Bài giảng Phát triển phần mềm phân tán - Bài 10: Exception handling - Vũ Thị Hương Giang
LESSON X. Exception
Handling
Vu Thi Huong Giang
Objectives
• After this lesson, students (learners) can:
– Learn what an exception is, what have to do to handle
exception
– Distinguish the checked and unchecked exceptions
– Implement the try-catch-finally blocks for catching and
handling exceptions
– Write methods that propagate exceptions.
– Write programmer-defined exception classes.
Content
I. Exception & Exception handling
II. Exception classes hierarchy
III. try/catch/finally blocks
IV. Throwing exceptions
V. Print a stack trace message
VI. Making custom exceptions
I. Exception and Exception
Handling
1. Exception
2. Exception Handling
1. Exception-What are errors?
• There are three categories or types of errors:
– (1) Syntax errors
– (2) Logic errors (sometimes called: semantic errors)
– (3) Runtime errors
• Syntax errors arise because the rules of the
language have not been followed. They are
detected by the compiler.
• Logic errors occur when a program doesn't
perform the way it was intended to.
• Runtime errors occur while the program is
running if the environment detects an operation
that is impossible to carry out
1. Exception-Error examples
• User Input Errors:
– connect to a URL which is incorrect
– network exception
• Device Errors:
– printer is off/ out of paper
– Network is down
• Physical limitations:
– Disks run out of memory,
– quotas fill up
– an infinite recursion causes a stack overflow.
• Code errors:
– divide by zero,
– array out of bound,
– integer overflow,
– access a null pointer
1. Exception-Definitions
• Exception: an occurrence of an undesirable
situation that can be detected during program
execution. Examples:
– Division by zero
– Trying to open an input file that does not exist
– An array index that goes out of bounds
• When an exception occurs, or is thrown, the
normal sequence of flow is terminated. The
exception-handling routine is then executed; we
say the thrown exception is caught
1. Exception-Process when exception
occurs
1. When an exception occurs, Java allows a
method to terminate abnormally
2. It throws an object that encapsulates error
information
3. It does not return a value
4. It exits immediately
5. Execution does not resume at the point of
method call
6. JVM searches for an exception handler
7. If none is found, the program crashes.
I. Exception and Exception
Handling
1. Exception
2. Exception Handling
2. Exception Handling
• Exception handling is generally used to handle
abnormal events in a program.
• When a method encounters a problem that
disrupts the normal flow, it can cause the
program to CRASH ! But with exception handling,
the method can throw an exception which
informs the caller that a problem occurred
allowing the caller to perform alternative actions.
• Exception handling enables more robust
programming by providing ways of recovering
from errors or problems
2. Exception Handling (within a
Program)
• Can use an if statement to handle an exception
Perform a task
If the preceding task did not execute correctly
Perform error processing
Perform next task
If the preceding task did not execute correctly
Perform error processing
2. Exception Handling
• Exception handling streamlines error handling by
allowing your program to define a block of code,
called an exception handler that is executed
automatically when an error occurs
• All exception handling has the same goals:
– 1. Anticipate the error by the user/system
– 2. Return the program to a safe state that enables the
user to execute other commands
– 3. Inform the user of the error's cause
– 4. Allow the user to save work and terminate the
program gracefully.
2. Exception Handling-Java's
Mechanism of Exception Handling
• When an exception occurs, an object of a
particular exception classis created
• Java provides a number of exception classes to
effectively handle certain common exceptions,
such as:
– Division by zero: handled by the ArithmeticException
class.
– Invalid input: handled by the InputMismatchException
class.
– File not found: handled by the FileNotFoundException
class.
2. Exception Handling-Techniques
• Techniques in Exception handling:
– Terminate program
– Fix error and continue
– Log error and continue
Example:
code without exception handling
import javax.swing.JOptionPane;
public class Sample {
public static void main(String[] args) {
// TODO Auto-generated method stub
System.out.println("Begin ...");
String inputStr;
int age;
inputStr = JOptionPane.showInputDialog(null, "Age:");
age = Integer.parseInt(inputStr);
System.out.println("Your age: " + age);
System.out.println("End ...");
}
}
Example:
code without exception handling
• What would happen if the
user enters an integer such
as 10?
• What would happen if the
user enters a text “ten”
instead of the number 10?
Content
I. Exception & Exception handling
II. Exception classes hierarchy
III. try/catch/finally blocks
IV. Throwing exceptions
V. Print a stack trace message
VI. Making custom exceptions
II. Exception class hierarchy
Throwable
Exception
Error
RuntimeException IOException AWTError
NullPointerException
ThreadDeath
OutofMemoryError
InputMismatchException
ClassCastException
ArrayIndexOutOfBoundException
ArithmethicException
Constructors and methods of class
Throwable
public Throwable()
//Default constructor
//Creates an instance of Throwable with an empty message string
public Throwable (String strMessage)
//Constructor with parameters
//Creates an instance of Throwable with message string
//specified by the parameter strMessage
public String getMessage( )
//Returns the detailed message stored in the object
public void printStackTrace( )
//Method to print the stack trace showing the sequence of
//method calls when an exception occurs
public void printStackTrace (PrintWriter stream)
//Method to print the stack trace showing the sequence of
//method calls when an exception occurs. Output is sent
//to the stream specified by the parameter stream.
public String toString( )
//Returns a string representation of the Throwable object
class Exception and its Constructors
public Exception( )
//Default constructor
//Creates a new instance of the class Exception
public Exception (String str)
//Constructor with parameters
//Creates a new instance of the class Exception.
//The parameter str specifies the message string.
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