Oop - Java - Pg-Desd - Session- 5v2

8/13/2019 Oop - Java - Pg-Desd - Session- 5v2

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J2SE Core Java

PG-DESD

Aug -2013


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Session 5:

Learning Objectives

Explain Packages

Explain Wrapper Classes

Describe Exception Handling Mechanism in Java


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Packages

A package is a grouping of related types (class, interface)providing access protection and name space management.


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Creating and Using Packages

Defining a package :

package package-name;

Importing a package :

import packagename.classname;

or

import packagename.*;

Naming convention :

double y = java.lang.Math.sqrt(x);


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package sadhu;

public class S{public void msg (){System.out.println("Hello i am from sadhu package!");

}

public static void main(String args[]){

S s=new S();s.msg();

}}

//compile // javac -d . S.java// run // java sadhu.S

Example


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package cdac;import sadhu.*;class R{

public void fun(){

System.out.println(" Hello i am from cdac package");}

public static void main(String[] args){

S s1=new S();s1.msg();R r1=new R();r1.fun();

}}

// javac -d . R.java---java cdac.R

Example


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package sadhu.sreenivas;public class X{public void method () {

System.out.println("Hello i am from sub package!");}public static void main(String args[]){

X x=new X();x.method();

}}

//compile // javac -d . X.java// run // java sadhu.sreenivas.X

Example


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Example

package cdac;import sadhu.*;

class SR{

public void fun(){

System.out.println(" Hello i am from cdac package");}

public static void main(String[] args)

{S s1=new S();s1.msg();SR sr1=new SR();sr1.fun();sadhu.sreenivas.X x1=new sadhu.sreenivas.X();x1.method();

}}

// javac -d . R.java---java cdac.R


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Access Modifiers

Example


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Wrapper Classes

An object of a wrapper class can be used in any situation where aprimitive value will not suffice

For example, some objects serve as containers of other objects.Primitive values could not be stored in such containers, butwrapper objects could be.

Another example is that some methods take an Object as aparameter. So a primitive cannot be used, but an object of anyclass can be used.


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Instantiating Wrapper Classes

All wrapper classes can be instantiated using the corresponding

primitive type as an argument.

Integer age = new Integer(40);Double num = new Double(8.2);Character ampersand = new Character(&);

Boolean isDone = new Boolean(false);

All wrapper classes EXCEPT Character and Void can beinstantiated using a String argument.

Integer age = new Integer(40);

Double num = new Double(8.2);Boolean isDone = new Boolean(true);

The Void wrapper class cannot be instantiated.


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Wrapper Class Methods

Wrapper classes all contain various methods that help convertfrom the associated type to another type.

Integer num = new Integer(4);float flt = num.floatValue();//stores 4.0 in flt

Double dbl = new Double(8.2);int val = dbl.intValue(); //stores 8 in val


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Wrapper Class Methods

Wrapper classes also contain static methodsthat help managethe associated type

Each numeric class contains a method to convert arepresentation in a String to the associated primitive:

int num1 = Integer.parseInt(3);double num2 = Double.parseDouble(4.7);long num3 = Long.parseLong(123456789);


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Wrapper Class Constants

The wrapper classes often contain useful constants as well

The Integer class contains MIN_VALUEand MAX_VALUEwhichhold the smallest and largest int values

Other numeric classes will also have MIN_VALUEand MAX_VALUEwhich hold the smallest and largest value for their correspondingtypes.


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Autoboxing

Autoboxingis the automatic conversionof a primitive value to acorresponding wrapper object:

Integer obj;int num = 42;obj = num;

The assignment creates the appropriate Integer object

The reverse conversion (called unboxing) also occursautomatically as needed


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Exception Handling


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Kinds of Errors

Compile time errors

Run time errors

Source of Errors

Input Errors

Device ErrorsPhysical Limitations

Code Errors

Exception Handling


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What should be done when an error occurred?

Notify the user if an error occurs

Save all work

Allow users to exit from the program


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What is Exception Handling?

An exception signifies an illegal, invalid or unexpected issue

during program execution.

Since exceptions are almost always assumed to beanticipated, you need to provide appropriate exceptionhandling.

An exception is an event that occurs during the execution ofa program that disrupts the normal flow of instructions.

