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