The Ubiquitous IOException

As far as computer operations go, input and output are unreliable. They are subject to problems completely outside the programmer's control. Disks can develop bad sectors while a file is being read. Construction workers drop their backhoes through the cables that connect your WAN. Users unexpectedly cancel their input. Telephone repair crews shut off your modem line while trying to repair someone else's. (This last one actually happened to me while writing this chapter. My modem kept dropping the connection and then not getting a dial tone; I had to hunt down the Verizon "repairman" in my building's basement and explain to him that he was working on the wrong line.)

Because of these potential problems and many more, almost every method that performs input or output is declared to throw an IOException. IOException is a checked exception, so you must either declare that your methods throw it or enclose the call that can throw it in a try/catch block. The only real exceptions to this rule are the PrintStream and PrintWriter classes. Because it would be inconvenient to wrap a try/catch block around each call to System.out.println( ), Sun decided to have PrintStream (and later PrintWriter) catch and eat any exceptions thrown inside a print( ) or println( ) method. If you do want to check for exceptions inside a print( ) or println( ) method, you can call checkError( ):

public boolean checkError( )

The checkError( ) method returns TRue if an exception has occurred on this print stream, false if one hasn't. It tells you only that an error occurred. It does not tell you what sort of error occurred. If you need to know more about the error, you'll have to use a different output stream or writer class.

IOException has many subclasses15 in java.io aloneand methods often throw a more specific exception that subclasses IOException; for instance, EOFException on an unexpected end of file or UnsupportedEncodingException when you try read text in an unknown character set. However, methods usually declare only that they throw an IOException.

The java.io.IOException class declares no public methods or fields of significancejust the usual two constructors you find in most exception classes:

public IOException( )
public IOException(String message)

The first constructor creates an IOException with an empty message. The second provides more details about what went wrong. Of course, IOException has the usual methods inherited by all exception classes such as toString( ) and printStackTrace( ).

Java 6 also adds an IOError class that is "Thrown when a serious I/O error has occurred." Xueming Shen snuck this class in the backdoor solely to avoid declaring that methods in the new Console class throw IOException like they should. I am not sure if this wart will remain in the final version of Java 6 or not. At the time of this writing, I am lobbying strenuously to get this removed, or at least replaced by a runtime exception instead of an error.