1. Exceptions in Java
OOSSE - Programming with Java
21 Dec 2012 OOSSE - Java Lecture 1
2. Objectives
In this lecture, we will
• Introduce exception handling
• Discuss try, catch and finally
• Discuss checked and unchecked exceptions
21 Dec 2012 OOSSE - Java Lecture 9 2
3. Exceptions
• The Sun Java tutorial defines an exception, an exceptional
event, as:
“an event that occurs during the execution of a program that
disrupts the normal flow of instructions”
• Exceptions are generated when something goes wrong
– try to divide a number by zero
• ArithmeticException
– try to access a position past either end of an array
• index is negative, or index >= the array length
• ArrayIndexOutOfBoundsException
– attempt to read data of the wrong type, for example with a
Scanner object
• InputMismatchException
21 Dec 2012 OOSSE - Java Lecture 9 3
4. Exceptions
• Some types of exceptions are unpredictable
– They may happen on some runs through the program but not
others
– Unpredictable exceptions do not generate compiler errors
• When they do occur, the program terminates and throws an
exception
• For example:
Exception in thread "main"
java.lang.ArithmeticException: / by zero
at ExceptionEx.main(ExceptionEx.java:6)
Exception in thread "main"
java.lang.ArrayIndexOutOfBoundsException: 3
21 Dec 2012 OOSSE - Java Lecture 9 4
5. Handling Exceptions
• A program crash can be avoided by handling the exception
• Code that could cause an error is wrapped in a try catch
block
Scanner kybd = new Scanner(System.in);
try
{
int number = kybd.nextInt();
double price = kybd.nextDouble();
}
catch (InputMismatchException ime)
{
System.out.println("Wrong type of input");
System.out.println(ime.toString());
}
21 Dec 2012 OOSSE - Java Lecture 9 5
6. Handling Exceptions
• If an exception is generated within the try block then
execution immediately jumps to the catch block
– the remaining statements in the try block are not executed
• The catch block can contain code to deal with the problem
and handle the exception
– or output a message explaining what went wrong
• Regardless of whether an exception occurs, execution
continues after the catch block without a crash
• There can be more than one catch block
– to deal with different types of exception
• A finally block can exist after all the catch blocks
– executed regardless of whether or not an exception occurs
21 Dec 2012 OOSSE - Java Lecture 9 6
7. Handling Exceptions
• Code sample:
try {
int number = kybd.nextInt();
int answer = number / 0;
}
catch (ArithmeticException ae){
System.out.println(“Divide by zero");
}
catch (InputMismatchException ime){
System.out.println("Wrong type of input");
}
finally {
System.out.println("Finally end up here");
}
21 Dec 2012 OOSSE - Java Lecture 9 7
8. Dealing with an Exception
Scanner kybd = new Scanner(System.in);
int num;
boolean done = false;
while (!done){
try {
System.out.println("Please enter an integer");
num = kybd.nextInt();
done = true;
}
catch (InputMismatchException e){
String invalidInput = kybd.next();
System.out.println("Wrong input, try again");
}
}
21 Dec 2012 OOSSE - Java Lecture 9 8
9. Unchecked Exceptions
• The examples we have looked at so far are unchecked
exceptions
– Also known as run-time exceptions
• The exceptions are unpredictable
– They may or may not occur
– They could happen anywhere
• The compiler does not insist that they are handled
– It is not compulsory to use a try-catch block
– If an exception occurs outside of a try-catch block then the
program will terminate
21 Dec 2012 OOSSE - Java Lecture 9 9
10. Checked Exceptions
• Some other exceptions are more predictable
– An attempt to open a file that does not exist
FileNotFoundException
– An attempt to read beyond the end of a file
EOFException
– Both of these are types of IOException but some other
types of exception may also be predictable
• Checked exceptions are predictable
• If a method call can generate a checked exception you
must handle it
– otherwise will get a compiler error
21 Dec 2012 OOSSE - Java Lecture 9 10
11. Handling File I/O Exceptions
• All code that involves opening and accessing a file must
be wrapped in a try-catch block
try
{
PrintWriter pw = new PrintWriter("Payroll.txt");
pw.print(name);
pw.printf("%6.2f", hourlyPay);
// more file-writing code omitted here
pw.close();
}
catch(IOException e)
{
System.out.println("Error writing to file");
}
21 Dec 2012 OOSSE - Java Lecture 9 11
12. Handling File I/O Exceptions
try
{
Scanner inFile = new Scanner(new File(fileName));
while (inFile.hasNext())
{
name = inFile.next();
hourlyPay = inFile.nextDouble();
// more code here to process input
}
inFile.close();
}
catch(IOException e)
{
System.out.println("Error reading from file");
}
21 Dec 2012 OOSSE - Java Lecture 9 12
13. What is an Exception?
• The Exception class in Java is the basis for exception
handling
• The Exception class is a subclass of the Throwable class
– java.lang.Throwable
• The Exception class defines several methods including:
– getMessage()
– toString()
– printStackTrace()
• A method can raise an Exception if something is not right
– throw new IOException(“Unable to read data”);
• A method can specify that it may throw an Exception
– public String myMethod ( ) throws IOException ….
21 Dec 2012 OOSSE - Java Lecture 9 13
14. Separating Error-Handling Code from "Regular"
Code
• readFile { try {
• open the file;
• determine its size;
• allocate that much memory;
• read the file into memory;
• close the file; }
• catch (fileOpenFailed) { doSomething;
} catch (sizeDeterminationFailed) {
doSomething; } catch (memoryAllocationFailed) {
doSomething; } catch (readFailed) {
doSomething; } catch (fileCloseFailed) {
doSomething; }}
21 Dec 2012 OOSSE - Java Lecture 9 14
16. Summary
In this lecture we have:
• Introduced exception handling
• Discussed try, catch and finally
• Discussed checked and unchecked exceptions
21 Dec 2012 OOSSE - Java Lecture 9 16
Notas do Editor
Part of this lecture will be reserved for working through solutions to selected exercises from last week. Notes relating to this do not appear in the slides.
