpreted and compiled. The code is complied to
a bytecode that is binary and platform independent. When the
program has to be executed, the code is fetched into the memory
and interpreted on the user’s machine. As an interpreted language,
Java has simple syntax.
When you compile a piece of code, all errors are listed together. You
can execute only when all the errors are rectified. An interpreter, on
the other hand, verifies the code and executes it line by line. Only
when the execution reaches the statement with error, the error is
reported. This makes it easy for a programmer to debug the code.
The drawback is that this takes more time than compilation.
Compilation is the process of converting the code that you
type, into a language that the computer understands – machine
language. When you compile a program using a compiler, the compiler
checks for syntactic errors in code and list all the errors on the
screen. You have to rectify the errors and recompile the program to
get the machine language code. The Java compiler compiles the code
to a bytecode that is understood by the Java environment.Bytecode is
the result of compiling a Java program. You can execute this code on
any platform. In other words, due to the bytecode compilation
process and interpretation by a browser, Java programs can be
executed on a variety of hardware and operating systems. The only
requirement is that the system should have a Java-enabled Internet
browser.

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The Java interpreter can execute Java code directly on any
machine on which a Java interpreter has been installed.a Java program
can run on any machine that has a Java interpreter. The bytecode
supports connection to multiple databases. Java code is portable.
Therefore, others can use the programs that you write in Java, even
if they have different machines with different operating systems.
Bytecode is a highly optimized set of
instructions designed to be executed by the Java run-time system,
which is called the Java Virtual Machine (JVM). That is, in its
standard form, the JVM is an interpreter for bytecode. This may
come as a bit of surprise.Translating a Java program into bytecode
helps it to run much easier in a wide variety of environments. The
reason is straightforward: only the JVM needs to be implemented for
each platform. Once the run-time package exists for a given system,
any Java program can run on it. Remember, although the details of
the JVM will differ from platform to platform, all interpret the same
Java bytecode. If a Java program was compiled to native code, then
different versions of the same program should exist for each type of
CPU connected to the Internet. This is, of course, not a feasible
solution. Thus, the interpretation of bytecode is the easiest way to
create truly portable programs.
2. How do you compile a Java program?
The programs that you write in Java should be saved in a
file, which has the following name format:

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<class_name>.java
Compiling
A program is a set of instructions. In order to execute a program,
the operating system needs to understand the language. The only
language an operating system understands is in terms of 0’s and 1’s
i.e. the binary language. Programs written in language such as C and
C++ are converted to binary code during the compilation process.
However, that binary code can be understood only by the operating
system for which the program is compiled. This makes the program or
application as operating system dependent.
In Java, the program is compiled into bytecode (.class file) that run
on the Java Virtual Machine, which can interpret and run the program
on any operating system. This makes Java programs platform-
independent.
At the command prompt, type
javac <filename>.java
to compile the Java program.

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3. What do you mean by operator precedence?
When more than one operator is used in an expression,
Java will use operator precedence rule to determine the order in
which the operators will be evaluated. For example, consider the
following expression:
Result=10+5*8-15/5
In the above expression, multiplication and division operations have
higher priority over the addition and subtraction. Hence they are
performed first. Now, Result = 10+40-3.
Addition and subtraction has the same priority. When the operators
are having the same priority, they are evaluated from left to right in
the order they appear in the expression. Hence the value of the
result will become 47. In general the following priority order is
followed when evaluating an expression:
· Increment and decrement operations.
· Arithmetic operations.
· Comparisons.
· Logical operations.
· Assignment operations.

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To change the order in which expressions are evaluated, parentheses
are placed around the expressions that are to be evaluated first.
When the parentheses are nested together, the expressions in the
innermost parentheses are evaluated first. Parentheses also improve
the readability of the expressions. When the operator precedence is
not clear, parentheses can be used to avoid any confusion.
4. What is an array? Explain with examples.
An array represents a number of variables which occupy
contiguous spaces in the memory. Each element in the array is
distinguished by its index. All elements in an array must be of the
same data type. For example, you cannot have one element with int
data type and another belonging to the boolean data type in the
same array. An array is a collection of elements of the same type
that are referenced by a common name. Each element of an array can
be referred to by an array name and a subscript or index. To create
and use an array in Java, you need to first declare the array and
then initialize it. The syntax for creating an array is:

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data- type [ ] variablename;
Example:
int [ ] numbers;
The above statement will declare a variable that can hold an array of
int type variables. After declaring the variable for the array, the
array needs to be allocated in memory. This can be done using the
new operator in the following way:
numbers = new int [10];
This statement assigns ten contiguous memory locations of the type
int to the variable numbers. The array can store ten elements.
Iteration can be used to access all the elements of the array, one by
one.
5. How will you implement inheritance in Java?

