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Exception Handling

Starring: ArrayIndexOutOfBOunds

Co-Starring: Ariane Rocket

IOException

RunTImeException

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Purpose:

In this lecture series we will learn about Exception Handling.

Our programs need to be able to handle run time problems that may arise due to logic or other flaws in our programs. We need to be able to recognize the common exception errors as well as throw our own exceptions to be able to handle unexpected run time problems.

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Resources:

Java Essentials Chapter 14 p.557

Java Essentials Study Guide Chapter 12 p.195

Lambert Comprehensive Appendix F

Big Java Chapter 14 p. 557

Deitel & Deitel “Java How to Program” Chapter 14 p.698

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Resources:

Barrons:

Page 15-16Pages 41, 79, 170, 180, 250, 252, 255,

375

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Intro:Exception handling is necessary as

many Java methods require you to deal with the possibility that the method will not work as specified.

We will discuss:

The AP AB Requirements

Errors vs Exceptions

Checked Exceptions

UnChecked Exceptions

Throwing our own Exceptions

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AP AB Subset Requirements:

Students are expected to understand the exceptions that occur when their programs contain errors , in particular:

NullPointerException ArrayIndexOutOfBoundsException

ArithmeticException ClassCastException

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Students are expected to be able to throw the unchecked IllegalStateException and NoSuchElementException in their own methods (principally when implementing collection ADTs).

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Checked exceptions are not in the subset. In particular, the try/catch/finally statements is not in the subset.

NOTE: Checked Exceptions & Try/Catch/Finally are NOT IN THE AP Subset

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Errors vs Exceptions

Errors are serious Run Time problems that usually are NOT practical to handle in a program

For example, an infinite loop results in Java throwing a StackOverflowError

Java defines a separate class for each kind of error in java.lang.Error

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Exceptions are divided into two categories:

Exceptions that Java REQUIRES the programmer to handle

IOException is one that MUST be handled

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Exceptions that the programmer has the option of handling

ArithmeticException

ArrayIndexOutOfBoundsException

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Examples of handling exceptions:

// catch division by zero

try

{

quotient = dividend / divisor;

System.out.println(“Successful division”);

}

catch (ArithmeticException e)

{

System.out.println(“ErrorA: “ + e.toString);

}

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// catch to catch an index out of range

try

{

a[x] = 0;;

System.out.println(“Successful Supscripting”);

}

catch (ArrayIndexOutOfBoundsException e)

{

System.out.println(“ErrorB: “ + e.toString);

}

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When Java detects and throws an exception, control is immediately transferred from the problem instruction in the try block to the catch statement.

Therefore, if no exception occurs then the remaining code in the try executes completely and the catch code never executes

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You can have these combined:// catch division by zerotry{

quotient = dividend / divisor;System.out.println(“Successful division”); a[x] = 0;;System.out.println(“Successful Supscripting”);

}catch (ArithmeticException e){

System.out.println(“ErrorA: “ + e.toString);}catch (ArrayIndexOutOfBoundsException e){

System.out.println(“ErrorB: “ + e.toString);}

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Checked Exceptions:

These are due to external circumstances that the programmer can not prevent

Therefore the compiler will make sure your program handles these exceptions (compiler forced)

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When you call a method that throws a checked exception, you MUST tell the system what to do if the exception is thrown

All children of IOException are checked exceptions and these are the most common types of Checked exceptions

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Look at our File I/O processing:

// instiantate a buffer to hold a chunk of the fileBufferedReader br = new BufferedReader(reader);

// read in 1 line of the file into the buffertry

{linein = br.readLine();

} catch (IOException e)

{linein = new String("-- An error has occurred

--");}

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We were forced to handle possible IO exceptions when we read from the buffered reader

This is an example of a checked exception

These exceptions describe a problem that is likely to occur at times regardless of how careful you are

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Checked errors ---- you must handle in your code methods that throw exception errors

TRY CATCH

try

{

// IO stuff

} catch (IOException e)

{

// do some S.O.P error msg // (use e)

} A Checked exception will occur only in the

context of a specific activity

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Students do not need to write their own checked exceptions but they need to be able to read and understand them when they occur in existing programs

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Unchecked Exceptions:

RunTime Exceptions can occur almost anywhere

These kind of errors are the programmers fault !!!

Unchecked exceptions halt the program

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While a file I/O error can occur for reasons out of your control, you are responsible for a NullPointerException because your code was badly designed in trying to access a NULL reference

Java’s compiler does not force you to handle these unchecked exceptions

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NullPointerException and IllegalArgumentException are examples of unchecked exceptions

All exceptions that extend the class java.lang.RuntimeException class are Unchecked exceptions

Here is the list of unchecked exceptions you are responsible for:

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NullPointerException --- attempt to access a Null Object

IllegalArgumentException --- An argument supplied is not legal for that method

ArrayIndexOutOfBoundsException --- Attempt to access an index element that is not in the Array’s range

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ClassCastException --- Occurs when an attempt is made to cast a variable to a class that it does not match

ArithmeticException --- division by ZERO for integers

IndexOutOfBoundsException --- thrown when an index is out of range (parent of

ArrayIndex and StringIndex)

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StringIndexOutOfBoundsException --- Thrown when an index is out of the range of the String’s size

