Question

While reading about exception, I will always come across checked exceptions and unchecked exceptions, So wanted to know how to distinguish that which is what?

Edit: I want to know if i create any exception class then how can i create as a checked or as an unchecked?

and what is the significance of each?

Answer 1

All Throwables except subclasses of java.lang.RuntimeException or java.lang.Error are checked. Properly, in Java, “exceptions” are subclasses of java.lang.Exception, “errors” are subclasses of java.lang.Error and java.lang.Throwable is not usually subclassed directly.

Programs are not supposed to create their own Error subclasses (though the documentation is rather ambiguous on that) so generally you always create Exceptions, using a RuntimeException if you don’t want it to be checked.

To know at run-time if you have a checked exception you could use:

if(throwable instanceof Exception && !(throwable instanceof RuntimeException)) {
    // this is a checked Exception
    }

A checked exception is one which must be either handled in a catch clause, or declared as being thrown in the method signature; the compiler enforces this. Generally, one uses checked exceptions for exceptions which should be handled by the calling code, while unchecked exceptions are for conditions which are the result of a programming error and should be fixed by correcting the code.

That said there is much debate in the Java community about the efficacy of using checked exceptions vs. unchecked exceptions everywhere – a subject way to deep to discuss in this answer.

EDIT 2012-10-23: In response to comments (which are quite valid), to clarify, the following would be what is required to determine if a captured Throwable is a checked Throwable as opposed to a checked Exception:

if(obj instanceof Throwable && !(obj instanceof RuntimeException) && !(obj instanceof Error)) {
    // this is a checked Throwable - i.e. Throwable, but not RuntimeException or Error
    }

If the object in question is known to be an instance of Throwable (e.g. it was caught), only the second part of the above ‘if’ is needed (e.g. testing for Throwable is redundant).