EXCEPTION HANDLING AND TEXT I/O - Introduction to Java Programming: Comprehensive Version - page 464

A test program that uses the new Circle class is given in Listing 12.8.

L ISTING 12.8

TestCircleWithException.java
1 public class TestCircleWithException {
2 public static void main(String[] args) {
3 try {
4 CircleWithException c1 = new CircleWithException( 5 );
5 CircleWithException c2 = new CircleWithException( -5 );
6 CircleWithException c3 = new CircleWithException( 0 );
7 }
8 catch (IllegalArgumentException ex) {
9 System.out.println(ex);
10

try

catch

}

11
12 System.out.println( "Number of objects created: " +
13 CircleWithException.getNumberOfObjects());
14 }
15 }

java.lang.IllegalArgumentException: Radius cannot be negative
Number of objects created: 1

The original Circle class remains intact except that the class name is changed to
CircleWithException , a new constructor CircleWithException(newRadius) is
added, and the setRadius method now declares an exception and throws it if the radius is
negative.
The setRadius method declares to throw IllegalArgumentException in the method
header (lines 25-32 in CircleWithException.java). The CircleWithException class would
still compile if the throws IllegalArgumentException clause (line 26) were removed from
the method declaration, since it is a subclass of RuntimeException and every method can
throw RuntimeException (an unchecked exception) regardless of whether it is declared in the
method header.
The test program creates three CircleWithException objects— c1 , c2 , and c3 —to test
how to handle exceptions. Invoking new CircleWithException(-5) (line 5 in Listing 12.8)
causes the setRadius method to be invoked, which throws an IllegalArgumentException ,
because the radius is negative. In the catch block, the type of the object ex is
IllegalArgumentException , which matches the exception object thrown by the setRadius
method, so this exception is caught by the catch block.
The exception handler prints a short message, ex.toString() (line 9 in Listing 12.8),
about the exception, using System.out.println(ex) .
Note that the execution continues in the event of the exception. If the handlers had not
caught the exception, the program would have abruptly terminated.
The test program would still compile if the try statement were not used, because the method
throws an instance of IllegalArgumentException , a subclass of RuntimeException (an
unchecked exception). If a method throws an exception other than RuntimeException or
Error , the method must be invoked within a try-catch block.

12.9

✓ ✓

What is the purpose of declaring exceptions? How do you declare an exception, and
where? Can you declare multiple exceptions in a method header?

Check

Point

12.10

What is a checked exception, and what is an unchecked exception?

12.11

How do you throw an exception? Can you throw multiple exceptions in one throw
statement?

12.12

What is the keyword throw used for? What is the keyword throws used for?