Approaches for dealing with error conditions

Using Conditional statements and return values

Use Javas exception handling mechanism


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Class division by zero

class DivisionByZeroHandled{

int c;

public int compute(int a, int b){

if(b==0) return 0;elsec=a/b;

return c;}

}


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Handling Exceptions - Java

Format:

try

{

// Code that may cause an error/exception to occur

}

catch (ExceptionType identifier)

{

// Code to handle the exception

}


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Handling Exceptions: DivisionByZero

class DivisionByZeroHandled{public static void main(String[] args){

int a=5,b=0,c=0;try{

c=a/b;}catch(Exception e){

System.out.println(e);}System.out.println(Handled Exception");

}}


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Handling Exceptions: Result Of Calling ReadLine ()

try

{

System.out.print("Type an integer: ");

BufferedReader br=new BufferedReader(new InputStreamReader(System.in));

String s =br.readLine();

System.out.println("You typed in..." + s);int num = Integer.parseInt (s);

System.out.println("Converted to an integer..." + num);

}

The exception

can occur here


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Handling Exceptions: Result Of Calling ParseInt ()

try

{

System.out.print("Type an integer: ");

BufferedReader br=new BufferedReader(new InputStreamReader(System.in));

String s =br.readLine();

System.out.println("You typed in..." + s);

num = Integer.parseInt (s);System.out.println("Converted to an integer..." + num);

}

The second exception

can occur here


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Where The Exceptions Occur In Class Integer

class Integer

{

public Integer (int value);

public Integer (String s) throwsNumberFormatException;

public static int parseInt (String s) throwsNumberFormatException;

}


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Handling Exceptions: Tracing The Example

Integer.parseInt (String s)

{

:

:

}main ()try

{

num = Integer.parseInt(s);

}

:

catch (NumberFormatException e)

{

:

}


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{

}main ()try

{

num = Integer.parseInt (s);

}

:


{

:

}

Oops!

The user didnt enter an integer



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{

}main ()try

{


}

:


{

:

}

NumberFormatException e =

new NumberFormatException ();



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{

}main ()try

{


}

:


{

:

}

NumberFormatException e =

new NumberFormatException ();



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{

}main ()try

{


}

:


{

}

Exception must be dealt with here



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Handling Exceptions: Catching The Exception


{

System.out.println(e);

}


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Catching The Exception: Error Messages


{

System.out.println(e.getMessage());


e.printStackTrace();

}


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{

System.out.println(e.getMessage());


e.printStackTrace();

}}

}

For input string: cdac"

java.lang.NumberFormatExceptio

For input string: cdac"

java.lang.NumberFormatException: For input string: cdac"

at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)

at java.lang.Integer.parseInt(Integer.java:426)

at java.lang.Integer.parseInt(Integer.java:476)

at Myclass.main(Myclass.java:39)

Catching The Exception: Error Messages


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R Ti E ti


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Run Time Exceptions


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Checked Exceptions

Must be handled if the potential for an error exists

must use a try-catchblock

Deal with problems that occur in a specific place

When a particular method invoked enclose it within a try-catch

block

Example:

SQLException, IOException


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Characteristics Of Unchecked Exceptions

The compiler doesnt require you to handle them if they arethrown.

No try-catch block required by the compiler

They can occur at any time in the program (not just for a specificmethod)

Typically they are fatal runtime errors that are beyond theprogrammerscontrol

Use conditional statements rather than the exception handlingmodel.

Examples:

NullPointerException,IndexOutOfBoundsException,ArithmeticException


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Common Unchecked Exceptions: NullPointerException

int [] arr = null;

arr[0] = 1;

arr = new int [4];

int i;

for (i = 0; i


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Common Unchecked Exceptions: ArrayIndexOutOfBoundsException

int [] arr = null;

arr[0] = 1;

arr = new int [4];

int i;

for (i = 0; i


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Common Unchecked Exceptions: ArithmeticExceptions

int [] arr = null;

arr[0] = 1;

arr = new int [4];

int i;

for (i = 0; i


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KeywordsException Handling in Java


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y p g


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Th Fi ll Cl


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The Finally Clause

An additional part of Javasexception handling model (try-catch-finally).

Used to enclose statements that must always be executed whether ornot an exception occurs.


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The Finally Clause: Exception Thrown

try

{

f.method();

}

catch

{

}

finally

{

}

4) A the end of the catch

block control transfers

to the finally clause

Example1.method ()

{

}

2) Exception thrown here


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The Finally Clause: No Exception Thrown

try

{

f.method();

}

catch

{

}

finally

{

}

Example1.method ()

{

}

2) Code runs okay here


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Compile:javac A.java

Run:java A cdachyd


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Compile :javac A.java

Run: java A CDAC PG-DAC AUG 2013 Batch

Output:

CDAC

PG-DAC

AUG

2013

Batch


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Next .

Arrays and String

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Oop - Java - Pg-Desd - Session- 5v2