EXCEPTION HANDLING AND TEXT I/O - Introduction to Java Programming: Comprehensive Version - page 459

must be nonnegative, but a negative argument is passed); the program can create an instance
of IllegalArgumentException and throw it, as follows:

IllegalArgumentException ex =
new IllegalArgumentException( "Wrong Argument" );
throw ex;

Or, if you prefer, you can use the following:

throw new IllegalArgumentException( "Wrong Argument" );

Note
IllegalArgumentException is an exception class in the Java API. In general,
each exception class in the Java API has at least two constructors: a no-arg con-
structor, and a constructor with a String argument that describes the exception.
This argument is called the exception message , which can be obtained using
getMessage() .

exception message

Tip
The keyword to declare an exception is throws , and the keyword to throw an exception
is throw .

throws vs. throw

12.4.3 Catching Exceptions
You now know how to declare an exception and how to throw an exception. When an excep-
tion is thrown, it can be caught and handled in a try-catch block, as follows: catch exception

try {
statements; // Statements that may throw exceptions
}
catch (Exception1 exVar1) {
handler for exception1;
}
catch (Exception2 exVar2) {
handler for exception2;
}
...
catch (ExceptionN exVarN) {
handler for exceptionN;
}

If no exceptions arise during the execution of the try block, the catch blocks are skipped.
If one of the statements inside the try block throws an exception, Java skips the remain-
ing statements in the try block and starts the process of finding the code to handle the
exception. The code that handles the exception is called the exception handler ; it is found
by propagating the exception backward through a chain of method calls, starting from the
current method. Each catch block is examined in turn, from first to last, to see whether
the type of the exception object is an instance of the exception class in the catch block.
If so, the exception object is assigned to the variable declared, and the code in the catch
block is executed. If no handler is found, Java exits this method, passes the exception to
the method that invoked the method, and continues the same process to find a handler. If
no handler is found in the chain of methods being invoked, the program terminates and
prints an error message on the console. The process of finding a handler is called catching
an exception .

exception handler
exception propagation