JP ASSIGNMENT SERIES PPT.ppt

1
EXCEPTION HANDELING
MECHANISM-2023EV080
BY
JAYAPRIYA R
CSE

Exception-Handling Overview
2
Show runtime error
Fix it using an if statement
With a method
Run
Quotient
Run
QuotientWithIf
Run
QuotientWithMethod

Exception Advantages
3
Now you see the advantages of using exception handling.
It enables a method to throw an exception to its caller.
Without this capability, a method must handle the
exception or terminate the program.
Run
QuotientWithException

Handling Input Mismatch Exception
4
By handling InputMismatchException, your program will
continuously read an input until it is correct.
Run
InputMismatchExceptionDemo

Exception Types
5
LinkageError
Error
Throwable
ClassNotFoundException
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
NullPointerException
IndexOutOfBoundsException
Many more classes
Many more classes
Many more classes
IllegalArgumentException

System Errors
6
LinkageError
Error
Throwable
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
Many more classes
Many more classes
Many more classes
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
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LinkageError
Error
Throwable
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
Many more classes
Many more classes
Many more classes
Exception describes errors
caused by your program
and external
circumstances. These
errors can be caught and
handled by your program.

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

Checked Exceptions vs. Unchecked
Exceptions
9
RuntimeException, Error and their subclasses are
known as unchecked exceptions. All other
exceptions are known as checked exceptions,
meaning that the compiler forces the programmer
to check and deal with the exceptions.

Unchecked Exceptions
10
In most cases, unchecked exceptions reflect programming
logic errors that are not recoverable. For example, a
NullPointerException is thrown if you access an object
through a reference variable before an object is assigned to
it; an IndexOutOfBoundsException is thrown if you access
an element in an array outside the bounds of the array.
These are the logic errors that should be corrected in the
program. Unchecked exceptions can occur anywhere in the
program. To avoid cumbersome overuse of try-catch
blocks, Java does not mandate you to write code to catch
unchecked exceptions.

Unchecked Exceptions
11
LinkageError
Error
Throwable
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
Many more classes
Many more classes
Many more classes
Unchecked
exception.

Declaring, Throwing, and Catching
Exceptions
12
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

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
13

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;
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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");
}
15

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;
}
16

Catching Exceptions
17
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
Suppose p2 is defined as follows:
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void p2() throws IOException {
if (a file does not exist) {
throw new IOException("File does not exist");
}
...
}

Catch or Declare Checked Exceptions
Java forces you to deal with checked exceptions. If a method
declares a checked exception (i.e., an exception other than Error
or RuntimeException), you must invoke it in a try-catch block or
declare to throw the exception in the calling method. For example,
suppose that method p1 invokes method p2 and p2 may throw a
checked exception (e.g., IOException), you have to write the code
as shown in (a) or (b).
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void p1() {
try {
p2();
}
catch (IOException ex) {
...
}
}
(a) (b)
void p1() throws IOException {
p2();
}

Rethrowing Exceptions
try {
statements;
}
catch(TheException ex) {
perform operations before exits;
throw ex;
}
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The finally Clause
try {
statements;
}
handling ex;
}
finally {
finalStatements;
}
21

Trace a Program Execution
try {
statements;
}
handling ex;
}
finally {
finalStatements;
}
Next statement;
22
animation
Suppose no
exceptions in the
statements

try {
statements;
}
handling ex;
}
finally {
finalStatements;
}
Next statement;
23
animation
The final block is
always executed

try {
statements;
}
handling ex;
}
finally {
finalStatements;
}
Next statement;
24
animation
Next statement in the
method is executed

try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
finally {
finalStatements;
}
Next statement;
25
animation
Suppose an exception
of type Exception1 is
thrown in statement2

try {
statement1;
statement2;
statement3;
}
handling ex;
}
finally {
finalStatements;
}
Next statement;
26
animation
The exception is
handled.

try {
statement1;
statement2;
statement3;
}
handling ex;
}
finally {
finalStatements;
}
Next statement;
27
animation
The final block is
always executed.

try {
statement1;
statement2;
statement3;
}
handling ex;
}
finally {
finalStatements;
}
Next statement;
28
animation
The next statement in
the method is now
executed.

try {
statement1;
statement2;
statement3;
}
handling ex;
}
handling ex;
throw ex;
}
finally {
finalStatements;
}
Next statement;
29
animation
statement2 throws an
exception of type
Exception2.

