More Data Types and Operators - Java: A Beginner's Guide - page 219

strings. It must be made clear, however, that String reference variables may, of course,
change the object to which they refer. It is just that the contents of a specific String object
cannot be changed after it is created.

Ask the Expert

Q :

You say that once created , String objects are immutable. I understand that, from
a practical point of view, this is not a serious restriction, but what if I want to
create a string that can be changed?

A : You're in luck. Java offers a class called StringBuffer , which creates string objects
that can be changed. For example, in addition to the charAt( ) method, which obtains
the character at a specific location, StringBuffer defines setCharAt( ) , which sets a
character within the string. Java also supplies StringBuilder , which is related to
StringBuffer , and also supports strings that can be changed. However, for most pur-
poses you will want to use String , not StringBuffer or StringBuilder .

To fully understand why immutable strings are not a hindrance, we will use another of
String 's methods: substring( ) . The substring( ) method returns a new string that contains
a specified portion of the invoking string. Because a new String object is manufactured
that contains the substring, the original string is unaltered, and the rule of immutability re-
mains intact. The form of substring( ) that we will be using is shown here:

String substring(int startIndex , int endIndex )

Here, startIndex specifies the beginning index, and endIndex specifies the stopping point.
Here is a program that demonstrates substring( ) and the principle of immutable strings: