Java utilities - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 351

10.11.2 java.lang.StringBuffer
String objects cannot be altered. Once they are created, they are immutable objects.
Concatenating two strings together creates a whole new string object in memory:

String str ="This is ";
str = str + " a new string object " ;

The str variable now references a new String object that holds “This is
a new string object „ . The String class maintains a pool of strings in
memory. String literals are saved there and new strings are added as they are
created. Extensive string manipulation with lots of new strings created with the
String append operation can therefore result in lots of memory taken up by
unneeded strings. Note, however, that if two string literals are the same, the
second string reference will point to the string already in the pool rather than
create a duplicate.
The class java.lang.StringBuffer provides a much more memory effi-
cient tool for building strings. The class works as follows:

StringBuffer strb = new StringBuffer ("This is ");
strb.append ("a new string object");
System.out.println (strb.toString ());

Yo u first create an instance of the StringBuffer class and then append new
strings to it using the append() method. Internally, the class holds a large
character array. Appending strings just involves filling places in the array with
new char values. It does not need to create any new objects. If the array is already
filled when you try to do an append() , then StringBuffer creates a larger
array and copies the old characters into the new one. All this happens internally,
completely transparently to you.

10.11.3 java.lang.StringBuilder
Java Version 5.0 added the StringBuilder class, which is a drop-in replace-
ment for StringBuffer in cases where thread safety is not an issue.
Because StringBuilder is not synchronized, it has better performance than
StringBuffer .Ingeneral, you should use StringBuilder in preference
over StringBuffer .Infact, the J2SE 5.0 javac compiler normally uses
StringBuilder instead of StringBuffer whenever you perform string con-
catenation, as in

System.out.println ("The result is " + result);

All the methods available on StringBuffer are also available on

StringBuilder ,soitreally is a drop-in replacement.