Introduction to Parameters and Objects - Building Java Programs: A Back to Basics Approach - page 160

There is no character at position 12 in s2 , but this call asks for characters starting
at position 8 that come before position 12, so this actually makes sense.
You have to be careful about what indexes you use, though. With the substring
method you can ask for the position just beyond the end of the String , but you can't
ask for anything beyond that. For example, if you ask for

s2.substring(8, 13) // out of bounds!

your program will generate an execution error. Similarly, if you ask for the charAt at a
nonexistent position, your program will generate an execution error. These errors are
known as exceptions.
Exceptions are runtime errors as mentioned in Chapter 1.

Exception
A runtime error that prevents a program from continuing its normal
execution.

We say that an exception is thrown when an error is encountered. When an excep-
tion is thrown, Java looks to see if you have written code to handle it. If not, program
execution is halted and you will see what is known as a stack trace or back trace. The
stack trace shows you the series of methods that have been called, in reverse order. In
the case of bad String indexes, the exception prints a message such as the following
to the console:

Exception in thread "main"
java.lang.StringIndexOutOfBoundsException:
String index out of range: 13
at java.lang.String.substring(Unknown Source)
at ExampleProgram.main(ExampleProgram.java:3)

You can use String s as parameters to methods. For example, the following pro-
gram uses String parameters to eliminate some of the redundancy in a popular chil-
dren's song:

1 public class BusSong {
2 public static void main(String[] args) {
3 verse("wheels", "go", "round and round");
4 verse("wipers", "go", "swish, swish, swish");
5 verse("horn", "goes", "beep, beep, beep");
6 }
7
8 public static void verse(String item, String verb, String sound) {
9 System.out.println("The " + item + " on the bus " +
10 verb + " " + sound + ",");