1. Testing and Exceptions

2. Exception Abnormal event occurring during program execution Examples
Manipulate nonexistent files
File file = new File(s); Scanner fileIn = new Scanner(file);
Improper array subscripting
int[] a = new int[3];
a[4] = 1000;
Improper arithmetic operations
a[2] = 1000 / 0;

3. Java treatment of an exception
If exception occurs and a handler is in effect
Flow of control is transferred to the handler
After handler completes flow of control continues with the statement following the handler
If exception occurs and there is no handler for it
The program terminates

4. Task Prompt and extract the name of a file
From that file, two integer values are to be extracted
Compute and display the quotient of the values

5. Implementation
public static void main(String[] args) throws IOException {
Scanner stdin = new Scanner(System.in);
System.out.print("Filename: ");
String s = stdin.nextLine();
File file = new File(s); Scanner fileIn = new Scanner(file);
int a = fileIn.nextInt();
int b = fileIn.nextInt();
System.out.println( a / b ); }

6. What can go wrong?
public static void main(String[] args) throws IOException {
Scanner stdin = new Scanner(System.in);
System.out.print("Filename: ");
String s = stdin.nextLine();
File file = new File(s); Scanner fileIn = new Scanner(file);
int a = fileIn.nextInt();
int b = fileIn.nextInt();
System.out.println( a / b ); }

7. How can we deal with the problems?
public static void main(String[] args) throws IOException {
Scanner stdin = new Scanner(System.in);
System.out.print("Filename: ");
String s = stdin.nextLine();
File File = new File(s);
Scanner fileIn = new Scanner(file);(
int a = fileIn.nextInt());
int b = fileIn.nextInt());
System.out.println( a / b ); }

8. Exception handlers
Code that might generate an exception is put in a try block
If there is no exception, then the handlers are ignored
For each potential exception type there is a catch handler
When handler finishes the program continues with statement after the handlers
try {
Code that might throw exceptions of types E or F }
catch (E e) {
Handle exception e
catch (F f) {
Handle exception f
More code

9. Introduce try-catch blocks
public static void main(String[] args) throws IOException {
Scanner stdin = new Scanner(System.in);
System.out.print("Filename: ");
String s = stdin.nextLine();
File file = new File(s); Scanner fileIn = new Scanner(file);
int a = fileIn.nextInt();
int b = fileIn.nextInt());
System.out.println( a / b ); }

10. Setting up the file stream processing
Scanner stdin = new Scanner(System.in);
System.out.print("Filename: ");
String s = stdin.nextLine();
// set up file stream for processing
Scanner fileIn = null;
try {
File file = new File(s);
fileIn = new Scanner(file); }
catch (FileNotFoundException e) {
System.err.println(s + “: Cannot be opened for reading");
System.exit(0);
How come the main() throws expression did not indicate it could throw a FileNotFoundException?

11. Run time exceptions Java designers realized
Runtime exceptions can occur throughout a program
Cost of implementing handlers for runtime exceptions typically exceeds the expected benefit
Java makes it optional for a method to catch them or to specify that it throws them
However, if a program does not handle its runtime exceptions it is terminated when one occurs

12. Getting the inputs try { int a = fileIn.nextInt();
int b = fileIn.nextInt();
System.out.println( a / b ); }
catch (InputMismatchException e) {
System.err.println(s + ": contains nonnumeric inputs");
System.exit(0);

13. Converting the inputs try { int a = fileIn.nextInt();
int b = fileIn.nextInt();
System.out.println( a / b ); }
catch (InputMismatchException e) {
System.err.println(s + ": contains non-integral inputs");
System.exit(0);
catch (NoSuchElementException e) {
System.err.println(s + ": does not contain two inputs");
catch (ArithmeticException e) {
System.err.println(s + ": unexpected 0 input value");