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The String and StringBuffer Classes (notes)

The java.lang package contains two string classes: String and StringBuffer. You've already seen the String class on several occasions in this tutorial. You use the String class when you are working with strings that cannot change. StringBuffer, on the other hand, is used when you want to manipulate the contents of the string on the fly.

The reverseIt method in the following code uses both the String and StringBuffer classes to reverse the characters of a string. If you have a list of words, you can use this method in conjunction with a sort program to create a list of rhyming words (a list of words sorted by ending syllables). Just reverse all the strings in the list, sort the list, and reverse the strings again. You can see the reverseIt method producing rhyming words in the example in Using Input and Output Streams that shows you how to use piped streams.

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();
    }
}

The reverseIt method accepts an argument of type String called source that contains the string data to be reversed. The method creates a StringBuffer, dest, the same size as source. It then loops backwards over all the characters in source and appends them to dest, thereby reversing the string. Finally, the method converts dest, a StringBuffer, to a String.

In addition to highlighting the differences between Strings and StringBuffers, this lesson illustrates several features of the String and StringBuffer classes: creating Strings and StringBuffers, using accessor methods to get information about a String or StringBuffer, modifying a StringBuffer, and converting one type of string to another.

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Note to C and C++ Programmers: Java strings are first-class objects, unlike C and C++ strings, which are simply null-terminated arrays of 8-bit characters.

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Why Two String Classes?

The Java development environment provides two classes that store and manipulate character data: String, for constant strings, and StringBuffer, for strings that can change.

Creating Strings and StringBuffers

The following statement taken from the reverseIt method creates a new StringBuffer in three steps: declaration, instantiation, and initialization.

StringBuffer dest = new StringBuffer(len);

These are the same steps for creating an object of any type.

Accessor Methods

The reverseIt method uses two accessor methods to obtain information about the source string: charAt and length. Both String and StringBuffer provide a number of other accessor methods, including some for inspecting substrings and getting the positions of a specific character.

Modifying StringBuffers

The reverseIt method uses StringBuffer's append method to add characters to dest. In addition to append, StringBuffer provides methods to insert characters into the buffer, modify a character at a specific location within the buffer, and so on.

Converting Objects to Strings

reverseIt converts the resulting StringBuffer to a String and returns the string. You can convert several different data types to Strings using String's valueOf method.

Converting Strings to Numbers

You can also use methods from the Integer, Float, Double, and Long classes to convert the contents of a String to a number.

Strings and the Java Compiler

The Java compiler uses Strings and StringBuffers behind the scenes to handle literal strings and concatenation.

Other Interesting Features

String and StringBuffer provide several other useful ways to manipulate string data, including concatenation, comparison, substitution, and conversion to upper and lower case. java.lang.String and java.lang.StringBuffer summarize and list all of the methods and variables supported by these two classes.

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