The String and StringBuffer Classes

Modifying StringBuffers

The reverseIt method uses StringBuffer's append method to add a character to the end of the destination string: dest.

class ReverseString {
    public static String reverseIt(String source) {
        int i, len = source.length();
        StringBuffer dest = new StringBuffer(len);

        for (i = (len - 1); i >= 0; i--) {
            dest.append(source.charAt(i));
        }
        return dest.toString();
    }
}

If the appended character causes the size of the StringBuffer to grow beyond its current capacity, the StringBuffer allocates more memory. Because memory allocation is a relatively expensive operation, you can make your code more efficient by initializing a StringBuffer's capacity to a reasonable first guess, thereby minimizing the number of times memory must be allocated for it. For example, the reverseIt method constructs the StringBuffer with an initial capacity equal to the length of the source string, ensuring only one memory allocation for dest.

The version of the append method used in reverseIt is only one of the StringBuffer methods that appends data to the end of a StringBuffer. There are several append methods that append data of various types, such as float, int, boolean, and even Object, to the end of the StringBuffer. The data is converted to a string before the append operation takes place.

Inserting Characters

At times, you may want to insert data into the middle of a StringBuffer. You do this with one of StringBufffer's insert methods. This example illustrates how you would insert a string into a StringBuffer:

StringBuffer sb = new StringBuffer("Drink Java!");
sb.insert(6, "Hot ");
System.out.println(sb.toString());

This code snippet prints:

Drink Hot Java!

With StringBuffer's many insert methods, you specify the index before which you want the data inserted. In the example, "Hot " needed to be inserted before the 'J' in "Java". Indices begin at 0, so the index for 'J' is 6. To insert data at the beginning of a StringBuffer, use an index of 0. To add data at the end of a StringBuffer, use an index equal to the current length of the StringBuffer or use append.

Setting Characters

Another useful StringBuffer modifier is setCharAt, which replaces the character at a specific location in the StringBuffer with the character specified in the argument list.. setCharAt is useful when you want to reuse a StringBuffer.

The String and StringBuffer Classes