Exception Handling

• Example: NullPointerException
  

  String name = null;
  int n = name.length(); // ERROR
  

• Cannot apply a method to null
• Virtual machine throws exception
• Unless there is a handler, program exits with stack trace
  

  Exception in thread "main" java.lang.NullPointerException 
   at Greeter.sayHello(Greeter.java:25) 
   at GreeterTest.main(GreeterTest.java:6)
  

• Indicate error conditions by throwing exception object.
  For instance, for parameter validation:
  

  public double sqrt(double x)
  {
    if (x < 0.0)
      throw new IllegalArgumentException("invalid parameter: negative value");
    ....
  }
  

• javadoc for java.lang.Exception