1. Throwing, Catching Defining
Exceptions
Throwing, Catching
Defining

2. Outcomes Find the exception that crashed a program
find where it happened in your code
Write a try-catch block to catch exceptions your program might throw
Design and place try-catch blocks so your program recovers gracefully
Create your own Exception class

3. Exceptional Circumstances
Reading numbers from user
User types in a word
not the sort of thing we expected
program crashes
Enter a number: 20
Enter another number: done
Exception in thread "main" java.util.InputMismatchException
at java.util.Scanner.throwFor(Scanner.java:840)
at java.util.Scanner.next(Scanner.java:1461)
at java.util.Scanner.nextDouble(Scanner.java:2387)
at SumNumbers.main(SumNumbers.java:26)
see SumNumbers.java

4. Exceptional Circumstances
Reading file name from user
User gives name of a file that doesn’t exist
can’t open that file for input
program crashes
Enter the name of a file and I will sum its numbers: noSuchFile.txt
Exception in thread "main" java.io.FileNotFoundException: noSuchFile.txt (No such file or directory)
at java.io.FileInputStream.open(Native Method)
at java.io.FileInputStream.<init>(FileInputStream.java:120)
at java.util.Scanner.<init>(Scanner.java:636)
at SumUsersFile.main(SumUsersFile.java:20)
see SumUsersFile.java

5. Exceptions Our Scanner has thrown an exception
Exception in thread “main” java.util.InputMismatchException
Exception in thread “main” java.io.FileNotFoundException
It ran into an error it couldn’t fix
program “crashed”
eventually!

6. What Exception?
Exception in thread "main" java.util.InputMismatchException
at java.util.Scanner.throwFor(Scanner.java:840)
at java.util.Scanner.next(Scanner.java:1461)
at java.util.Scanner.nextDouble(Scanner.java:2387)
at SumNumbers.main(SumNumbers.java:26)
Exception in thread "main" java.io.FileNotFoundException: noSuchFile.txt (No such file or directory)
at java.io.FileInputStream.open(Native Method)
at java.io.FileInputStream.<init>(FileInputStream.java:120)
at java.util.Scanner.<init>(Scanner.java:636)
at SumUsersFile.main(SumUsersFile.java:20)

7. Where? Exception in thread "main" java.util.InputMismatchException
at java.util.Scanner.throwFor(Scanner.java:840)
at java.util.Scanner.next(Scanner.java:1461)
at java.util.Scanner.nextDouble(Scanner.java:2387)
at SumNumbers.main(SumNumbers.java:26)
Exception in thread "main" java.io.FileNotFoundException: noSuchFile.txt (No such file or directory)
at java.io.FileInputStream.open(Native Method)
at java.io.FileInputStream.<init>(FileInputStream.java:120)
at java.util.Scanner.<init>(Scanner.java:636)
at SumUsersFile.main(SumUsersFile.java:20)

8. Where??? Methods call other methods One method throws the exception
which may call other methods in turn
One method throws the exception
error message shows whole “stack” of calls
main
nextDouble
Exception in thread "main" java.util.InputMismatchException
at java.util.Scanner.throwFor(Scanner.java:840)
at java.util.Scanner.next(Scanner.java:1461)
at java.util.Scanner.nextDouble(Scanner.java:2387)
at SumNumbers.main(SumNumbers.java:26)
next
throwFor

9. Look for Your Program Many of the methods don’t belong to you
you can’t do anything about them
Some of the methods are yours
that’s where you can do something
line 26 of SumNumbers.java
Exception in thread "main" java.util.InputMismatchException
at java.util.Scanner.throwFor(Scanner.java:840)
at java.util.Scanner.next(Scanner.java:1461)
at java.util.Scanner.nextDouble(Scanner.java:2387)
at SumNumbers.main(SumNumbers.java:26)

10. Exercise Where did this exception get thrown from?
What methods were called?
What line(s) of your program are relevant?
Exception in thread "main" java.util.InputMismatchException
at java.util.Scanner.throwFor(Scanner.java:840)
at java.util.Scanner.next(Scanner.java:1461)
at java.util.Scanner.nextInt(Scanner.java:2091)
at java.util.Scanner.nextInt(Scanner.java:2050)
at csci1227.Scanner.nextInt(Scanner.java:169)
at ArraySearcher.search(ArraySearcher.java:178)
at ArraySearcher.main(ArraySearcher.java:58)

11. Exception Hierarchy Exception IOException RuntimeException
EOFException
ClassCastException
FileNotFoundException
ConcurrentModificationException
IndexOutOfBoundsException
IllegalArgumentException
ArrayIndexOutOfBoundsException
NoSuchElementException
StringIndexOutOfBoundsException
NullPointerException
InputMismatchException

12. Throwing Exceptions Throw an exception when errors happen
and you can’t deal with it properly
Throw an appropriate type of exception
illegal argument  IllegalArgumentException
out of bounds  IndexOutOfBoundsException
bad time for request  IllegalStateException
If necessary, make your own exception type
extend RuntimeException

13. Setters Throwing Exceptions
Client tries to set property to illegal value
length & width of recatngle must be >= 0
public void setWidth(double newWidth) {
// object to negative widths
if (newWidth < 0) {
String message = "Illegal width: " + newWidth;
throw new IllegalArgumentException(message); }
// new width OK – make the change
width = newWidth;

14. The throw Command Command to throw an exception is throw
throw new IllegalArgumentException(message);
Exception to throw usually created there
Exception constructor given a message
String message = "Illegal width: " + newWidth;
message is printed out if program crashes

