Exception Handling

Object – Oriented Programming
Week 13– Exception Handling
Ferdin Joe John Joseph, PhD
Faculty of Information Technology
Thai-Nichi Institute of Technology

Motivations
When a program runs into a runtime error, the
program terminates abnormally.
How can you handle the runtime error so that
the program can continue to run or terminate
gracefully?
3

Exceptions
• Unexpected conditions
• Disrupt normal flow of program
• With exception handling, we can develop
more robust programs
4

Exceptions
Exceptions are thrown
And can be caught.
5

Where do exceptions come
from?
• JVM
OR
• Java Programs – can throw an exception
6

Exception Types
7
LinkageError
Error
AWTError
AWTException
Throwable
ClassNotFoundException
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
NullPointerException
IndexOutOfBoundsException
Several more classes
IllegalArgumentException

System Errors
8
LinkageError
Error
AWTError
AWTException
Throwable
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
System errors are thrown by JVM
and represented in the Error class.
The Error class describes internal
system errors. Such errors rarely
occur. If one does, there is little
you can do beyond notifying the
user and trying to terminate the
program gracefully.

Exceptions
9
LinkageError
Error
AWTError
AWTException
Throwable
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
Exception describes errors
caused by your program
and external
circumstances. These
errors can be caught and
handled by your program.

Runtime Exceptions
10
LinkageError
Error
AWTError
AWTException
Throwable
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
RuntimeException is caused by
programming errors, such as bad
casting, accessing an out-of-bounds
array, and numeric errors.

Types of Exceptions
• Errors - serious and fatal problems
• Exceptions - can be thrown by any
program, user can extend
• RuntimException - caused by illegal
operations, thrown by JVM (unchecked)
11

Checked Exceptions vs.
Unchecked Exceptions
12
RuntimeException, Error and their subclasses are
known as unchecked exceptions.
All other exceptions are known as checked
exceptions - the compiler forces the programmer to
check and deal with the exceptions.

Checked or Unchecked Exceptions
13
LinkageError
Error
AWTError
AWTException
Throwable
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
Unchecked
exception.

Declaring, Throwing, and
Catching Exceptions
14
method1() {
try {
invoke method2;
}
catch (Exception ex) {
Process exception;
}
}
method2() throws Exception {
if (an error occurs) {
throw new Exception();
}
}
catch exception throw exception
declare exception

Checked Exceptions
Checked exceptions that can be thrown in a
method must be
• caught and handled
OR
• declared in the throws clause
15

Declaring Exceptions
Every method must state the types of
checked exceptions it might throw. This is
known as declaring exceptions.
public void myMethod()
throws IOException
public void myMethod()
throws IOException, OtherException
16

Throwing Exceptions
When the program detects an error, the
program can create an instance of an
appropriate exception type and throw it. This
is known as throwing an exception. Here is
an example,
throw new TheException();
TheException ex = new TheException();
throw ex;
17

Throwing Exceptions Example
/** Set a new radius */
public void setRadius(double newRadius)
throws IllegalArgumentException {
if (newRadius >= 0)
radius = newRadius;
else
throw new IllegalArgumentException(
"Radius cannot be negative");
}
18

Catching Exceptions
try {
statements; // Statements that may throw exceptions
}
catch (Exception1 exVar1) {
handler for exception1;
}
catch (Exception2 exVar2) {
handler for exception2;
}
...
catch (ExceptionN exVar3) {
handler for exceptionN;
}
19

Catching Exceptions
20
try
catch
try
catch
try
catch
An exception
is thrown in
method3
Call Stack
main method main method
method1
main method
method1
main method
method1
method2 method2
method3

Catch or Declare Checked
Exceptions
If a method declares a checked exception, you must invoke it in a try-
catch block or declare to throw the exception in the calling method.
In this example, method p1 invokes method p2 and p2 may throw a
checked exception (e.g., IOException)
21
void p1() {
try {
p2();
}
catch (IOException ex) {
...
}
}
(a) (b)
void p1() throws IOException {
p2();
}

Syntax
try {
// Normal execution path
throw new EmptyStackException();
} catch (ExampleException ee) {
// deal with the ExampleException
} finally {
// This optional section is executed upon termination of
any of the try or catch blocks above,
// except when System.exit() is called in "try" or "catch"
blocks;
}
Faculty of Information Technology,
22

Rethrowing Exceptions
try {
statements;
}
catch(TheException ex) {
perform operations before exits;
throw ex;
}
23

The finally Clause
try {
statements;
}
handling ex;
}
finally {
finalStatements;
}
24

Trace a Program Execution
try {
statements;
}
handling ex;
}
finally {
finalStatements;
}
Next statement;
25
animation
Suppose no
exceptions in the
statements

try {
statements;
}
handling ex;
}
finally {
finalStatements;
}
Next statement;
26
animation
The final block is
always executed

try {
statements;
}
handling ex;
}
finally {
finalStatements;
}
Next statement;
27
animation
Next statement in the
method is executed

try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
finally {
finalStatements;
}
Next statement;
28
animation
Suppose an exception
of type Exception1 is
thrown in statement2

try {
statement1;
statement2;
statement3;
}
handling ex;
}
finally {
finalStatements;
}
Next statement;
29
animation
The exception is
handled.

try {
statement1;
statement2;
statement3;
}
handling ex;
}
finally {
finalStatements;
}
Next statement;
30
animation
The final block is
always executed.

try {
statement1;
statement2;
statement3;
}
handling ex;
}
finally {
finalStatements;
}
Next statement;
31
animation
The next statement in
the method is now
executed.

try {
statement1;
statement2;
statement3;
}
handling ex;
}
handling ex;
throw ex;
}
finally {
finalStatements;
}
Next statement;
32
animation
statement2 throws an
exception of type
Exception2.

try {
statement1;
statement2;
statement3;
}
handling ex;
}
handling ex;
throw ex;
}
finally {
finalStatements;
}
Next statement;
33
animation
Handling exception

try {
statement1;
statement2;
statement3;
}
handling ex;
}
handling ex;
throw ex;
}
finally {
finalStatements;
}
Next statement;
34
animation
Execute the final block

try {
statement1;
statement2;
statement3;
}
handling ex;
}
handling ex;
throw ex;
}
finally {
finalStatements;
}
Next statement;
35
animation
Rethrow the exception
and control is
transferred to the caller

Cautions When Using
Exceptions
• Exception handling separates error-
handling code from normal programming
tasks, thus making programs easier to read
and to modify.
• 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 calling methods.
36

When to Throw Exceptions
• An exception occurs in a method.
• If you want the exception to be processed
by its caller, you should create an exception
object and throw it.
• If you can handle the exception in the
method where it occurs, there is no need to
throw it.
37

Creating Custom Exception
Classes
38
 Use the exception classes in the API whenever possible.
 Create custom exception classes if the predefined
classes are not sufficient.
 Declare custom exception classes by extending
Exception or a subclass of Exception.

Exceptions and Generics
• A generic class cannot extend Throwable.
• An Exception cannot be of a generic type.
39

Example 1
40

Example 1
41

Example 2
42

Exception Handling

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