EXCEPTIONS

1
EXCEPTIONS

• Def An exception is a run-time error.
• Examples include attempting to divide by zero,
  or manipulate invalid data.

2
Handling Exceptions within Applications

• The try-catch statement
• This includes a block of statements that may
  throw an exception the try block.
• That is, where an exception may occur.
• This also includes 1 or more catch clauses.
• Each catch clause is a handler for a different
  type exception.
• If no exceptions are thrown, what happens?
• The catch clauses are skipped.

3

• When an exception occurs, the catch block
  handling the corresponding exception type is
  executed, and then control is transferred to the
  statement immediately following the try-catch
  statement.
• The try-catch statement may be augmented by a
  finally block.
• The finally block is executed whether the try
  statements execute successfully or a catch clause
  is executed.
• See examples bottom p. 536 (LewLoftus)

4

• If an exception is not caught and handled where
  it occurs, control is returned to each preceding
  method that was called.
• Refer to pp.539-541. What lines of code are
  never executed because the exception was not
  caught?
• We can write our own exception classes by having
  them extend the class Exception, or one of its
  descendants.
• Hierarchy of Java exception classes p.542

5
The throws clause in a method header.

• Exceptions are classified as either checked or
  unchecked.
• Checked exceptions must be either caught or must
  be listed in the throws clause of every method
  that may either catch it or propagate it.
• Example IOExceptions (see IOSample.java)
• An unchecked exception requirres no throws
  clause. These are only of RuntimeException or
  its descendants.

6
I/O Concepts

• Def Stream an ordered sequence of bytes that
  flows from a source to a destination.
• A stream is either an input stream (for reading
  information) or an output stream (for writing
  information).
• A program can handle multiple streams of both
  types but a particular store of data, or file,
  can serve as only an input stream or an output
  stream, but not both at the same time.

7
The class System

• The class System contains three public static
  object variables for I/O
• System.in, System.out, System.err.
• These objects represent the standard IO devices
  keyboard for input and monitor for output.
• Many other classes exist in the Java standard
  class library for defining I/O streams for
  dealing with streams of data for files and for
  data that is not treated as raw bytes but as
  characters.
• Most I/O class operations require the throws
  IOException in the method headers.