String Buffer

In some programming languages, a String Buffer is an alternative to a String. It has the ability to be altered through adding or appending, whereas a String is normally fixed or immutable.

In Java

Theory

Java's standard way to handle text is to use its String class. Any given String in Java is an immutable object, which means its state cannot be changed. A String has an array of characters. Whenever a String must be manipulated, any changes require the creation of a new String (which, in turn, involves the creation of a new array of characters, and copying of the original array). This happens even if the original String's value or intermediate Strings used for the manipulation are not kept.

Java provides an alternate class for string manipulation, called a StringBuffer. A StringBuffer, like a String, has an array to hold characters. It, however, is mutable (its state can be altered). Its array of characters is not necessarily completely filled (as oppose to a String, whose array is always the exact required length for its contents). Thus, it has the capability to add, remove, or change its state without creating a new object (and without the creation of a new array, and array copying). The exception to this is when its array is no longer of suitable length to hold its content. In this case, it is required to create a new array, and copy contents.

For these reasons, Java would handle an expression like

String newString = aString + anInt + aChar + aDouble;

like this:

String newString = (new StringBuffer(aString)).append(anInt).append(aChar).append(aDouble).toString();

Implications

Generally, a StringBuffer is more efficient than a String in string handling. However, this is not necessarily the case, since a StringBuffer will be required to recreate its character array when it runs out of space. Theoretically, this is possible to happen the same number of times as a new String would be required, although this is unlikely (and the programmer can provide length hints to prevent this). Either way, the effect is not noticeable in modern desktop computers.

As well, the shortcomings of arrays are inherent in a StringBuffer. In order to insert or remove characters at arbitrary positions, whole sections of arrays must be moved.

The method by which a StringBuffer is attractive in an environment with low processing power takes this ability by using too much memory, which is likely also at a premium in this environment. This point, however, is trivial, considering the space required for creating many instances of Strings in order to process them. As well, the StringBuffer can be optimized to "waste" as little memory as possible.

The StringBuilder class, introduced in J2SE 5.0, differs from StringBuffer in that it is unsynchronized. When only a single thread at a time will access the object, using a StringBuilder processes more efficiently than using a StringBuffer.

StringBuffer and StringBuilder are included in the java.lang package.

In .NET

Microsoft's .NET Framework has a StringBuilder class in its Base Class Library.

In other languages

In C++ and Ruby, the standard string class is already mutable, with the ability to change the contents and append strings, etc., so a separate mutable string class is unnecessary.
In Objective-C (Cocoa/OpenStep frameworks), the NSMutableString class is the mutable version of the NSString class.

External links

The JavaDocs of StringBuffer, StringBuilder and String.
The source code of these classes
Urban Performance Legends - An article which involves a discussion of immutable objects with respect to object-oriented design