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1.13 Using a StringBuffer
One of the things
you may have noticed about the String class that
is used to represent strings in Java is that it is immutable. In
other words, there are no methods that allow you to change the
contents of a string. Methods that operate on a string return a new
string, not a modified copy of the old one. When you want to operate
on a string in place, you must use a StringBuffer
object instead.Example 1-13
demonstrates the use of a StringBuffer. It
interactively reads a line of user input, as Example 1-12 did, and creates a
StringBuffer to contain the line. The program then
encodes each character of the line using the
rot13 substitution cipher, which simply
"rotates" each letter 13 places
through the alphabet, wrapping around from Z back to A when
necessary. Because a StringBuffer object is being
used, you can replace each character in the line one-by-one. A
session with this Rot13Input program might look
like this:
% java je3.basics.Rot13Input
> Hello there. Testing, testing!
Uryyb gurer. Grfgvat, grfgvat!
> quit
%
The main( ) method of Example 1-13
calls another method, rot13( ), to perform the
actual encoding of a character. This method demonstrates the use of
the primitive Java char type and character
literals (i.e., characters that are used literally in a program
within single quotes).
Example 1-13. Rot13Input.java
package je3.basics;
import java.io.*; // We're doing input, so import I/O classes
/**
* This program reads lines of text from the user,
encodes them using the
* trivial "Rot13" substitution cipher, and then prints out the
encoded lines.
**/
public class Rot13Input {
public static void main(String[ ] args) throws IOException {
// Get set up to read lines of text from the user
BufferedReader in =
new BufferedReader(new InputStreamReader(System.in));
for(;;) { // Loop forever
System.out.print("> "); // Print a prompt
String line = in.readLine( ); // Read a line
if ((line == null) || line.equals("quit")) // If EOF or "quit"...
break; // ...break out of loop
StringBuffer buf = new StringBuffer(line); // Use a StringBuffer
for(int i = 0; i < buf.length( ); i++) // For each character...
buf.setCharAt(i, rot13(buf.charAt(i)));// ..read, encode, store
System.out.println(buf); // Print encoded line
}
}
/**
* This method performs the Rot13 substitution cipher. It "rotates"
* each letter 13 places through the alphabet. Since the Latin alphabet
* has 26 letters, this method both encodes and decodes.
**/
public static char rot13(char c) {
if ((c >= 'A') && (c <= 'Z')) { // For uppercase letters
c += 13; // Rotate forward 13
if (c > 'Z') c -= 26; // And subtract 26 if necessary
}
if ((c >= 'a') && (c <= 'z')) { // Do the same for lowercase letters
c += 13;
if (c > 'z') c -= 26;
}
return c; // Return the modified letter
}
}