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Inheritance can create a general class that defines traits
common to a set of related items. This class can then be inherited by
other, more specific classes, each adding those things that are unique
to it. In the terminology of Java, a class that is inherited is called a
superclass. The class that does the inheriting is called a subclass.
Therefore, a subclass is a specialized version of a superclass. Java
provides a mechanism for partitioning the class name space into more
manageable chunks. This mechanism is the package. The package is
both a naming and a visibility control mechanism. You can define
classes inside a package that are not accessible by code outside that
package. You can also define class members that are only exposed to
other members of the same package. Using the keyword interface,
Inheritance is one of the
cornerstones of object-oriented programming, because it allows the
creation of hierarchical classifications. Using inheritance, you can
create a general class that defines traits common to a set of related
items. This class can then be inherited by other, more specific classes,
each adding those things that are unique to it. In the terminology of
Java, a class that is inherited is called a superclass. The class that
does the inheriting is called a subclass. Therefore, a subclass is a
specialized version of a superclass. It inherits all of the instance
variables and methods defined by the superclass and add its own,
unique elements.
The extends keyword is used to derive a class from a
superclass, or in other words, extend the functionality of a
superclass.
Syntax
public class <subclass_name> extends <superclass_name>

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Example
public class Confirmed extends Ticket
{
}
Rules for Overriding Methods
· The method name and the order of arguments should be identical
to that of the superclass method.
· The return type of both the methods must be the same.
· The overriding method cannot be less accessible than the method it
overrides. For example, if the method to override is declared as
public in the superclass, you cannot override it with the private
keyword in the subclass.
· An overriding method cannot raise more exceptions than those
raised by the superclass.
6. Explain different kinds of Exceptions in Java.

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The term exception denotes an exceptional event. It
can be defined as an abnormal event that occurs during program
execution and disrupts the normal flow of instruction.
The class at the top of the exception classes hierarchy is Throwable
class. Two classes are derived from the Throwable class – Error and
Exception. The Exception class is used for the exceptional conditions
that has to be trapped in a program. The Error class defines a
condition that does not occur under normal circumstances. In other
words, the Error class is used for catastrophic failures such as
VirtualMachineError
Java has several predefined exceptions. The most common
exceptions that you may encounter are described below.
· Arithmetic Exception
This exception is thrown when an exceptional arithmetic condition has
occurred. For example, a division by zero generates such an
exception.
· NullPointer Exception
This exception is thrown when an application attempts to use null
where an object is required. An object that has not been allocated
memory holds a null value. The situations in which an exception is
thrown include:

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- Using an object without allocating memory for it.
- Calling the methods of a null object.
- Accessing or modifying the attributes of a null object.
· ArrayIndexOutOfBounds Exception
This exception is thrown when an attempt is made to access an array
element beyond the index of the array. For example, if you try to
access the eleventh element of an array that has only ten elements,
the exception will be thrown.
7. What are the uses of stream class?
Stream Classes are classified as FileInputStream,
FileOutputStream, BufferedInputStream, BufferedOutputStream,
DataInputStream, and DataOutputStream classes.
The FileInputStream and FileOutputStream Classes
These streams are classified as mode streams as they read and write
data from disk files. The classes associated with these streams have
constructors that allow you to specify the path of the file to which
they are connected. The FileInputStream class allows you to read
input from a file in the form of a stream. The FileOutputStream
class allows you to write output to a file stream.
Example:
FileInputStream inputfile = new FileInputStream (“Employee.dat”);

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FileOutputStream outputfile = new FileOutputStream (“binus.dat”);
The BufferedInputStream and BufferedOutputStream Classes
The BufferedInputStream class creates and maintains a buffer for an
input stream. This class is used to increase the efficiency of input
operations. This is done by reading data from the stream one byte at
a time. The BufferedOutputStream class creates and maintains a
buffer for the output stream. Both the classes represent filter
streams.
The DataInputStream and DataOutputStream Classes
The DataInputStream and DataOutputStream classes are the filter
streams that allow the reading and writing of Java primitive data
types.
The DataInputStream class provides the capability to read primitive
data types from an input stream. It implements the methods presents
in the DataInput interface.
8. What is AWT? Explain.
The Abstract Windowing Toolkit, also called as AWT is a
set of classes, enabling the user to create a user friendly, Graphical
User Interface (GUI). It will also facilitate receiving user input
from the mouse and keyboard. The AWT classes are part of the

