Let us have some illustrations to get hold of Exception Handling..
Exceptions Methods:
Following is the list of important medthods available in the Throwable class.
| SN | Methods with Description |
|---|---|
| 1 | public String getMessage() Returns a detailed message about the exception that has occurred. This message is initialized in the Throwable constructor. |
| 2 | public Throwable getCause() Returns the cause of the exception as represented by a Throwable object. |
| 3 | public String toString() Returns the name of the class concatenated with the result of getMessage() |
| 4 | public void printStackTrace() Prints the result of toString() along with the stack trace to System.err, the error output stream. |
| 5 | public StackTraceElement [] getStackTrace() Returns an array containing each element on the stack trace. The element at index 0 represents the top of the call stack, and the last element in the array represents the method at the bottom of the call stack. |
| 6 | public Throwable fillInStackTrace() Fills the stack trace of this Throwable object with the current stack trace, adding to any previous information in the stack trace. |
Catching Exceptions:
A method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch looks like the following:
try
{
//Protected code
}catch(ExceptionName e1)
{
//Catch block
}
|
A catch statement involves declaring the type of exception you are trying to catch. If an exception occurs in protected code, the catch block (or blocks) that follows the try is checked. If the type of exception that occurred is listed in a catch block, the exception is passed to the catch block much as an argument is passed into a method parameter.
Example:
The following is an array is declared with 2 elements. Then the code tries to access the 3rd element of the array which throws an exception.
// File Name : ExcepTest.java
import java.io.*;
public class ExcepTest{
public static void main(String args[]){
try{
int a[] = new int[2];
System.out.println("Access element three :" + a[3]);
}catch(ArrayIndexOutOfBoundsException e){
System.out.println("Exception thrown :" + e);
}
System.out.println("Out of the block");
}
}
|
This would produce following result:
Exception thrown :java.lang.ArrayIndexOutOfBoundsException: 3 Out of the block |
Multiple catch Blocks:
A try block can be followed by multiple catch blocks. The syntax for multiple catch blocks looks like the following:
try
{
//Protected code
}catch(ExceptionType1 e1)
{
//Catch block
}catch(ExceptionType2 e2)
{
//Catch block
}catch(ExceptionType3 e3)
{
//Catch block
}
|
The previous statements demonstrate three catch blocks, but you can have any number of them after a single try. If an exception occurs in the protected code, the exception is thrown to the first catch block in the list. If the data type of the exception thrown matches ExceptionType1, it gets caught there. If not, the exception passes down to the second catch statement. This continues until the exception either is caught or falls through all catches, in which case the current method stops execution and the exception is thrown down to the previous method on the call stack.
Example:
Here is code segment showing how to use multiple try/catch statements.
try
{
file = new FileInputStream(fileName);
x = (byte) file.read();
}catch(IOException i)
{
i.printStackTrace();
return -1;
}catch(FileNotFoundException f) //Not valid!
{
f.printStackTrace();
return -1;
}
|
The throws/throw Keywords:
If a method does not handle a checked exception, the method must declare it using the throws keyword. The throws keyword appears at the end of a method’s signature.
You can throw an exception, either a newly instantiated one or an exception that you just caught, by using the throwkeyword. Try to understand the different in throws and throw keywords.
The following method declares that it throws a RemoteException:
import java.io.*;
public class className
{
public void deposit(double amount) throws RemoteException
{
// Method implementation
throw new RemoteException();
}
//Remainder of class definition
}
|
Amethod can declare that it throws more than one exception, in which case the exceptions are declared in a list separated by commas. For example, the following method declares that it throws a RemoteException and an InsufficientFundsException:
import java.io.*;
public class className
{
public void withdraw(double amount) throws RemoteException,
InsufficientFundsException
{
// Method implementation
}
//Remainder of class definition
}
|
The finally Keyword
The finally keyword is used to create a block of code that follows a try block. A finally block of code always executes, whether or not an exception has occurred.
Using a finally block allows you to run any cleanup-type statements that you want to execute, no matter what happens in the protected code.
A finally block appears at the end of the catch blocks and has the following syntax:
try
{
//Protected code
}catch(ExceptionType1 e1)
{
//Catch block
}catch(ExceptionType2 e2)
{
//Catch block
}catch(ExceptionType3 e3)
{
//Catch block
}finally
{
//The finally block always executes.