try {
statement1;
statement2;
statement3;
}
handling ex;
}
handling ex;
throw ex;
}
finally {
finalStatements;
}
Next statement;
30
animation
Handling exception

try {
statement1;
statement2;
statement3;
}
handling ex;
}
handling ex;
throw ex;
}
finally {
finalStatements;
}
Next statement;
31
animation
Execute the final block

try {
statement1;
statement2;
statement3;
}
handling ex;
}
handling ex;
throw ex;
}
finally {
finalStatements;
}
Next statement;
32
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. 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 calling methods.
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.
33

When to Use Exceptions
When should you use the try-catch block in the code?
You should use it to deal with unexpected error
conditions. Do not use it to deal with simple, expected
situations. For example, the following code
34
try {
System.out.println(refVar.toString());
}
catch (NullPointerException ex) {
System.out.println("refVar is null");
}

When to Use Exceptions
35
if (refVar != null)
System.out.println(refVar.toString());
else
System.out.println("refVar is null");

Defining Custom Exception Classes
36
 Use the exception classes in the API whenever possible.
 Define custom exception classes if the predefined
classes are not sufficient.
 Define custom exception classes by extending
Exception or a subclass of Exception.

Custom Exception Class Example
37
The setRadius method throws an exception if the radius is negative.
Suppose you wish to pass the radius to the handler, you have to
create a custom exception class.
Run
TestCircleWithRadiusException
CircleWithRadiusException
InvalidRadiusException

Assertions
An assertion is a Java statement that
enables you to assert an assumption
about your program. An assertion
contains a Boolean expression that
should be true during program
execution. Assertions can be used to
assure program correctness and avoid
logic errors.
38

Declaring Assertions
An assertion is declared using the new Java
keyword assert in JDK 1.4 as follows:
assert assertion; or
assert assertion : detailMessage;
where assertion is a Boolean expression and
detailMessage is a primitive-type or an Object
value.
39

Executing Assertions
When an assertion statement is executed, Java evaluates
the assertion. If it is false, an AssertionError will be
thrown. The AssertionError class has a no-arg constructor
and seven overloaded single-argument constructors of
type int, long, float, double, boolean, char, and Object.
For the first assert statement with no detail message, the
no-arg constructor of AssertionError is used. For the
second assert statement with a detail message, an
appropriate AssertionError constructor is used to match
the data type of the message. Since AssertionError is a
subclass of Error, when an assertion becomes false, the
program displays a message on the console and exits.
40

Executing Assertions Example
public class AssertionDemo {
public static void main(String[] args) {
int i; int sum = 0;
for (i = 0; i < 10; i++) {
sum += i;
}
assert i == 10;
assert sum > 10 && sum < 5 * 10 : "sum is " + sum;
}
}
41

Compiling Programs with Assertions
Since assert is a new Java keyword
introduced in JDK 1.4, you have to compile
the program using a JDK 1.4 compiler.
Furthermore, you need to include the
switch –source 1.4 in the compiler command
as follows:
javac –source 1.4 AssertionDemo.java
NOTE: If you use JDK 1.5, there is no need
to use the –source 1.4 option in the
command. 42

Running Programs with Assertions
By default, the assertions are disabled at runtime.
To enable it, use the switch –enableassertions, or
–ea for short, as follows:
java –ea AssertionDemo
Assertions can be selectively enabled or disabled
at class level or package level. The disable switch
is –disableassertions or –da for short. For example,
the following command enables assertions in
package package1 and disables assertions in class
Class1.
java –ea:package1 –da:Class1 AssertionDemo
43

Using Exception Handling or Assertions
Assertion should not be used to replace exception
handling. Exception handling deals with unusual
circumstances during program execution.
Assertions are to assure the correctness of the
program. Exception handling addresses robustness
and assertion addresses correctness. Like
exception handling, assertions are not used for
normal tests, but for internal consistency and
validity checks. Assertions are checked at runtime
and can be turned on or off at startup time.
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Another good use of assertions is place assertions in
a switch statement without a default case. For
example,
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switch (month) {
case 1: ... ; break;
...
default: assert false : "Invalid month: " + month
}

The File Class
The File class is intended to provide an abstraction
that deals with most of the machine-dependent
complexities of files and path names in a machine-
independent fashion. The filename is a string. The File
class is a wrapper class for the file name and its
directory path.
46

Obtaining file properties and manipulating file
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Reading Data from the Web
Just like you can read data from a file on
your computer, you can read data from a
file on the Web.
48

Reading Data from the Web
URL url = new URL("www.google.com/index.html");
After a URL object is created, you can use the
openStream() method defined in the URL class to
open an input stream and use this stream to create a
Scanner object as follows:
Scanner input = new Scanner(url.openStream());
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Run
ReadFileFromURL

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