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System Development with Java
Budditha Hettige
Department of Statistics and Computer Science
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Exception Handling
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Errors
• Errors can be categorized as several ways;
– Syntax Errors
– Logical Errors
– Runtime Errors
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Syntax Errors
• Error in the syntax of a sequence of characters
or tokens that is intended to be written in a
particular programming language
• All syntax errors can be reliably detected until
run-time
• Many Syntax errors can be detected at
compile-time
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Common Java syntax errors
• Capitalization of key words
Line xx: class or interface declaration expected
• Writing a string over a new line
Line xx: ';' expected
• Missing brackets in a no-argument message
Line xx: Invalid expression statement
• Forgetting to import a package
Line nn: Method yyyy not found in class xxxx.
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Syntax Error correction
1. Go to the error line
2. Identify the error
3. Correct the error
• You must have a good working knowledge of error messages to discover the cause of the error
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Error line Error line
Error description Error description
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Logical Errors
• Errors that indicate the logic used when coding
the program failed to solve the problem
• You do not get error messages with logic
errors
• Your only clue to the existence of logic errors
is the production of wrong solutions
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Common Logic Errors in Java
• Using a variable before it is given a value
int x;
x = x + 1;
System.out.println("X = " + x);
• Misplaced Semi-colon (usually with a loop or if statement) if ( x > y) ; {
System.out.println("X is bigger");
}
• Confusing the equivalence operator == with the assignment operator =
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Logical error correction
• Debugging can be used to find what the logical
errors in your program
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Debug line Debug line
Results Results
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Runtime Errors
• low-level errors – dereference of a null pointer
– out-of-bounds array access
– divide by zero
– attempt to open a non-existent file for reading
– bad cast (e.g., casting an Object that is actually a Boolean to Integer)
• higher-level – call to Stack's "pop" method for an empty stack
– call to "factorial" function with a negative number
– call to List's nextElement method when hasMoreElements is false
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Runtime Errors contd..
Errors can arise due to
• User error
– providing a bad file name or a poorly formatted
input file
• Programmer error
– These errors should be detected as early as
possible to provide good feedback.
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Exceptions can occur at many
levels
• Hardware/operating system level.
• Arithmetic exceptions; divide by 0, under/overflow.
• Memory access violations; segfault, stack over/underflow.
• Language level.
• Type conversion; illegal values, improper casts.
• Bounds violations; illegal array indices.
• Bad references; null pointers.
• Program level.
• User defined exceptions.
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Ways to handle errors
• Write an error message and quit.
– This doesn't provide any recovery
• Return a special value to indicate that an error occurred
– calling code check for an error. This can reduce the efficiency of the code
• Use a reference parameter or a global variable to hold an error code
• Use exceptions. This seems to be the method of choice for modern programming languages.
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What Is an Exception?
• An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions.
• Example
– Divide by zero errors
– Accessing the elements of an array beyond its range
– Invalid input
– Hard disk crash
– Opening a non-existent file
– Heap memory exhausted
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Example
class DivByZero {
public static void main(String args[]) {
System.out.println(3/0);
System.out.println(“Pls. print me.”);
}
}
Error Message
Exception in thread "main"
java.lang.ArithmeticException: / by zero at
DivByZero.main(DivByZero.java:3)
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Divide by zero Divide by zero
Where the error is occured
Where the error is occured
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Default exception handler
1. Provided by Java runtime
2. Prints out exception description
3. Prints the stack trace
4. Hierarchy of methods where the exception occurred
5. Causes the program to terminate
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Exception in thread "main" java.lang.ArithmeticException: / by zero at DivByZero.main(DivByZero.java:3
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What Happens When an Exception
Occurs?
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When an Exception Occurs?
• When an exception occurs within a method,
the method creates an exception object and
hands it off to the runtime system
– Creating an exception object and handing it to the
runtime system is called “throwing an exception”
– Exception object contains information about the
error, including its type and the state of the
program when the error occurred
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When an Exception Occurs? (1)
• The runtime system searches the call stack for
a method that contains an exception handler
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When an Exception Occurs? (2)
• When an appropriate handler is found, the runtime system passes the exception to the handler
– An exception handler is considered appropriate if the type of the exception object thrown matches the type that can be handled by the handler
– The exception handler chosen is said to catch the exception.
• If the runtime system exhaustively searches all the methods on the call stack without finding an appropriate exception handler, the runtime system (and, consequently, the program) terminates and uses the default exception handler
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Searching the Call Stack for
an Exception Handler
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How exception handler works?
Exception "thrown" here
Exception handler
Exception handler
Thrown exception matched against first set of exception handlers
Thrown exception matched against first set of exception handlers
If it fails to match, it is matched against next set of handlers, etc.
If it fails to match, it is matched against next set of handlers, etc.
