LECTURE
NOVEMBER 1 2004

ASSIGNMENTS
Fri 11/5 CodeLab #9
Mon 11/8 Assignment 3 Code
- Questions about Assignment 3?

CATCHING EXCEPTIONS

What?
- reliable program
How?
- correct results for all valid input
- robust: doesn't crash easily even for invalid input

What?
- *exception*
When?
- normal course of program execution

When?
- an exception occurs
What?
- exception is thrown

What?
- handling exceptions
How?
- so far, left to system

Example:
- AgeInputVer1
Results?
- java.lang.NumberFormatException: nine
     at java.lang.Integer.parseInt(Integer.java:405)
     at java.lang.Integer.parseInt(Integer.java:454)
     at AgeInput.getAge(AgeInput.java:46)
     at Ch8AgeInputMain.main(Ch8AgeInputMain.java:24)

What?
- exception is caught
How?
- *exception-handling* routine is executed
- syntax
    try {
        // block including a statement that could throw an exception
    } catch ( /* declaration of exception to catch */) {
        // exception-handling block
    }
  - catch clause must declare an exception type and identifier
    - e.g. Exception e
  - catch block only catches particular type of exception
    - e.g. if declared IOException ioe, won't catch NullPointerException
    - but does catch all children; e.g. Exception will catch IOException

What?
- control flow
When?
- exception occurs
How?
- try block executed through the statement that causes the exception
- skips the remainder of the try block
- executes the catch block
- continues after the try-catch block

What?
- control flow
When?
- no exception
How?
- try block executed completely
- catch block skipped
- continues after the try-catch block

Example:
  myAgeInputVer2.java

MULTIPLE CATCH BLOCKS

Why?
- code generates more than one type of exception
What?
- multiple catch blocks
How?
- syntax:
  try {
  } catch () {
  } catch () {
  ...
  }
- only one will be executed for a given exception
- checked in sequence: the first one that matches is executed
- put the most specialized at the top

What?
- no matching catch statement
How?
- system handles

Example:
  try {
    inputStr = bf.readLine();
    int age = Integer.parseInt(inputStr);
  } catch (IOException ioe) {
  } catch (NumberFormatException nfe) {
  }

RESERVED WORD FINALLY

Why?
- designate code that must always be executed, whether or not an exception occurs
- cleanup code
What?
- reserved word finally
How?
- syntax:
    try {
        // exception-throwing block
    } catch (<exception type> <identifier>) {
        // catch block
    } finally {
        // finally block
    }

Example:
  finallyEg.java

THROWING EXCEPTIONS

What?
- throwing exceptions
How?
- syntax:

  throw <a throwable object>;

  - <a throwable object>
    - can be any Exception subclass
    - typically use the most specialized subclass

Example:
  AgeInputVer3.java

TYPES OF EXCEPTIONS

What?
- exception hierarchy
How?
- Throwable