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java.awt package. The user interface consists of the following three:
· Components – Anything that can be put on the user interface. This
includes buttons, check boxes, pop-up menus, text fields, etc.
· Containers – This is a component that can contain other
components.
· Layout Manager – These define how the components will be
arranged in a container.
The statement import java.awt.*; imports all the components,
containers and layout managers necessary for designing the user
interface.
The AWT supplies the following components.
· Labels (java.awt.Label)
· Buttons (java.awt.Button)
· Checkboxes (java.awt.Checkbox)
· Single- line text field (java.awt.TextField)
· Larger text display and editing areas (java.awt.TextArea)
· Pop-up lists of choices (java.awt.Choice)
· Lists (java.awt.List)
· Sliders and scrollbars (java.awt.Scrollbar )
· Drawing areas (java.awt.Canvas)
· Menus (java.awt.Menu, java.awt.MenuItem,
java.awt.CheckboxMenuItem )
· Containers (java.awt.Panel, java.awt.Window and its subclasses)

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9. What are the different components of an event?
An event comprises of three components:
· Event Object – When the user interacts with the application by
pressing a key or clicking a mouse button, an event is generated. The
operating system traps this event and the data associated with it, for
example, the time at which the event occurred, the event type (like a
keypress or a mouseclick). This data is then passed on to the
application to which the event belongs.
In Java, events are represented by objects that describe the events
themselves. Java has a number of classes that describe and handle
different categories of event.
· Event Source – An event source is an object that generates an
event. For example, if you click on a button, an ActionEvent object
is generated. The object of the ActionEvent class contains
information about the event.
· Event-handler – An event-handler is a method that understands
the event and processes it. The event-handler method takes an event
object as a parameter.

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10.Draw and explain the JDBC Application Architecture.
Connection to a Database
The java.sql package contains classes that help in connecting to a

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database, sending SQL statements to the database, and processing
query results.
The Connection Objects
The Connection object represents a connection with a database. You
may have several Connection objects in an application that connects to
one or more databases.
Loading the JDBC-ODBC Bridge and Establishing Connection
To establish a connection with a database, you need to register the
ODBC-JDBC Driver by calling the forName() method from the Class
class and then calling the getConnection() method from the
DriverManager class.
The getConnection() method of the DriverManager class attempts
to locate the driver that can connect to the database represented by
the JDBC URL passed to the getConnection() method.
The JDBC URL
The JDBC URL is a string that provides a way of identifying a
database. A JDBC URL is divided into three parts:
<protocol>:<subprotocol>:<subname>
· <protocol> in a JDBC URL is always jdbc.
· <subprotocol> is the name of the database connectivity mechanism.
If the mechanism of retrieving the data is ODBC-JDBC bridge, the
subprotocol must be odbc.
· <subname> is used to identify the database.
Example: JDBC URL
String url = “jdbc:odbc:MyDataSource”;

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Class.forName (“sun.jdbc.odbc.JdbcOdbcDriver“);
Connection con = DriverManager.getConnection(url);
Using the Statement Object You can use the statement object to
send simple queries to the database as shown in the sample QueryApp
program.
The Statement object allows you to execute simple queries. It has
the following three methods that can be used for the purpose of
querying:
§ The executeQuery() method executes a simple query and returns a
single ResultSet object.
§ The executeUpdate() method executes an SQL INSERT, UPDATE or
DELETE statement.
§ The execute() method executes an SQL statement that may return
multiple results.
The ResultSet Object
The ResultSet object provides you with methods to access data from
the table. Executing a statement usually generates a ResultSet object.
It maintains a cursor pointing to its current row of data. Initially the
cursor is positioned before the first row. The next() method moves
the cursor to the next row. You can access data from the ResultSet
rows by calling the getXXX() method where XXX is the data type.
The following code queries the database and process the ResultSet.
Using the PreparedStatement Object

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You have to develop an application that queries the database
according to the search criteria specified by a user. For example, the
user supplies the publisher ID and wants to see the details of that
publisher.
select * from publishers where pub_id=?
To make it possible, you need to prepare a query statement at
runtime with an appropriate value in the where clause.
The PreparedStatement object allows you to execute parameterized
queries. The PreparedStatement object is created using the
prepareStatement() method of the Connection object.
stat=con.prepareStatement (“select * from publishers where
pub_id=?”);
The prepareStatement(), method of the Connection object takes an
SQL statement as a parameter. The SQL statement can contain
placeholders that can be replaced by INPUT parameters at runtime.
The ‘?’ symbols is a placeholder that can be replaced by the INPUT
parameters at runtime.
Passing INPUT Parameters:
Before executing a PreparedStatement object, you must set the
value of each ‘?’ parameter. This is done by calling an appropriate
setXXX() method, where XXX is the data type of the parameter.
stat.setString(1, pid.getText());
ResultSet result=stat.executeQuery();