Transcript PPTX - UF CISE

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COP 3503 FALL 2012
SHAYAN JAVED
LECTURE 11
Programming Fundamentals using Java
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Exception Handling
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Errors

Syntax Errors

Logic Errors

Runtime Errors
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Syntax Errors

Arise because language rules weren’t followed.
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Syntax Errors

Arise because language rules weren’t followed.

Detected by the compiler
 javac
for Java
 g++ for C++
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Logic Errors

Program compiles and runs, but results are wrong.
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Logic Errors


Program compiles and runs, but results are wrong.
Detected and fixed through testing.
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Logic Errors



Program compiles and runs, but results are wrong.
Detected and fixed through testing.
Arise because logic coded by the programmer was
incorrect.
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Logic Errors



Program compiles and runs, but results are wrong.
Detected and fixed through testing.
Arise because logic coded by the programmer was
incorrect.
 Example:
wrote c = a - b instead of c = a + b
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Runtime Errors

Occur when program is running – environment
detects it and can’t carry it out
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Runtime Errors


Occur when program is running – environment
detects it and can’t carry it out
Examples of Code Errors:
 Divide
by zero
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Runtime Errors


Occur when program is running – environment
detects it and can’t carry it out
Examples of Code Errors:
 Divide
by zero
 Array out of bounds
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Runtime Errors


Occur when program is running – environment
detects it and can’t carry it out
Examples of Code Errors:
 Divide
by zero
 Array out of bounds
 Accessing a null pointer (reference)
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Runtime Errors


Occur when program is running – environment
detects it and can’t carry it out
Examples of Code Errors:
 Divide
by zero
 Array out of bounds
 Accessing a null pointer (reference)
 Integer overflow
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Runtime Errors


Occur when program is running – environment
detects it and can’t carry it out
Examples of Code Errors:
 Divide
by zero
 Array out of bounds
 Accessing a null pointer (reference)
 Integer overflow

Programs crash when such exceptions are not
handled
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Errors
int[] numbers = { 1.5, 5, 7 };
System.out.prntln(numbers[numbers.length]);
Try to point out all the errors in this code
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Errors
int[] numbers = { 1.5, 5, 7 };
System.out.prntln(numbers[numbers.length]);
Syntax Error(s)
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Errors
int[] numbers = { 1.5, 5, 7 };
System.out.prntln(numbers[numbers.length]);
Syntax Error(s)
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Errors
int[] numbers = { 1.5, 5, 7 };
System.out.prntln(numbers[numbers.length]);
Syntax Error(s)
Runtime Error(s)
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Exception
An exception is an event, which occurs during the
execution of a program, that disrupts the normal
flow of the program’s instructions.
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Exception Handling in Java

Mechanism for handling exceptions by detecting
and responding to them in a systematic, uniform
and reliable manner.
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Exception Handling in Java

Mechanism for handling exceptions by detecting
and responding to them in a systematic, uniform
and reliable manner.

Any exceptions not handled within the Java
program are “caught” by the Java Runtime
Environment (JRE)
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Exception

A method in Java throws Exceptions
 “Something
went wrong”
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Exception

A method in Java throws Exceptions
 “Something

went wrong”
Exceptions are Objects
 Every
Exception is a subclass of the Exception class
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Unchecked Exceptions/Errors
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
LinkageError
VirtualMachineError
Error
AWTError
Several more classes
Several more classes
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System Errors
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
LinkageError
VirtualMachineError
Error
AWTError
Several more classes
Several more classes
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System Errors

Thrown by the Java Virtual Machine (JVM)
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System Errors

Thrown by the Java Virtual Machine (JVM)

Represented by the Error class
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System Errors

Thrown by the Java Virtual Machine (JVM)

Represented by the Error class

Describes internal system errors
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System Errors

Thrown by the Java Virtual Machine (JVM)

Represented by the Error class

Describes internal system errors

Rarely occur – if they do you can’t do much other
than terminating
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Runtime Exceptions (Unchecked)
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
LinkageError
VirtualMachineError
Error
AWTError
Several more classes
Several more classes
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Checked Exceptions
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
LinkageError
VirtualMachineError
Error
AWTError
Several more classes
Several more classes
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Checked Exceptions
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
Need to explicitly deal
with
Checked Exceptions:
try and catch them, or
throw them
LinkageError
VirtualMachineError
Error
AWTError
Several more classes
Several more classes
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Exception Handling

Keywords:
 try
some code, catch any Exceptions
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Exception Handling

Keywords:
 try
 or
some code, catch any Exceptions
throw an Exception
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Exception Handling

Keywords:
 try
 or
some code, catch any Exceptions
throw an Exception
 finally
execute some code
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Exception Handling

Java forces you to deal with checked Exceptions
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Exception Handling


Java forces you to deal with checked Exceptions
Two ways to deal with them:
void p1 () {
try {
riskyMethod();
}
catch (Exception ex) {
....
}
}
(a)
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Exception Handling


