Download exception handler

Lecture10
Exception Handling
Jaeki Song
Introduction
• Categories of errors
– Compilation error
• The rules of language have not been followed
– Runtime error
• The program is running if the environment detects
an operation that is impossible to carry out
– Logic error
• A program doesn’t perform the way it was intended
to
Exception
• Runtime errors causes exceptions
– Events that occur during the execution of a program
and disrupt the normal flow of control
• A program that does not provide the code to handle exceptions
may terminate abnormally, causing serious problem
• E.g.
– The user may enter invalid input
– The program may attempt to open file that doesn’t exist
– The program may attempt to access an out-of-bound array
element
Java and Exception
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
Several more classes
LinkageError
VirtualMachineError
Error
AWTError
Several more classes
Java and Exception
• A java exception is an instance of a class
derived from Throwable
– Errors related to graphic are included in the
java.awt package
– Numeric exceptions are included in the
java.lang package
Exception Classes
• The Exception class describes errors caused
by your program and external circumstances
– ClassNotFoundException
• If you attempt to use a class that does not exist
– RuntimeException
• Describes programming errors, such as bad casting,
accessing an out-of-bound array, and numeric errors
Common Exceptions
Exception
Purpose
ArithmetricException
Error caused by a calculation, such as division by zero
NumberFormatException
Problem converting a string to a number; occurs when the
text field is blank or contains a fraction when an integer is
required
IllegalArgumentException
Unable to format the value passed to one of the format
methods
FileNotFoundException
File does not exist in path specified
IOException
Failure of an input or output operation such as reading from
a file
OutOfMemoryException
Not enough memory to create an object
Exception Handling
• The exception handling in Java consists of
claiming exception, throwing exception,
and catching and processing exceptions
method1()
{
try
{
invoke method2;
}
catch (Exception ex)
{
Process exception;
}
catch exception
}
method2() throws Exception
{
if (an error occurs)
{
throw new Exception();
}
}
claim exception
throw exception
Claiming Exceptions
• In general, every method states the types of
exceptions it might encounter
– Simply tells the compiler what might go wrong when
the method is executing
• To claim an exception in a method, you use the
throws keyword
public void myMethod( ) throws IOException
public void myMethod( ) throws IOException,
OtherExceptions
Throwing Exceptions
• When a statement causes errors, the method
containing the statement creates an exception
object and passes it to the system
– The exception object contains information about
exception, including its type when the error occurred
• In the method that has claimed the exception, the
following is the syntax to throw an exception:
TheException ex = new TheException( );
throw ex
Catching Exceptions
• After a method throws an exception, the
Java runtime systems begins the process of
finding the code to handle the error
– The code that handles the error is called the
exception handler
– The handler must match the type of exception
thrown
• If no handler is found, the program terminates
Catching Exceptions
• When calling a method that explicitly claims an
exception, you must use the try-catch block
try
{ statement; } //statements that may throw exceptions
catch (Exception1 ex)
{ handler for exception1;}
catch (Exception2 ex)
{handler for exception2;}
Catching Exceptions
• If no exception arise during the execution of
the try clause, the catch clauses are skipped
• If one of the statement inside the try block
throws an exception, Java skips the
remaining statements and starts to search
for a handler for the exception
Catching Exceptions
main method
{
...
try
{
...
invoke method1;
statement1;
}
catch (Exception1 ex1)
{
Process ex1;
}
method1
{
...
try
{
...
invoke method2;
statement2;
}
catch (Exception2 ex2)
{
Process ex2;
}
method2
{
...
try
{
...
invoke method3;
statement3;
}
catch (Exception3 ex3)
{
Process ex3;
}
}
}
}
Example
• Example 1
• Example 2
The Exception Object
• If an exception of a subclass of Exception
occurs in a graphics program, Java prints
the error message
– The exception object contains valuable
information about the exception
The Exception Object
• public String getMessage()
– Returns the detailed message of the Throwable object
• public String toString()
– Returns a short description of the Throwable object
• public void printStackTrace( )
– Prints the Throwable object and its trace information
The finally Clause
try
{ statements;}
catch(TheException e)
{ handling e; }
finally
{ finalStatements; }
Cautions When Using Exceptions
• Exception handling separates
error-handling code from normal
programming tasks, thus making
programs easier to read and to
modify. Be aware, however, that
exception handling usually
requires more time and resources
because it requires instantiating
a new exception object, rolling
back the call stack, and
propagating the errors to the
Customized Exception Handling
• Procedures
– Step 1: Create own exception class by extending
Exception and creating its own constructors
• Your exception class can inherit from the Exception class
public class NegativeAmountException extends Exception
{
public NegativeAmountException()
{
super (“Negative amount “);
statement;
}
}
Customized Exception Handling
• Step2: Throws exception when certain
conditions happens
public void deposit (double amount) throws
NegativeException
{
statement;
}
Customized Exception Handling
• Step3: Provide the try and catch clauses for the
thrown exception
try
{
statement;
}
catch
{
statement;
}