If exception matches none of handlers, program is abandoned
If exception matches none of handlers, program is abandoned
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Advantages
• Separating Error-Handling code from
“regular” business logic code
• Propagating errors up the call stack
• Grouping and differentiating error types
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Java Exception Handling
• A method can duck any exceptions thrown within it,
thereby allowing a method farther up the call stack to
catch it. Hence, only the methods that care about
errors have to worry about detecting errors
• Any checked exceptions that can be thrown within a
method must be specified in its throws clause
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Grouping and Differentiating
Error Types
• Because all exceptions thrown within a program are objects, the grouping or categorizing of exceptions is a natural outcome of the class hierarchy
• An example of a group of related exception classes in the Java platform are those defined in java.io – IOException and its descendants
– IOException is the most general and represents any type of error that can occur when performing I/O
– Its descendants represent more specific errors. For
– example, • FileNotFoundException means that a file could not be located on
disk.
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Catching Exceptions
try
{
<code to be monitored for exceptions>
}
catch (<ExceptionType1> <ObjName>)
{
<handler if ExceptionType1 occurs>
}
...
}
catch (<ExceptionTypeN> <ObjName>)
{
<handler if ExceptionTypeN occurs>
}
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Example
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Multiple catch
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How try and catch works?
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Try { }
Try { }
Catch { }
Catch { }
Catch { }
Catch { }
Statement after the last catch
NO Match
Leaves the method
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Example
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1. Execute This 1. Execute This
2. Goto this catch()
2. Goto this catch()
3. Execute This 3. Execute This
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No any catch for matching ???
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Catching Exceptions with finally
Syntax: try
{
<code to be monitored for exceptions>
}
catch (<ExceptionType1> <ObjName>)
{
<handler if ExceptionType1 occurs>
} ...
}
finally
{
<code to be executed before the try block ends>
}
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Catching Exceptions the finally Keyword
• Block of code is always executed despite of different scenarios:
– Forced exit occurs using a return, a continue or a break statement
– Normal completion
– Caught exception thrown
• Exception was thrown and caught in the method
– Uncaught exception thrown
• Exception thrown was not specified in any catch block in the method
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How try and catch –finaly works?
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Try { }
Try { }
Catch { }
Catch { }
Catch { }
Catch { }
Statement after the last catch
NO Match
Finally() { }
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Sequence of Events for finally clause
Preceding step
try block
throw statement
matching catch
next step
finally
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Throwing Exceptions
• Java allows you to throw exceptions (generate exceptions)
– throw <exception object>;
• An exception you throw is an object
– You have to create an exception object in the same way
– you create any other object
• Example:
– throw new ArithmeticException(“testing...”);
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Rules in Exception Handling
• A method is required to either catch or list all exceptions it might throw – Except for Error or RuntimeException, or their subclasses
• If a method may cause an exception to occur but does not catch it, then it must say so using the throws keyword – Applies to checked exceptions only
– Syntax: <type> <methodName> (<parameterList>)
throws <exceptionList>
{
<methodBody>
}
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Sequence of Events for No throw
Preceding step
try block
throw statement
unmatched catch
matching catch
unmatched catch
next step
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Sequence of Events for throw
Preceding step
try block
throw statement
unmatched catch
matching catch
unmatched catch
next step
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The exception hierarchy
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The Error and Exception Classes
• Throwable class
– Root class of exception classes
– Immediate subclasses
• Error
• Exception
• Exception class
– Conditions that user programs can reasonably deal with
– Usually the result of some flaws in the user program code
– Examples
• Division by zero error
• Array out-of-bounds error
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The Error and Exception Classes
• Error class
– Used by the Java run-time system to handle errors
occurring in the run-time environment
– Generally beyond the control of user programs
– Examples
• Out of memory errors
• Hard disk crash
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User define Exception
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Creating Your Own Exception Class
• Steps to follow
– Create a class that extends the RuntimeException or the Exception class
– Customize the class
• Members and constructors may be added to the class
– Example: class HateStringExp extends RuntimeException
{
/* some code */
}
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Sample Code
class TestHateString {
public static void main(String args[]) {
String input = "invalid input";
try {
if (input.equals("invalid input")) {
throw new HateStringExp();
}
System.out.println("Accept string.");
}
catch (HateStringExp e) {
System.out.println("Hate string!”);
}
}
}
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class HateStringExp extends RuntimeException
{
/* some code */
}
class HateStringExp extends RuntimeException
{
/* some code */
}
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Example
public class CustomExceptionTest
{
public static void main(String[] args) throws Exception
{
int age = getAge();
if (age < 0)
{
throw new NegativeAgeException(age);
}
Else
{
System.out.println("Age entered is " + age);
}
}
static int getAge()
{
return -10;
}
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Exception class
public class NegativeAgeException extends Exception
{
private int age;
public NegativeAgeException(int newAge)
{
age = newAge;
}
public String toString()
{
return "Age cannot be negative" + " " +age ;
}
}
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