Java forces you to deal with checked Exceptions
Two ways to deal with them:
void p1 () {
try {
riskyMethod();
}
catch (Exception ex) {
....
}
void p1 () throws Exception {
riskyMethod();
}
}
(a)
(b)
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Exception Handling

Remember the clone() method?
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Exception Handling


Remember the clone() method?
Can be written in two ways:
Object clone() {
try {
return super.clone();
}
catch
(CloneNotSupportedException
ex) {
....
}
}
(a)
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Exception Handling


Remember the clone() method?
Can be written in two ways:
Object clone() {
try {
return super.clone();
}
catch
(CloneNotSupportedException
ex) {
....
}
}
(a)
Object clone() throws
CloneNotSupportedException {
return super.clone();
}
(b)
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Exception Handling

In the first case, we are catching and handling the
Exception
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Exception Handling

In the first case, we are catching and handling the
Exception

In the second case we are throwing it – needs to
be caught and handled by the calling method
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Catching Exceptions

A try-catch statement:
try {
// Statement(s) which throw Exceptions
} catch (Exception1 exception1) {
// Handles Exceptions of type Exception1
} catch (Exception2 exception2) {
// Handles Exceptions of type Exception2
} catch (Exception exception) {
// Handles Exceptions of type Exception
// ALL Exceptions
}
// Any code after the try-catch block
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Catching Exceptions

A try-catch statement:
try {
// Statement(s) which throw Exceptions
} catch (CloneNotSupportedException exception1) {
// Handles Exceptions of type CloneNotSupportedException
} catch (NullPointerException exception2) {
// Handles Exceptions of type NullPointerException
} catch (Exception exception) {
// Handles Exceptions of type Exception
// ALL Exceptions
}
// Any code after the try-catch block
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Catching Exceptions

A try-catch statement:
try {
Circle clone = circle1.clone();
} catch (CloneNotSupportedException exception1) {
// Handles Exceptions of type CloneNotSupportedException
} catch (NullPointerException exception2) {
// Handles Exceptions of type NullPointerException
} catch (Exception exception) {
// Handles Exceptions of type Exception
// ALL Exceptions
}
// Any code after the try-catch block
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Catching Exceptions

Thrown Exceptions have to be eventually caught
somewhere in your code
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Exception Information

So an Exception has been caught – what can we do
with it?
try {
// Statements which throw Exceptions
} catch (Exception exception) {
// ALL Exceptions
}
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Exception Information

Some useful methods in the Throwable class:
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Exception Information

Some useful methods in the Throwable class:

String toString():
 Returns a short description of the Exception
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Exception Information

Some useful methods in the Throwable class:

String toString():
 Returns a short description of the Exception

String getMessage():
 Returns a detailed description of the Exception
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Exception Information

Some useful methods in the Throwable class:

String toString():
 Returns a short description of the Exception

String getMessage():
 Returns a detailed description of the Exception

void printStackTrace():
 Prints the stacktrace information on the console
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Exception Information

Some useful methods in the Throwable class:

void printStackTrace():
 Prints the stacktrace information on the console
Sample output:
java.lang.NullPointerException at
MyClass.method2(MyClass.java:9) at
MyClass.method1(MyClass.java:6) at
MyClass.main(MyClass.java:3)
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Exception Information

Example:
java.io.PrintWriter output = null;
try {
output = new java.io.PrintWriter(“text.txt”);
output.println(“Welcome to Java”);
output.close();
}
catch (java.io.IOException ex){
System.out.println(ex.toString());
ex.printStackTrace() ;
}
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Problems

Example:
java.io.PrintWriter output = null;
try {
output = new java.io.PrintWriter(“text.txt”);
output.println(“Welcome to Java”);
output.close();
}
catch (java.io.IOException ex){
System.out.println(ex.toString());
ex.printStackTrace() ;
}
Must execute output.close() even if Exception occurs
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Solution

Use the finally clause – for code that must be
executed “no matter what”
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Solution

Use the finally clause – for code that must be
executed “no matter what”
try {
// Statement(s) which throw Exceptions
} catch (Exception1 exception1) {
// Handles Exceptions of type Exception1
} catch (Exception exception) {
// Handles Exceptions of type Exception
} finally {
// code executed whether there is an Exception or not
}
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Solution

Example:
java.io.PrintWriter output = null;
try {
output = new java.io.PrintWriter(“text.txt”);
output.println(“Welcome to Java”);
}
catch (java.io.IOException ex){
ex.printStackTrace() ;
} finally {
if (output != null)
output.close();
}
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The finally block

Executed when try block is exited in these ways:
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The finally block

Executed when try block is exited in these ways:
 After
last statement of try block
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The finally block

Executed when try block is exited in these ways:
 After
 After
last statement of try block
last statement of the catch block (if an Exception
was caught)
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The finally block

Executed when try block is exited in these ways:
 After
last statement of try block
 After
last statement of the catch block (if an Exception
was caught)
 When
an Exception is thrown in try but NOT caught
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The finally block

