What is an exception?

• An exception is an event that causes an interruption in the normal flow of a running program.
• In java, occurrance of an exception is represented by an object
• The object is of type Exception or a subclass of it.

When are exception objects created?

• when an exceptional condition is detected at runtime, the method that detects it creates the exception object
• exceptional conditions, like other conditions, are detected using if-then-else.
• information can be placed into the exception object by using constructor arguments, as allowed by the exception (sub)class
• many methods from Java API library classes are defined to create exception objects
• you can write methods that create exception objects

What happens once an exception object is created?

• It must be passed along the call chain until coming to a method which will handle it
• the call chain starts with the currently running method, then goes to the method that called the currently running method, then goes to the method that called THAT method, and so forth "up the line".
• if none of the methods along the call chain will handle it, it is reported to the user by the JVM and program execution halts.
• The analogy used is to imagine the exception to be a ball, or better yet a "hot potato".
• The method that creates the exception object then throws it.
• A method interested in handling an exception contains code to catch it.
• throws and catch are Java keywords.

Is a method I write required to catch an exception thrown by a method it calls?

• No. (wait, there's more...)
• Exceptions actually come in two general flavors:
• unchecked exceptions require nothing of the caller
• these are of type RuntimeException or a subclass of it
• examples are IndexOutOfBoundsException and NullPointerException
• these kinds of exceptions can occur in nearly any method, so the caller should not be obliged to handle them
• the caller is, however, allowed to handle such exceptions
• checked exceptions must either be caught or else re-thrown by the caller
• these are of all exception classes other than the unchecked ones.
• this requirement is enforced ("checked") by the compiler; a compiler error results if not met
• In short, the caller of a method that throws a checked exception is required to either catch it or throw it

How will I know whether a method I am calling requires me to catch to re-throw an exception?

• Check the specification given in Method Detail of the method you want to call.
• If it contains a Throws: clause, then your method has to catch or throw the exception type(s) described there.
• Note: this information is not contained in the Method Summary section.
• Note: unchecked exceptions are generally not documented in the specification. Don't worry about them.

Enough questions and lists! I need some examples!

1. catching an exception thrown by a called method
2. re-throwing (propagating) an exception thrown by a called method
3. a method that creates and throws an exception
4. a method that creates and throws a customized exception
5. a method that both catches and throws an exception

Catching an exception thrown by a called method

/*
 *  Method to that simply returns true if given file exists and false otherwise
 */
 public boolean fileExists(String fileName) {
    try {
       FileReader myReader = new FileReader(fileName);
    } catch (FileNotFoundException e) {
       return false;
    }
    return true;
 }

Re-throwing (propagating) an exception thrown by a called method

/*
 *  Method that returns value read from an existing FileReader
 */
 public int getValue(FileReader myReader) throws IOException {
    return myReader.read();
 }

A method that creates and throws an exception

/*
 *  Method to yield a secret code, provided the supplied password is correct.
 *  If the password is incorrect, it throws a general Exception.
 */
 public int getCode(String password) throws Exception {
    if (password.equals("where's my code?")) {
       return 8397201;
    } else {
       throw new Exception("Invalid password: "+password);
    }
 }

A method that creates and throws a customized exception

/*
 *  Customized exception subclass, which must be stored in file BadPasswordException.java
 */
 public class BadPasswordException extends Exception {
    public BadPasswordException(String badPassword) {
       super("Invalid password: "+badPassword);
    }
 }

/*
 *  Method to yield a secret code, provided the supplied password is correct.
 *  If the password is incorrect, it throws a BadPasswordException.
 */
 public int getSecretCode(String password) throws BadPasswordException {
    if (password.equals("where's my code?")) {
       return 8397201;
    } else {
       throw new BadPasswordException(password);
    }
 }

A method that both catches and throws an exception

/*
 *  Method that stores and returns value read from an existing FileReader
 *  If an IOException occurs, store MAX_VALUE then propagate the exception.
 */
 public int setAndGetValue(FileReader myReader) throws IOException {
    try {
       this.value = myReader.read();
    } catch (IOException e) {
       this.value = Integer.MAX_VALUE;
       throw e;
    }
    return this.value;
 }

I think I understand the basics.