NumberFormatException --- Thrown when an attempt to convert a String to one of the numeric types is made when the String does not have the appropriate format (child of IllegalArgument)

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Students need to be able to throw the Unchecked exceptions:

IllegalStateException --- Signals that a method has been invoked at an illegal or inappropriate time

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NoSuchElementException --- Thrown when an attempt to access a nonexistent element is made. Thrown by the nextElement method of an Enumeration to indicate that there are no more elements in the enumeration

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In these Examples of Exceptions, what type of exception would occur ?

int num = 21;

int count = 0;

System.out.println(num / count);

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In these Examples of Exceptions, what type of exception would occur ?

int num = 21;

int count = 0;

System.out.println(num / count);

ANS: ArithmeticException

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(Manager is a subclass of an Employee class)

Employee worker1 = new Employee( );

System.out.println((Manager)worker1);

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(Manager is a subclass of an Employee class)

Employee worker1 = new Employee( );

System.out.println((Manager)worker1);

ANS: ClassCastException as worker1 is an Employee NOT a Manager

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int[ ] anArray = new int[10];

anArray[11] = o;

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int[ ] anArray = new int[10];

anArray[11] = o;

ANS:ArrayIndexOutOfBoundsException

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String s = “string”;

System.out.println(s.substring(0,8));

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String s = “string”;

System.out.println(s.substring(0,8));

ANS:StringIndexOutOfBOundsException

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(I know we have not discussed Iterator yet, but try and reason this one out)

ArrayList friends = new ArrayList( );

Iterator it2;

for (it2 = friends.iterator( ); it2.hasNext( ) ; )

{

String temp2 = (String)it2.next( );

System.out.println(temp2);

}

System.out.println(it2.next( ) );

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(I know we have not discussed Iterator yet, but try and reason this one out)

ArrayList friends = new ArrayList( );Iterator it2;for (it2 = friends.iterator( ); it2.hasNext( ) ; )

{String temp2 = (String)it2.next( );System.out.println(temp2);

}System.out.println(it2.next( ) );

ANS:NoSuchElementException on the last output

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The for loop iterator traverses the entire array so the final output asks for the next element in the ArrayList that does not exist

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Manager worker1 = null;

String temp = worker1.getName( );

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Manager worler1 = null;

String temp = worker1.getName( );

ANS: NullPointerException;

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int units = Integer.parseInt(“1234a”);

System.out.println(units + 1);

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int units = Integer.parseInt(“1234a”);

System.out.println(units + 1);

ANS: NumberFormatException

as “1234a” is not a valid number

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Throwing Exceptions:

Throwing exceptions can ensure that certain conditions exist when your programs are executed

Frequently, especially in the AP exam, PRECONDITIONS are set for any caller of a method to meet

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However, we can ensure proper data by checking ourselves

When you detect an error, you may throw an appropriate exception

The throw statement is used to invoke the exception handling mechanism

To throw an exception, you need to create an instance of an Exception Object with the new operator and then throw it

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if (withDrawAmount > balance)

{

IllegalArgumentException exception = new

IllegalArgumentException(“Amount exceeds Balance”);

throw exception;

}

else

balance -= wothDrawAmount;

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You can simplify the exception creation:

throw new IllegalArgumentException(“Amount exceeds Balance”);

At that point the current flow of your code is interrupted and control passes to the exception handler

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Students must be able to THROW exceptions:

if (radius < 0)

throw new IllegalArgumentException (“bad radius value”);

else

I = Math.Pi * r * r;

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Unchecked errors do not require try / catch

The exceptions students will be required to throw are:

IllegalArgumentException IllegalStateException NoSuchElementException

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Examples:public void race(double raceLength){

if (raceLength < = 0){ throw new IllegalArgumentException(“Race length must be greater than zero”);}numberOfRaces++;milesRaced += raceLength;

}

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public BankAccount (double initialBalance)

{

if (initialBalance < 0)

{

throw new IllegalStateException(“Initial balance must be non-negative”);

}

balance = initialBalance;

}

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The NoSuchElementException will be discussed when we learn about Linked Lists

However, this error will result when a Linked List attempts to remove the first element from an empty List

Lets look at the Java Doc for java.util.LinkedList and examine the removeFirst method

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LinkedList myLL = new LinkedList;

myLL.remove( );

will result in a

NoSuchElementException

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What is critical for the AP Exam:

Know the Conditions under which the common (listed in the previous section) exceptions are thrown

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What is critical for the AP Exam:

Be able to throw the IllegalStateException and the NoSuchElementException in your own projects

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What is critical for the AP Exam:

Students should understand these unchecked exceptions:

NullPointerException ArrayIndexOutOfBOundsException ArithmeticException (divide by

zero) ClassCastException IllegalArgumentException

IOException is also helpful to know

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Java Docs:

Open up Java Docs and look at the exceptions the ArrayList, for example, throws

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Ariane Rocket Incident:

Discuss this ESA Story from Java Essentials Chapter 14 pp.576-577

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Projects:

Various Multiple Choice and Free Response Questions

All Barrons M/C and Free Response Questions on Exception Handling

From this point forward, all potential unchecked exceptions MUST be handled

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NO Specific Test for This Section !!!