How to center a string using String.format?

Question

public class Divers {
  public static void main(String args[]){

     String format = \"|%1$-10s|%2$-10s|%3$-20s|\\n\";
     System.out.format(format, \"FirstName\",         


        

Answer 1

I was playing around with Mertuarez's elegant answer above and decided to post my version.

public class CenterString {

    public static String center(String text, int len){
        if (len <= text.length())
            return text.substring(0, len);
        int before = (len - text.length())/2;
        if (before == 0)
            return String.format("%-" + len + "s", text);
        int rest = len - before;
        return String.format("%" + before + "s%-" + rest + "s", "", text);  
    }

    // Test
    public static void main(String[] args) {
        String s = "abcde";
        for (int i = 1; i < 10; i++){
            int max = Math.min(i,  s.length());
            for (int j = 1; j <= max; j++){
                System.out.println(center(s.substring(0, j), i) + "|");
            }
        }
    }
}

Output:

a|
a |
ab|
 a |
ab |
abc|
 a  |
 ab |
abc |
abcd|
  a  |
 ab  |
 abc |
abcd |
abcde|
  a   |
  ab  |
 abc  |
 abcd |
abcde |
   a   |
  ab   |
  abc  |
 abcd  |
 abcde |
   a    |
   ab   |
  abc   |
  abcd  |
 abcde  |
    a    |
   ab    |
   abc   |
  abcd   |
  abcde  | 

Practical differences from Mertuarez's code:

1. Mine does the math up-front and makes the final centered string in one shot instead of making a too-long string and then taking a substring from it. I assume this is slightly more performant, but I did not test it.
2. In the case of text that can't be perfectly centered, mine consistently puts it half a character to the left rather than putting it half a character to the right half of the time.
3. In the case of text that's longer than the specified length, mine consistently returns a substring of the specified length that's rooted at the beginning of the original text.