Chapter 8

Section 8.1

• an exception is an object that defines an unusual or erroneous situation
• it is thrown by a program or runtime environment, and can be caught and handled appropriately
• Java has a predefined set of exceptions and errors that may occur during execution of a program – listed in Appendix K
• programs can not handle an exception at all, handle it where it occurs, or handle it somewhere else in the program
• if an exception isn’t handled at all, the program terminates abnormally and produces a message describing what exception occurred and where
• first line indicates exception thrown and some information about why, then remaining lines are call stack trace to show where it occurred
• listing 8.1 p.379
• to catch and handle an exception, use try-catch block
• try identifies a block of statements that may throw an exception
• catch clause follows try block and defines how to handle the exception
• a try block can have several catch clauses, each called an exception handler
• statements in a try block are executed, then if no exception is thrown, execution continues after try-catch block – this is normal flow that we’d like to happen
• if exception is thrown in try block, control immediately transfers to appropriate catch clause – control transfers to first catch clause whose exception class corresponds to the class of the exception thrown
• after whole catch clause is executed, control continues after try block
• listing 8.2, p. 381
• try statement can have optional finally clause, a section of code executed no matter how try block is exited
• finally clause is listed after all catch clauses, and is executed after try if no exception is thrown, or after catch if an exception is thrown
• if an exception is not caught and handled immediately, control returns to the method that called the method that caused the exception
• we could handle exception there, otherwise control goes back to method that called it – this is exception propagation – it continues until control propagates back to main, then the program ends
• listing 8.3, p. 384
• listing 8.4, p. 385
• we can define our own exception classes by deriving them from Exception or its descendants
• listing 8.5, 8.6 p. 388, 389
• uses throw statement to begin exception propagation
• exceptions can be checked or not
• checked exceptions must be caught or listed in the throws clause of any method that may throw or propagate it
• unchecked exceptions are RuntimeException or any of its descendants and should not be caught or have a throws clause
• errors are like unchecked exceptions

Section 8.2

• a stream is an ordered sequence of bytes
• comes from analogy that we read from a source and write to a sink
• we treat a stream as an input stream from which we read, or an output stream to which we write
• a particular file, for example, can be either, but not both at the same time
• classes for streams are divided into input and output streams, but then also into the type of information on which they operate – character data versus byte data, and the role they play – data streams versus processing streams
• character stream manages 16-bit Unicode characters – it is just a long series of characters read or written in chunks
• byte stream manages 8-byt bytes of raw binary data, typically used for reading and writing binary data like sounds and images
• InputStream and OutputStream classes represent byte streams
• Reader and Writer classes represent character streams
• data stream represents a source or destination stream, like a string in memory or a file on disk
• processing stream or filtering stream performs manipulation on the data in a stream
• these streams are the same classes mentioned above, just characterized differently
• IOExceptions can be thrown during I/O operations

Section 8.3

• standard I/O streams are System.in, System.out and System.err
• System class has three object references, in, out and err, that are public and static as we’ve seen
• System.in usually represents keyboard input, while the other two map output to the monitor, usually
• System.out and System.err are declared to be PrintStream references, and System.In is a generic InputStream reference
• PrintStream objects have print and println defined already

Section 8.4

• FileReader class represents an input file with character data that we can read from
• its constructors link it to the program and open a stream from which data can be read
• listing 8.7, p.397
• FileWriter class represents an output file that we can write to
• listing 8.9, p. 402

Section 8.5

• when a program ends, data it was using is destroyed unless we store it externally, like writing it to a file
• persistence is the concept of objects living outside of the program where they were created
• can create persistent objects in Java by object serialization
• transform it into a series of bytes that represents the object, then store it into a file
• it can later be read from the file and restored to the original object
• must implement Serialize interface
• to serialize, call writeObject method of an ObjectOutputStream
• to deserialize, call readObject method of an ObjectInputStream
• if don’t want to serialize all the information in an object, like passwords for example, use transient when declare that variable