1 / 89 cop 3503 fall 2012 shayan javed lecture 11 programming fundamentals using java 1
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COP 3503 FALL 2012SHAYAN JAVED
LECTURE 11
Programming Fundamentals using Java
1
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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
LinkageError
Error
AWTError
AWTException
Throwable
ClassNotFoundException
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
NullPointerException
IndexOutOfBoundsException
Several more classes
Several more classes
Several more classes
IllegalArgumentException
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System Errors
LinkageError
Error
AWTError
AWTException
Throwable
ClassNotFoundException
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
NullPointerException
IndexOutOfBoundsException
Several more classes
Several more classes
Several more classes
IllegalArgumentException
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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)
LinkageError
Error
AWTError
AWTException
Throwable
ClassNotFoundException
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
NullPointerException
IndexOutOfBoundsException
Several more classes
Several more classes
Several more classes
IllegalArgumentException
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Checked Exceptions
LinkageError
Error
AWTError
AWTException
Throwable
ClassNotFoundException
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
NullPointerException
IndexOutOfBoundsException
Several more classes
Several more classes
Several more classes
IllegalArgumentException
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Checked Exceptions
LinkageError
Error
AWTError
AWTException
Throwable
ClassNotFoundException
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
NullPointerException
IndexOutOfBoundsException
Several more classes
Several more classes
Several more classes
IllegalArgumentException
Need to explicitly deal with Checked Exceptions:
try and catch them, or throw them
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Exception Handling
Keywords: try some code, catch any Exceptions
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Exception Handling
Keywords: try some code, catch any Exceptions
or throw an Exception
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Exception Handling
Keywords: try some code, catch any Exceptions
or 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) { .... }
}
(a)
void p1 () throws Exception {
riskyMethod();
}
(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 last statement of try block
After 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 Example:
try { System.out.println(refVar.toString());}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 Example:
Replace with:
try { System.out.println(refVar.toString());}catch (NullPointerException ex) { System.out.println("refVar is null");}
if (refVar != null) System.out.println(refVar.toString());else System.out.println("refVar is null");
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