Silicon Magic: May 2009

Java by Example: Exception handling – throw custom Exception

Exception indicates illegal operation. So for your program you can define an operation as illegal. Say in your login program you allow maxim 3 times retry with wrong user/password. Then you can define your exception for attempt to login for more than thrice. And if this case occurs you can throw your exception.

Here is an example of throw custom exception to check if a number is a Fibonacci number or not. When a non Fibonacci number found exception 'Invalid fibonacci number' will be thrown..

Here is the example . .

public
class ExceptionExp4 {

public
static
void main(String args[]){

ExceptionExp4 excpExp = new ExceptionExp4();

try{

excpExp.tryCustomException();

}catch(Exception ce){

ce.printStackTrace();

}

public
void tryCustomException() throws Exception{

int fibs[]={1,2,3,5,8,10,13};

for(int i=0;i<fibs.length;i++){

if(isFibs(fibs[i]))

System.out.println(fibs[i]+" - OK");

else

throw
new Exception("Invalid fibonacci number "+fibs[i]);

}

private
boolean isFibs(int num){

int f0 = 1;

int f1 = 1;

int fib = 1;

while(num>=fib){

if(num==fib)

return
true;

fib = f0 + f1;

f0 = f1;

f1 = fib;

}

return
false;

}

Fibonacci number series is

1 2 3 5 8 13 21 34 . . . .

So if you try to execute exception should occur when 10 is found..

Lets see the output of this program...

1 - OK

2 - OK

3 - OK

5 - OK

8 - OK

java.lang.Exception: Invalid fibonacci number 10

at example.java.lang.ExceptionExp4.tryCustomException(ExceptionExp4.java:18)

at example.java.lang.ExceptionExp4.main(ExceptionExp4.java:7)

So in this way we can easily create our own exception with our own message as per our needs.

Java By Example: Exception Handling – Throwing exception from method

The second way of exception handling is throwing the exception. This is done by adding throws with method name.

Syntax of throwing exception from method is

access_modifier return_type method_name(arguments) throws exception_list{

}

Here is a practical example of doing this..

public
class ExceptionExp3 {

public
static
void main(String args[]){

ExceptionExp3 excpExp = new ExceptionExp3();

try{

excpExp.tryThrows();

}catch(ArrayIndexOutOfBoundsException ae){

System.out.println("Exception Caught");

}

public
void tryThrows() throws ArrayIndexOutOfBoundsException{

String strArray[] = {"This","is", "an", "example", "of", "Java","exception" };

System.out.println(strArray[0]);

System.out.println(strArray[1]);

System.out.println(strArray[3]);

System.out.println(strArray[4]);

System.out.println(strArray[5]);

System.out.println(strArray[6]);

System.out.println(strArray[7]);

System.out.println("This statement wont execute");

}

The ArrayIndexOutOfBoundsException is thrown from method tryThrows() instead of handling the exception. In this case when exception occurred in tryThrows() method instead of terminating the program this method throw the exception to caller.

This program will give the following output..

This

example

Java

exception

Exception Caught

Java by Example: Exception handling – Basic Exception Handling example ‘try catch’

Exception Handling: The basic way to cope with exception is using try ...catch.

The Syntax for using try...catch is

try{

...

Code have to monitor for exception

...

}catch(Exception e){

...

Exception action code

...

}

Here is an easy example to demonstrate the above case..

public
class ExceptionExp2 {

public
static
void main(String args[]){

String strArray[] = {"This","is","an","example","of","Java","exception" };

try{

System.out.println(strArray[0]);

System.out.println(strArray[1]);

System.out.println(strArray[3]);

System.out.println(strArray[4]);

System.out.println(strArray[5]);

System.out.println(strArray[6]);

System.out.println(strArray[7]);

}catch(ArrayIndexOutOfBoundsException exp){

System.out.println("Exception Occurred");

}

System.out.println("This will execute");

}

If you run the above code it will give the following output …

This

example

Java

exception

Exception Occurred

This will execute

So exception is handled here by printing a message "Exception Occurred" and the program continues to execute the codes after the try ... catch block instead of terminating.

Java by Example: Exception Handling – Java Exception Basics

Java provides strong way to cope with Errors and Exceptions. When an illegal operation occurred Java throws exception. And When exception occurs program execution ends if exception is not handled properly. There is a class below to understand Java Exception..

public
class ExceptionExp1 {

public
static
void main(String args[]){

String strArray[] = {"This","is", "an", "example", "of", "Java","exception" };

System.out.println(strArray[0]);

System.out.println(strArray[1]);

System.out.println(strArray[3]);

System.out.println(strArray[4]);

System.out.println(strArray[5]);

System.out.println(strArray[6]);

System.out.println(strArray[7]);

System.out.println("Program will terminate before executing this line");

}

If you execute this program, the output will be-

This

example

Java

exception

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 7

at example.java.lang.ExceptionExp1.main(ExceptionExp1.java:13)

Here exception occurs before ends the program and terminates.

Oracle : Hierarchical Data/ Tree in Oracle

Oracle provides strong way to query tree like data or hierarchical data.

The syntax for query hierarchical data is-

SELECT columns

FROM table_name

WHERE where_condition

CONNECT BY PRIOR condition_for_next_level

START WITH start_condition

ORDER SIBLINGS BY columns

Let's try it with a practical example.

