Why am I getting whitespaces in StringBuilder in Java?

Question

I wrote a method that returns a rotated StringBuilder with a given key. However, although it is working fine, it's adding white spaces within the StringBuilder.

public static StringBuilder rotateCipher(String plain, int key) {
    int keyTemp = key;
    char[] rotatedChar = new char[plain.length()];
    StringBuilder builder = new StringBuilder();
    
    for (int i = 0; i < plain.length(); i++) {
        rotatedChar[i] = plain.charAt(key);
        key++;
        if (key == plain.length()) {
            builder.append(String.valueOf(rotatedChar));
            builder.append(plain.substring(0, keyTemp + 1));
            break;
        }
    }
    
    return builder;
}

Output: nopqrstuvwxyz            abcdefghijklmn

Answer 1

The reason for the whitespace is that the array rotatedChar does not have all it's elements filled. By default, a char[] contains only (char)0 elements.

When you call this method with the parameters "abcdefghijklmnopqrstuvwxyz", 13 then only the first 13 elements of rotatedChar get filled, then you hit the if-condition and break out of the loop. That means you have the remaining 13 elements left as 0s, which is a nonprintable character, so it appears as whitespace.

It's a bit hard to suggest which part to change here because as Gabe pointed out in the comments, the solution only requires 2 calls to substring.

If you really want to use loops, maybe this can be an approach:

for (int i = 0; i < plain.length(); i++) {
    rotatedChar[i] = plain.charAt(key);
    key++;
    if (key == plain.length()) {
        //this "restarts" taking chars from the beginning of the string
        key = 0;
    }
}
return String.valueOf(rotatedChar);