2.2 StringBuffer

StringBuffer objects are similar to String objects in that they both represent sequences of characters. The main difference is that a StringBuffer is mutable, while a String is not. The StringBuffer class defines a number of append() methods for adding characters to the end of the sequence, as well as a number of insert() methods for inserting characters anywhere in the sequence.

Many computations that produce a String object use a StringBuffer internally to build the string. For example, to write a method that takes a String object and returns a new String that contains the sequence of characters in reverse, you use a StringBuffer as follows:

public static String reverse(String s) {
    StringBuffer buf = new StringBuffer(s.length());
    for (int i = s.length()-1; i >= 0; i--) {
        buf.append(s.charAt(i));
    }
    return buf.toString();
}

After creating a new StringBuffer object, the method loops over the given string from the last character to the first character and appends each character to the end of the StringBuffer object. When the loop finishes, the StringBuffer object contains a sequence of characters that is the reverse of the sequence of characters in the given String object. The method finishes by calling the toString() method of the StringBuffer; this method returns a String object that contains the same sequence of characters as the StringBuffer object.