1. Lab 1
Exception Handling

2. Lab 1: Exception Handling
Introduction
Exceptions in Java
Types of Exceptions
Handling Exceptions
Causes of Exceptions

3. Introduction
Exception handling is a mechanism for dealing with runtime errors.
Syntactically-correct programs do not mean they are error-free during runtime.
For example, a program trying to read data from a non-existent file.
In languages such as C, the programmer is responsible for error checking but is not rigidly enforced by the language.

4. Java provides support for exception handling.
The programmer is
not forced to check
whether f is null.
For example (C),
FILE *f = fopen("list.dat", "r");
fscanf(f, "%d", &qty);
Java provides support for exception handling.

5. Exceptions in Java In Java, exceptions are represented as objects.
An exception object representing a particular exception contains data about that exception. This makes it possible to obtain clues as to why it occurred and hopefully diagnose the cause of the problem.

6. When an exception occurs, an exception object is created and "thrown" to the calling method.
The calling method may choose to handle ("catch") the exception and/or "throw" the exception to its calling method, and so on.

7. EXCEPTION
OCCURS
Method C
throws the
exception object
Method C
: Exception
Method B
throws the
exception object
calls
Method B
calls
Method B
catches the
exception object
and handles the
exception
Method A
Method A catches the exception object and handles the exception

8. Each method involved in the calling chain must:
provide error handling code (if the method wants to) for each type of exception object thrown by the methods it calls.
rethrow the exception objects it receives to its calling method.

9. Types of Exceptions java.lang.Throwable java.lang.Error
java.lang.Exception

10. java.lang.Error Exceptions
These are serious errors which should be handled by the Java system.
Examples of exception objects in this category:
AWTError - serious error occurs in the AWT
NoClassDefFoundError
OutOfMemoryError

11. java.lang.RuntimeException Exceptions
java.lang.RuntimeException is a subclass of java.lang.Exception.
It usually represents fairly serious errors which are difficult to recover from.
They are automatically handled by the Java system although the programmer may handle them himself if there is a need.
Examples: NullPointerException, SecurityException

12. Handling Exception try and catch statements throws keyword
Throwing an exception
finally statement

13. try and catch statements…
x.method(); }
catch (ExceptionType e) {
Calling a method that
may throw an ExceptionType
exception
If the exception occurs,
it will be caught here and
statements in this block
will be executed.

14. throws keyword
A method may rethrow an exception without catching the exception.
Example:
public void method() throws IOException { … }

15. Throwing an Exception
A method may catch an exception and rethrow it to its caller.
Example:
public void op() throws IOException {
try {
x.method(); }
catch (IOException e) {
System.out.println("Error");
throw e;

16. finally statement
This statement is optional. Statements in the finally block will be executed whether or not an exception occurs.
try { …
} catch (ExceptionType e) {
} finally { }