}
|
Example:
public class ExcepTest{
public static void main(String args[]){
int a[] = new int[2];
try{
System.out.println("Access element three :" + a[3]);
}catch(ArrayIndexOutOfBoundsException e){
System.out.println("Exception thrown :" + e);
}
finally{
a[0] = 6;
System.out.println("First element value: " +a[0]);
System.out.println("The finally statement is executed");
}
}
}
|
This would produce following result:
Exception thrown :java.lang.ArrayIndexOutOfBoundsException: 3 First element value: 6 The finally statement is executed |
Note the followings:
- A catch clause cannot exist without a try statement.
- It is not compulsory to have finally clauses when ever a try/catch block is present.
- The try block cannot be present without either catch clause or finally clause.
- Any code cannot be present in between the try, catch, finally blocks.
Declaring you own Exception:
You can create your own exceptions in Java. Keep the following points in mind when writing your own exception classes:
- All exceptions must be a child of Throwable.
- If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class.
- If you want to write a runtime exception, you need to extend the RuntimeException class.
We can define our own Exception class as below:
class MyException extends Exception{
}
|
You just need to extend the Exception class to create your own Exception class. These are considered to be checked exceptions. The following InsufficientFundsException class is a user-defined exception that extends the Exception class, making it a checked exception. An exception class is like any other class, containing useful fields and methods.
Example:
// File Name InsufficientFundsException.java
import java.io.*;
public class InsufficientFundsException extends Exception
{
private double amount;
public InsufficientFundsException(double amount)
{
this.amount = amount;
}
public double getAmount()
{
return amount;
}
}
|
To demonstrate using our user-defined exception, the following CheckingAccount class contains a withdraw() method that throws an InsufficientFundsException.
// File Name CheckingAccount.java
import java.io.*;
public class CheckingAccount
{
private double balance;
private int number;
public CheckingAccount(int number)
{
this.number = number;
}
public void deposit(double amount)
{
balance += amount;
}
public void withdraw(double amount) throws
InsufficientFundsException
{
if(amount <= balance)
{
balance -= amount;
}
else
{
double needs = amount - balance;
throw new InsufficientFundsException(needs);
}
}
public double getBalance()
{
return balance;
}
public int getNumber()
{
return number;
}
}
|
The following BankDemo program demonstrates invoking the deposit() and withdraw() methods of CheckingAccount.
// File Name BankDemo.java
public class BankDemo
{
public static void main(String [] args)
{
CheckingAccount c = new CheckingAccount(101);
System.out.println("Depositing $500...");
c.deposit(500.00);
try
{
System.out.println("\nWithdrawing $100...");
c.withdraw(100.00);
System.out.println("\nWithdrawing $600...");
c.withdraw(600.00);
}catch(InsufficientFundsException e)
{
System.out.println("Sorry, but you are short $"
+ e.getAmount());
e.printStackTrace();
}
}
}
|
Compile all the above three files and run BankDemo, this would produce following result:
Depositing $500...
Withdrawing $100...
Withdrawing $600...
Sorry, but you are short $200.0
InsufficientFundsException
at CheckingAccount.withdraw(CheckingAccount.java:25)
at BankDemo.main(BankDemo.java:13)
|
Common Exceptions:
In java it is possible to define two catergories of Exceptions and Errors.
- JVM Exceptions: – These are exceptions/errors that are exclusively or logically thrown by the JVM. Examples : NullPointerException, ArrayIndexOutOfBoundsException, ClassCastException,
- Programmatic exceptions . These exceptions are thrown explicitly by the application or the API programmers Examples: IllegalArgumentException, IllegalStateException.
As you can see from the JVM’s error message, the error is a NullPointerException and the problem occurs in RuntimeError‘s 
Java’s VM throws an ArtithmeticException whenever you ask it to perform an illegal artithmetic operation. As you can see from the message, the error occurred in the 
Following the line numbers, you can see that the error occurs in the 
![Error messages after compiling with Java 2 compiler 1.3. [click to enlarge]](http://sofia.fhda.edu/gallery/java/unit07/error_messages_J2_lg.gif)
The graphics are quite good.
aprimit 