exceptions

Question 1

run-time errors

Answer 1

> Illegal/impossible operations to execute
Detected during program execution
But not detectable at compile time
Treated asexceptionsin Java
(examples: division by zero, using null reference, illegal format conversion)

Question 2

what to do when an error occurs…

alternatives

Answer 2

> Ignore error
Print error message and terminate
Have the method handle the error in the code where the problem lies as best as you can
Have the method pass it off to someone else to handle
Usually an error code is returned to whoever called the method

> Modern language approach:Exception!
(note: in language like C, a “core dump” takes place)

Question 3

how to indicate when a program generates an exception

Answer 3

“a program throws an exception”

In Java an exception is represented by an object

Question 4

what to do when an exception occurs…

Answer 4

do nothing:
>Usually your program will be terminated by the JVM
>Stack trace is printed showing where exception was generated (Red and blue in Eclipse window)

do something:
>In some applications aborting the program is not an option (Email processor, web/database server)
> We can “handle” the exception by “catching the exception” and defining code to be executed

Question 5

try

catch

Answer 5

Use try { } to enclose code that can potentially throw an exception

Use catch(EXCEPTIONTYPE e) { } to “catch” the exception and define the code to execute when the exception occurs

Question 6

why is try catch somewhat similar to if else

Answer 6

> when an exception occurs control “jumps” to the catch clause and code after the point where the exception occurred is not executed
If the exception is not thrown the code of the catch clause is ignored

Question 7

exception object

Answer 7

When an exception is thrown, a new exception object is created, which encodes information about the nature of the exception

Question 8

Exception(String message)

Answer 8

Constructor taking an explanation as an argument

Question 9

String getMessage()

Answer 9

Returns a message describing the exception

Question 10

void printStackTrace()

Answer 10

Prints the contents of the Java call stack at the time of the exception

Question 11

ArithmeticException

Answer 11

divide by zero

Question 12

NullPointerException

Answer 12

attempt to access an object with a null reference

Question 13

IndexOutOfBoundsException

Answer 13

array or string index out of range

Question 14

ArrayStoreException

Answer 14

attempting to store an object of type X in an array of type Y

Question 15

EmptyStackException

Answer 15

attempt to pop an empty Stack (java.util)

Question 16

IOException

Answer 16

attempt to perform an illegal input/output operation (java.io)

Question 17

NumberFormatException

Answer 17

attempt to convert an invalid string into a number (e.g., when calling Integer.parseInt( ) )

Question 18

Exception

Answer 18

The most generic type of exception.
All of these are Objects in Java’s class library (most in java.lang)
Every Exception object supports a group of methods (these methods are defined in this stack of flashcards)

Question 19

exception propagation

Answer 19

(looks for exception handlers)
Java pops back up the call stack to each of the calling methods to see whether the exception is being handled in a catch block of the method

Question 20

what happens when your code does not catch your exceptions

Answer 20

java aborts your program

Question 21

try

finally

Answer 21

similar to try catch, however finally always runs (whether there is an error or not)
(For example if the return occurs before the finally clause, the clause will be executed and then we will return)

Question 22

write your own exceptions…

Answer 22

> You can throw exceptions using throw

>Never leave the catch clause empty (when in doubt call printStackTrace)