CREATE
TABLE location(

id NUMBER,

name
VARCHAR2(100),

parent NUMBER

);

INSERT
INTO location(id,name,parent) VALUES(1,'Earth',0);

INSERT
INTO location(id,name,parent) VALUES(2,'Asia',1);

INSERT
INTO location(id,name,parent) VALUES(3,'Europe',1);

INSERT
INTO location(id,name,parent) VALUES(4,'America',1);

INSERT
INTO location(id,name,parent) VALUES(5,'Africa',1);

INSERT
INTO location(id,name,parent) VALUES(6,'Oceania',1);

INSERT
INTO location(id,name,parent) VALUES(7,'Bangladesh',2);

INSERT
INTO location(id,name,parent) VALUES(8,'India',2);

INSERT
INTO location(id,name,parent) VALUES(9,'Pakistan',2);

INSERT
INTO location(id,name,parent) VALUES(10,'Singapore',2);

INSERT
INTO location(id,name,parent) VALUES(11,'Malaysia',2);

INSERT
INTO location(id,name,parent) VALUES(12,'England',3);

INSERT
INTO location(id,name,parent) VALUES(13,'France',3);

INSERT
INTO location(id,name,parent) VALUES(14,'Turkey',3);

INSERT
INTO location(id,name,parent) VALUES(15,'Scotland',3);

INSERT
INTO location(id,name,parent) VALUES(16,'Mexico',4);

INSERT
INTO location(id,name,parent) VALUES(17,'Canada',4);

INSERT
INTO location(id,name,parent) VALUES(18,'USA',4);

INSERT
INTO location(id,name,parent) VALUES(19,'Dhaka',7);

INSERT
INTO location(id,name,parent) VALUES(20,'Rajshahi',7);

INSERT
INTO location(id,name,parent) VALUES(21,'Chittagong',7);

INSERT
INTO location(id,name,parent) VALUES(22,'Las Vegas',18);

INSERT
INTO location(id,name,parent) VALUES(23,'New Orlince',18);

INSERT
INTO location(id,name,parent) VALUES(24,'New York',18);

Now

SELECT * FROM location;

Will give..

ID NAME PARENT

-----------------------------------------------

1 Earth 0

2 Asia 1

3 Europe 1

4 America 1

5 Africa 1

6 Oceania 1

7 Bangladesh 2

8 India 2

9 Pakistan 2

10 Singapore 2

11 Malaysia 2

12 England 3

13 France 3

14 Turkey 3

15 Scotland 3

16 Mexico 4

17 Canada 4

18 USA 4

19 Dhaka 7

20 Rajshahi 7

21 Chittagong 7

22 Las Vegas 18

23 New Orlince 18

24 New York 18

Now

SELECT
lpad(' ',8*(level-1))||NAME AS name
FROM LOCATION

CONNECT
BY
PRIOR id = parent

START
WITH parent = 0

Will give the following result

NAME

------------

Earth

Asia

Bangladesh

Dhaka

Rajshahi

Chittagong

India

Pakistan

Singapore

Malaysia

Europe

England

France

Turkey

Scotland

America

Mexico

Canada

USA

Las Vegas

New Orlince

New York

Africa

Oceania

You can also use ORDER SIBLINGS BY to sort siblings

ORACLE - ALTER TABLE: Rename Table

Syntax:

ALTER TABLE table_name
RENAME TO new_table_name;

Example:
ALTER TABLE emp
RENAME TO emp_records;

This will change the table name from 'emp' to emp_record.

Oracle : ALTER TABLE: Add Columns

Syntax:

ALTER
TABLE

table_name

ADD(

column1 datatype1 constraint1,

column2 datatype2 constraint2,

column3 datatype3 constraint3

);

ALTER
TABLE

emp

ADD(

designation VARCHAR2(100) UNIQUE,

rank NUMBER
NOT
NULL

);

This will add 2 columns to the table 'emp' named 'designation' and 'rank' with given constraints and data types

Java Difference between Hashtable and Hashmap / Hashtable versus Hashmap

Both Hashtable and Hashmap are Hashtable implementation of java classes. Hashtable is a basic class of java 'Collection Framework' and Hashmap is a Hashtable implementa from Map interface.
The majore differences are described below...

1. Synchronous/asynchronous: Hashtable is synchronous and Hashmap is not synchronous by default.

2. null value as key: Hashtable does not support null value as hash key but Hasmap support it.

3. Faile-safe: Iterator of Hashmap is fail-safe but Enumeration of Hashtable is not failsafe.

Perhaps this will be able to eliminate any misconception about Hashtable and Hashmap

Java Dangling meta character, PatternSyntaxException

PatternSyntaxException: Unmatched closing ')',

PatternSyntaxException: Unclosed group near,

PatternSyntaxException: Dangling meta character

The cause of the above Exceptions are you're trying to use meta character in Java Pattern processing, like regular expression.

The meta characters in this case are ([{\^-$|]})?*+

So if you want to process a String containing any of the meta characters in regular expression procession using java regex you'll get Exception like above. To process String containing a meta character just use escape character, a fore slash('/') before that character.

Any java method that use java regex also give such errors.

For example

Let's see the code below

String input = "I live in Dhaka(agargaon), Bangladesh";

String arr[] = input.split(")");

This will give the Exception PatternSyntaxException: Unmatched closing ')',

One more such method is indexOf().

In this case you have ti change the above code as

String arr[] = input.split("\\)");

Or for indexOf

input.indexOf("\\)");

Silicon Magic

Java by Example: Exception handling – throw custom Exception

Java By Example: Exception Handling – Throwing exception from method

Java by Example: Exception handling – Basic Exception Handling example ‘try catch’

Java by Example: Exception Handling – Java Exception Basics

Oracle : Hierarchical Data/ Tree in Oracle

ORACLE - ALTER TABLE: Rename Table

Oracle : ALTER TABLE: Add Columns

Java Difference between Hashtable and Hashmap / Hashtable versus Hashmap

Java Dangling meta character, PatternSyntaxException

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