Fastest way to add spaces to a String?

Question

I need to have strings with a fixed width containing variable length text. The following (working) code does this with stupid if() and for() statements:
        StringBuffer levelName = new StringBuffer(record.getLevel().getName());
        if (levelName.length() < LEVEL_WIDTH) {
            int g = LEVEL_WIDTH - levelName.length();
            for (int i = 0; i < g; i++) {
                levelName.append(' ');
        sb.append(levelName); As this code is called a lot of time, i'm looking for a more efficient way to expand strings to a fixed width (by adding spaces). is there a better performing solution?

Answer 1

In this simple padding situation, there's no real advantage to using a StringBuffer over just a char array:
//(1) do this once:
char[] spaces = new char[LEVEL_WIDTH];
char[] buffer = new char[LEVEL_WIDTH];
java.util.Arrays.fill(spaces, ' ');
//(2) Do this whenever you pad:
int len = yourString.length();
if (len < LEVEL_WIDTH) {
    System.arraycopy(yourString, 0, buffer, 0, len);
    System.arraycopy(spaces, 0, buffer, len, LEVEL_WIDTH-len);
    yourString = new String(buffer); //(*)
}Notes:
1. First off, it should be noted that trying to optimize before profiling could be a waste of time: what if the next operation is a database query that takes 100,000 times are long as this?
2. arraycopy is a native op, so it should run faster than a loop. Arrays.fill calls arraycopy, btw.
3. For efficiency, I reuse arrays buffer and spaces.
4. The char array is copied in line (*). It would be nice to avoid that copying: to copy the original string and append padding directly into the strings buffer, but I don't see how that's possible. Using a StringBuffer involves at least as much copying, as far as I can tell (he says, staring at the source code)...