Using Exceptions

Introduction
When the JVM finds a run-time error in executing a program, it throws an exception. This means it terminates the execution and prints a trace of the contents of the run-time stack. This stack is used to store local variables and addresses of the next statment to be executed when a method is invoked. There are several types of exceptions; but they are all subclasses of Throwable. The two direct subclasses of Throwable are Error and Exception.

• The Error class covers errors in the system that you cannot correct, such as errors in the run-time system. You cannot alter your program to detect these errors.
• The Exception class covers two types of errors. The first are called checked errors and they extend Exception. You must alter your program to recognize these errors. An example of a checked error is the IOException which monitors input/output. So for instance if you read bytes from the console, with for example System.in.read(b), you must place this statement in a try-catch block or write throws IOException after the method header. This is called checking the exception. A class that inherits from Exception must be checked. You can write your own class that inherits from the Exception class, for instance:

  public class NegativeNumber extends Exception
  {
        public NegativeNumber()
        {
           super();
        }
  
        public NegativeNumber(String s)
        {
           super(s);
        }
  }
  

  
  
  You can throw this exception when a negative integer ais encountered by writing
  
  

  if( a <0) throw new  NegativeNumber(a + " a negative value was encountered");
  

  However, the method containing this would need throws NegativeNumber to immediately follow the heading.

• The second type of exception is called an unchecked exception. These exception classes extend RuntimeException which itself extends Exception. The JVM automatically throws the API versions of these exceptions when certain run-time exceptions are encountered. For instance the JVM throws an IndexOutOfBoundsException exception when an array index exceeds the limit indicted by the array dimension. You don't have to check for this, hence the term unchecked exception. If you write your own unchecked exception you don't indicate in the heading of the method in which the exception is thrown that you are throwing the exception. Of course you must throw the exception. For example, you would write if(stack.isEmpty() ) throw StackIsEmptyException().