Strings are immutable. So we cannot change their letters directly—instead, we can convert them to a character array. And then, for each letter in it, we apply the shifting logic.
Caesar method. This accepts a String, and an int that indicates how many places each character is to be shifted. It handles out-of-range characters by subtracting, or adding, 26.
ToCharArray: We first convert the String to a char array. This is a mutable data structure: an element can be changed.
String: We return a String by converting the char array into one. We use the String constructor.
Strings Java program that applies Caesar cipher
public class Program {
static String caesar(String value, int shift) {
// Convert to char array.
char[] buffer = value.toCharArray();
// Loop over characters.
for (int i = 0; i < buffer.length; i++) {
// Shift letter, moving back or forward 26 places if needed.
char letter = buffer[i];
letter = (char) (letter + shift);
if (letter > 'z') {
letter = (char) (letter - 26);
} else if (letter < 'a') {
letter = (char) (letter + 26);
}
buffer[i] = letter;
}
// Return final string.
return new String(buffer);
}
public static void main(String[] args) {
// Test the cipher method.
String a = "test";
System.out.println(a);
System.out.println();
String b = caesar(a, 18);
String c = caesar(b, -18);
System.out.println(b);
System.out.println(c);
System.out.println();
String d = caesar(a, 1);
String e = caesar(d, -1);
System.out.println(d);
System.out.println(e);
System.out.println();
String f = "exxegoexsrgi";
String g = caesar(f, -4);
System.out.println(f);
System.out.println(g);
}
}
Output
test
lwkl
test
uftu
test
exxegoexsrgi
attackatonce
Espionage is part of human civilization. Even in 40 BC, messages were being intercepted. The Caesar cipher, unlike modern systems, is easy to break—given the right knowledge.