checked and unchecked exceptions

[Cameron Park]

Hi, can someone tell me what are checked exceptions and what are unchecked exceptions? Thanks.

[Jimmy Blakely]

Checked exceptions are exceptions that must be encased within a try/catch block or the compiler will complain. For example...
public static void main(String argv[])
{
RandomAccessFile r = new RandomAccessFile("iotest.txt","rw");
}
...would cause a compile time error. RandomAccessFile's constructor throws an IOException, and since IOException is not a subclass of RuntimeException, it is a checked exception. And since I'm not encasing it in a try/catch block, the compiler will complain.
The remedy is ...
public static void main(String argv[])
{
try {
RandomAccessFile r = new RandomAccessFile("iotest.txt","rw");
} catch (IOException e)
{
System.out.println("Checked Exception Caught");
}
}

On the other hand, the compiler doesn't check for runtime exceptions (unchecked exceptions). For example, since ArithmeticException is a subclass of RuntimeException, throwing it won't cause a compile time error.
For example...
public static void main(String argv[])
{
throw new ArithmeticException();
}
...would compile successful but give you an exception during runtime.
Hope that helped.

Jimmy Blakely,
Sun Certified Java Programmer
Travis County Certified Defensive Driver

[This message has been edited by Jimmy Blakely (edited August 29, 2001).]

[Cameron Park]

Hi, Jimmy, I was wondering there is is a list of checked exceptions and unchecked exceptions. There are times I get confused as to what type of exception a particular exception is. I know ArithmaticException is a unchecked. So is ArrayOutOfBounds. But it's because I worked with them alot. Is there more?

[Jimmy Blakely]

I'm not too sure, and hopefully somebody will correct me if I'm wrong.
Try this....whenever the instanceof operator returns true, it is a runtime exception and NOT a checked exception.
boolean b = new ArithmeticException() instanceof RuntimeException;
System.out.println(b);
Using this snippet of code, I've found the following...
Unchecked Exceptions (Runtime)
ArithmeticException
IllegalArgumentException
IndexOutOfBoundsException
StringIndexOutOfBoundsException
SecurityException
IllegalMonitorStateException

Checked Exceptions (Compile Time)
IOException
FileNotFoundException
InterruptedException
Those are all I could think of at the moment. Just use that code snippet to test any exceptions I haven't included. I
Hope I was of assistance.
Jimmy Blakely,
Sun Certified Java Programmer
Travis County Certified Defensive Driver

[Gurpreet Sachdeva]

Hello Cameron,
Jimmy is perfectly right.
RuntimeException and all its sub classes(including those which you made by extending RuntimeException class) are UNCHECKED exceptions.
IOException and all its sub classes(including those which you made by extending IOException or Exception class) are CHECKED exceptions.
Regards
Gurpreet Sachdeva
For Mock Exams and some useful information about Bitshift operator, inner classes, garbage collection,etc please visit: http://www.go4java.20m.com

[Cameron Park]

Thank you Jimmy and Gurpreet.