Java Exceptions
An exception is a problem that arises during the execution of a program. An exception can occur for many different reasons, including the following:
Ø A user has entered invalid data.
Ø 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.
Exceptions Methods:
Following is the list of important methods available in the Throwable class.
Ø public String getMessage()
Returns a detailed message about the exception that has occurred. This message is initialized in the Throwable constructor.
Ø public Throwable getCause()
Returns the cause of the exception as represented by a Throwable object.
Ø public String toString()
Returns the name of the class concatenated with the result of getMessage()
Ø public void printStackTrace()
Prints the result of toString() along with the stack trace to System.err, the error output stream.
Ø 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.
Ø 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.
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.
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.
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.
}
An exception is a problem that arises during the execution of a program. An exception can occur for many different reasons, including the following:
Ø A user has entered invalid data.
Ø 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.
Exceptions Methods:
Following is the list of important methods available in the Throwable class.
Ø public String getMessage()
Returns a detailed message about the exception that has occurred. This message is initialized in the Throwable constructor.
Ø public Throwable getCause()
Returns the cause of the exception as represented by a Throwable object.
Ø public String toString()
Returns the name of the class concatenated with the result of getMessage()
Ø public void printStackTrace()
Prints the result of toString() along with the stack trace to System.err, the error output stream.
Ø 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.
Ø 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.
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.
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.
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.
}