What does the "18" mean in

        at IndexPractice.main(IndexPractice.java:18)

A good answer might be:

It is the line number in the program where the exception occured.

Remember that it is really bytecode that is executing, not acutal Java source code. However the bytecode file contains information about how lines of source code and bytecodes.correspond. The other line numbers (for parseInt(), for example) are not very useful for us because they refer to the source code that is not part of our file.

Some exception types are checked exceptions, which means that a method must do something about them. The compiler checks each method to be sure that it does. There are two things a method can do with a checked exception:

1. Handle it in a catch{} block, or
2. throw it to the method that called this one.

For example, an IOException is a checked exception. In the past your programs have done something like this:

public class DoesIO
{
  public static void main ( String[] a ) throws IOException
  {
    BufferedReader stdin = 
        new BufferedReader ( new InputStreamReader( System.in ) );

    // More Java statements ...
  }
}

The reserved word throws is used to say that this method does not catch the IOException, and that if one occurs in this method it will be thrown to the method that called this one. (In this example, it will be thrown back to the Java runtime system.)

QUESTION 5:

(Thought question: ) What happens to the exception when it is thrown to the Java runtime system?