Executed when try block is exited in these ways:
 After
last statement of try block
 After
last statement of the catch block (if an Exception
was caught)
 When
an Exception is thrown in try but NOT caught
Executed even if there is a return statement prior to
reaching the finally block
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Throwing Exceptions

If written code could encounter a runtime error:
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Throwing Exceptions

If written code could encounter a runtime error:

It creates an Exception object and throws it
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Throwing Exceptions

If written code could encounter a runtime error:

It creates an Exception object and throws it

and must also declare it in the method description
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Throwing Exceptions

If written code could encounter a runtime error:

It creates an Exception object and throws it

and must also declare it in the method description
 Only
if the Exception is a checked Exception
 Optional for unchecked
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Throwing Exceptions

Example:
public void setRadius(double newRadius)
{
if (newRadius >= 0)
radius =
newRadius;
else
throw new IllegalArgumentException(
"Radius cannot be negative");
}
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Throwing Exceptions

Example:
OPTIONAL
public void setRadius(double newRadius) throws
IllegalArgumentException
{
if (newRadius >= 0)
radius =
newRadius;
else
throw new IllegalArgumentException(
"Radius cannot be negative");
}
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Throwing Exceptions
try {
Circle c1 = new Circle(5);
c1.setRadius(-5);
}
catch (IllegalArgumentException ex) {
System.out.println(ex);
}
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Throwing Exceptions
try {
Circle c1 = new Circle(5);
c1.setRadius(-5);
}
catch (IllegalArgumentException ex) {
System.out.println(ex);
}
Output:
Radius cannot be negative
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Creating Custom Exceptions

Create custom Exception classes if predefined
classes not sufficient
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Creating Custom Exceptions

Create custom Exception classes if predefined
classes not sufficient

To create a custom class:
 class
should extend Exception
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Creating Custom Exceptions

Create custom Exception classes if predefined
classes not sufficient

To create a custom class:
 class
should extend Exception
 Good practice to add:
A
default (empty) constructor
 A constructor with one String parameter
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Creating Custom Exceptions
public class InvalidRadiusException extends Exception {
private double radius;
public InvalidRadiusException() {
super(“Invalid radius!”);
}
public InvalidRadiusException(double radius) {
super("Invalid radius!”);
this.radius = radius;
}
public double getRadius() {
return radius;
}
}
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Creating Custom Exceptions

Example:
public void setRadius(double newRadius) throws
IllegalArgumentException
{
if (newRadius >= 0)
radius =
newRadius;
else
throw new IllegalArgumentException(
"Radius cannot be negative");
}
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Creating Custom Exceptions

Example:
public void setRadius(double newRadius) throws
InvalidRadiusException
{
if (newRadius >= 0)
radius =
newRadius;
else
throw new InvalidRadiusException(radius);
}
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Creating Custom Exceptions
try {
Circle c1 = new Circle(5);
c1.setRadius(-5);
}
catch (IllegalArgumentException ex) {
System.out.println(ex);
}
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Creating Custom Exceptions
try {
Circle c1 = new Circle(5);
c1.setRadius(-5);
}
catch (InvalidRadiusException ex) {
System.out.println(“Invalid Radius: “ +
ex.getRadius());
}
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Creating Custom Exceptions
try {
Circle c1 = new Circle(5);
c1.setRadius(-5);
}
catch (InvalidRadiusException ex) {
System.out.println(“Invalid Radius: “ +
ex.getRadius());
}
Output:
Invalid Radius: -5.0
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When to create Custom Exceptions

Use the exception classes in the API whenever possible.
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When to create Custom Exceptions

Use the exception classes in the API whenever possible.

Write your own custom exception class if you answer yes to
one of the following:
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When to create Custom Exceptions

Use the exception classes in the API whenever possible.

Write your own custom exception class if you answer yes to
one of the following:

Do you need an exception type that isn’t represented by Java?
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When to create Custom Exceptions

Use the exception classes in the API whenever possible.

Write your own custom exception class if you answer yes to
one of the following:

Do you need an exception type that isn’t represented by Java?

Would it help users if they can differentiate your exceptions from
those thrown by classes from other vendors?
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When to create Custom Exceptions

Use the exception classes in the API whenever possible.

Write your own custom exception class if you answer yes to
one of the following:

Do you need an exception type that isn’t represented by Java?

Would it help users if they can differentiate your exceptions from
those thrown by classes from other vendors?

Do you want to pass more than just a string to the exception handler?
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When to use Exceptions

Use if the event is exception and truly an error
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When to use Exceptions



Use if the event is exception and truly an error
Do not use it to deal with simple, expected
situations
try {
System.out.println(refVar.toString());
Example:
}
catch (NullPointerException ex) {
System.out.println("refVar is null");
}
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When to use Exceptions



Use if the event is exception and truly an error
Do not use it to deal with simple, expected
situations
try {
System.out.println(refVar.toString());
Example:
}
catch (NullPointerException ex) {
System.out.println("refVar is null");
}

Replace with:
if (refVar != null)
System.out.println(refVar.toString());
else
System.out.println("refVar is null");