EXCEPTION:
An exception is a problem that arises during the execution
of a program. An exception can occur for many different reasons, including the following:
• A user has
entered invalid data.
• A file that
needs to be opened cannot be found.
• A network
connection has been lost in the middle of communications, or the JVM has run
out of memory.
Some of these exceptions are caused by user error, others
by programmer error, and others by physical resources that have failed in some
manner.
To understand how exception handling works in Java, you
need to understand the three categories of exceptions:
• Checked
exceptions: A checked exception is an exception that is typically a user error
or a problem that cannot be foreseen by the programmer. For example, if a file
is to be opened, but the file cannot be found, an exception occurs. These
exceptions cannot simply be ignored at the time of compilation.
• Runtime
exceptions: A runtime exception is an exception that occurs that probably could
have been avoided by the programmer. As opposed to checked exceptions, runtime
exceptions are ignored at the time of compliation.
• Errors:
These are not exceptions at all, but problems that arise beyond the control of
the user or the programmer. Errors are typically ignored in your code because
you can rarely do anything about an error. For example, if a stack overflow
occurs, an error will arise. They are also ignored at the time of compilation.
Exception Hierarchy:
All exception classes are subtypes of the
java.lang.Exception class. The exception class is a subclass of the Throwable
class. Other than the exception class there is another subclass called Error
which is derived from the Throwable class.
Errors are not normally trapped form the Java programs.
These conditions normally happen in case of severe failures, which are not
handled by the java programs. Errors are generated to indicate errors generated
by the runtime environment. Example : JVM is out of Memory. Normally programs
cannot recover from errors.
The Exception class has two main subclasses : IOException
class and RuntimeException Class.
Here is a list of most common checked and unchecked Java's
Built-in Exceptions.
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 throw keyword. 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.
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