15. Exercise
Rewrite the following constructor to throw an exception if either argument is negative
public Rectangle(double reqWidth, double reqHeight) {
if (reqWidth >= 0) {
width = reqWidth;
} else {
width = 0; }
if (reqHeight >= 0) {
height = reqHeight;
height = 0;

16. Catching Exceptions try-catch block
… // code that may throw an exception
} catch(ExceptionClass exceptionObject) {
… // code to handle the exception }
Try to do this, and if something goes wrong, do this to deal with it

17. Catching Exceptions For catching InputMismatchException
inside the number-reading loop
try {
num = kbd.nextDouble(); // may throw an exception
kbd.nextLine(); }
catch(InputMismatchException ime) {
System.out.println(“That’s not a number!”);
break; // exit from loop
Note: it’s a parameter!
see CatchInputMismatch.java

18. When No Exceptions Occur
while (num >= 0.0) {
sum += num;
System.out.print(“Enter another number: ”);
try {
num = kbd.nextDouble();
kbd.nextLine(); }
catch(InputMismatchException e) {
System.out.println(“That’s not a number!”);
break; // exit from loop

19. When An Exception Occurs
while (num >= 0.0) {
sum += num;
System.out.print(“Enter another number: ”);
try {
num = kbd.nextDouble(); // doesn’t even get to finish!
kbd.nextLine(); }
catch(InputMismatchException e) {
System.out.println(“That’s not a number!”);
break; // exit from loop

20. Notes InputMismatchException part of java.util
needs to be imported along with Scanner
import java.util.Scanner;
import java.util.InputMismatchException;
Our program exits the loop because of the break command
normally would continue on inside the loop
(or whatever comes after the catch block)

21. When an Exception Occurs…
Code in try block gets skipped
right out from where the exception occurred
// read an integer and save it in num
num = kbd.nextInt();
// skip the rest of the input line
kbd.nextLine();
if next thing isn’t an int, NONE of that happens
doesn’t read an integer (no integer there to be read!)
doesn’t save anything in num (num unchanged!)
doesn’t skip rest of line

22. Dealing with Exceptions
After catching the exception, the body of the catch block is executed
does whatever it says to do
print a message; break out of a loop
Question to ask yourself:
What do I want my program to do when it catches this exception?

23. What Do I Want to Do? Reading int values; non-int value entered
try { num = kbd.nextInt(); }
catch (InputMismatchException ime) {
end the loop?
break;
skip invalid data?
kbd.next(); }
but if you skip, what else might go wrong?

24. Exercise
What happens with the following code when the user enters a non-number?
while (num >= 0.0) {
sum += num;
System.out.print(“Enter another number: ”);
try {
num = kbd.nextDouble(); kbd.nextLine(); }
catch(InputMismatchException e) {
System.out.println(“That’s not a number!”); kbd.nextLine();
What goes wrong?
How might we fix it?

25. Bigger Try Blocks Enter a word instead of the first number
program crashes!
Catch blocks only catch exceptions from the corresponding try blocks
exceptions from outside don’t get caught
Try block can be as big as you want
so long as it stays inside the method

26. A Loop inside a try Block
Exception  stop reading numbers
even if it’s the first number!
int sum = 0;
try {
int num = kbd.nextInt();
while (num >= 0) {
sum += num;
num = kbd.nextInt(); }
catch (InputMismatchException ime) {}
System.out.println(“The sum is ” + sum);
What does the catch block do here?

27. Exercise
Revise the code so that the sum is only printed if the loop ends “normally”
that is, by the user entering a negative number
int sum = 0;
try {
int num = kbd.nextInt();
while (num >= 0) {
sum += num;
num = kbd.nextInt(); }
} catch (InputMismatchException ime) {
System.out.println(“The sum is ” + sum);
NOTE: You don’t need to add any code. Just move a piece of it!

28. Our Own Exception Classes
Usually use a built-in exception class
e.g. IllegalArgumentException
Can create our own exception classes
if none of the available ones is exactly right
or want to distinguish different problems
We will extend RuntimeException
extending Exception leads to complications
or some other suitable RuntimeException

29. Our Own Exception Class
NotEnufMilkException
represents not having enuf milk
public class NotEnufMilkException extends RuntimeException {
public NotEnufMilkException() {
super(“Not enuf milk!”); }
public NotEnufMilkException(String message) {
super(message);
see NotEnufMilkException.java

30. Our Own Exception Classes
Name ends with Exception
just so we’re clear about what it is!
Consists of two constructors
default constructor calls super with a default message (or no message)
super("Not enuf milk!");
constructor with String parameter calls super with that String
super(message);

31. Another Exception Class
NotEnufCookiesException
public class NotEnufCookiesException extends RuntimeException {
public NotEnufCookiesException() {
super(“Not enuf cookies!”); }
public NotEnufCookiesException(String message) {
super(message);
What exactly is the difference between this exception class definition and the previous one?
see NotEnufCookiesException.java

32. Catching Different Exceptions
Can have more than one catch block
each has a different exception it catches
try { … }
catch(NotEnufMilkException e) {…}
catch(NotEnufCookiesException e) {…}
if an exception is thrown, it goes to the matching catch block
if no matching block, it keeps going
until it is caught, or until it gets out the top (crash!)
see RoomReports.java

33. More Constructors
Every Exception class should have constructors for () and (String)
every built in Exception class does
Can add more of our own
assignment number is an int
public IllegalAssignmentException(int asgn) {
super("No such assignment: " + asgn); }
need to call super(String) – no super(char)!

34. Questions?