cop 3503 fall 2012 shayan javed lecture 11

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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


Arise because logic coded by the programmer was incorrect.

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Logic Errors


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


Examples of Code Errors: Divide by zero

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Runtime Errors


Examples of Code Errors: Divide by zero Array out of bounds

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Runtime Errors


Examples of Code Errors: Divide by zero Array out of bounds Accessing a null pointer (reference)

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Runtime Errors


Examples of Code Errors: Divide by zero Array out of bounds Accessing a null pointer (reference) Integer overflow

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Runtime Errors


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 };


Syntax Error(s)

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Errors

int[] numbers = { 1.5, 5, 7 };


Syntax Error(s)

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Errors

int[] numbers = { 1.5, 5, 7 };


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



IllegalArgumentException

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System Errors

LinkageError

Error

AWTError

AWTException

Throwable


VirtualMachineError

IOException

Exception

RuntimeException

Object

ArithmeticException







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System Errors

Thrown by the Java Virtual Machine (JVM)

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System Errors


Represented by the Error class

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System Errors



Describes internal system errors

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System Errors



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


VirtualMachineError

IOException

Exception

RuntimeException

Object

ArithmeticException







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Checked Exceptions

LinkageError

Error

AWTError

AWTException

Throwable


VirtualMachineError

IOException

Exception

RuntimeException

Object

ArithmeticException







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Checked Exceptions

LinkageError

Error

AWTError

AWTException

Throwable


VirtualMachineError

IOException

Exception

RuntimeException

Object

ArithmeticException







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


or throw an Exception

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Exception Handling


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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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

}


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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

}


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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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Some useful methods in the Throwable class:

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Some useful methods in the Throwable class: String toString():

Returns a short description of the Exception

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String getMessage(): Returns a detailed description of the Exception

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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:


try {output = new java.io.PrintWriter(“text.txt”);output.println(“Welcome to Java”);output.close();


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:


try {output = new java.io.PrintWriter(“text.txt”);output.println(“Welcome to Java”);


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


After last statement of the catch block (if an Exception was caught)

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The finally block



When an Exception is thrown in try but NOT caught

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The finally block



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


It creates an Exception object and throws it

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Throwing Exceptions



and must also declare it in the method description

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Throwing Exceptions



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




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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To create a custom class: class should extend Exception

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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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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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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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Example:

public void setRadius(double newRadius) throws InvalidRadiusException

{ if (newRadius >= 0) radius = newRadius; else throw new InvalidRadiusException(radius); }

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} catch (InvalidRadiusException ex) {

System.out.println(“Invalid Radius: “ + ex.getRadius());

}

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} 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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Write your own custom exception class if you answer yes to one of the following:

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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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Would it help users if they can differentiate your exceptions from those thrown by classes from other vendors?

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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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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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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");

cop 3503 fall 2012 shayan javed lecture 11

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