Review (from CS 170):   exception handling

  • Error reporting in Java is by "throwing" exceptions

  • How to handle exceptions in Java 

       try
   {
       // block of code that may throw exception
   }
   catch (Exception e)
   {
       // block of code that deal with exception
   }

  • Unhandled exceptions will cause the Java program to terminate

  • Why do we discuss exception here ?

    • Java has an exception hierarchy that is similar to the class hierarchy

Exception handling example

  • Example Java program that throws the ArrayIndexOutOfBoundsException exception: 

        public static void main(String[] args)
    {
        int[] a = new int[10];

        try
        {
            a[99] = 1;  // Index out of range
        }
        catch (ArrayIndexOutOfBoundsException e)
        {
	    System.out.println(e);
        }
    }

DEMO: 04-inheritance/25-exception/Demo.java

The inheritance hierarchy of exceptions

  • The exception types in Java is also organized as an inheritance hierarchy (just like classes)   --   the root is Ecception 

      ArrayIndexOutOfBoundsException is-a IndexOutOfBoundsException
  IndexOutOfBoundsException      is-a RunTimeException
  RunTimeException               is-a Exception

Exceptions and polymorphism

  • We can catch more general exceptions using exception types higher up in the exception hierarchy.

    Example: 

        public static void main(String[] args)
    {
        int[] a = new int[10];

        // Index out of range
        try
        {
            a[99] = 1;
        }
        catch (Exception e)   // Higher up in hierarchy
        {
	    System.out.println(e);
        }
    }

  • Use Exception will catch all types of exceptions

DEMO: 04-inheritance/25-exception/Demo2.java

Exceptions and polymorphism

  • You can use multiple catch clauses: 

        public static void main(String[] args)
    {
        int[] a = new int[10];

        // Index out of range
        try
        {
            if ( Math.random() < 0.5 )
	        a = null;
            a[99] = 1;
        }
        catch (ArrayOndexOutOfBoundsException e)
        {
	    System.out.println(e);
        }
        catch (Exception e)
        {
	    System.out.println(e);
        }
    }

  • You must use more specific exceptions first

DEMO: 04-inheritance/25-exception/Demo3.java