How to center a string using String.format?

I quickly hacked this up. You can now use StringUtils.center(String s, int size) in String.format.

import static org.hamcrest.CoreMatchers.*;
import static org.junit.Assert.assertThat;

import org.junit.Test;

public class TestCenter {
    @Test
    public void centersString() {
        assertThat(StringUtils.center(null, 0), equalTo(null));
        assertThat(StringUtils.center("foo", 3), is("foo"));
        assertThat(StringUtils.center("foo", -1), is("foo"));
        assertThat(StringUtils.center("moon", 10), is("   moon   "));
        assertThat(StringUtils.center("phone", 14, '*'), is("****phone*****"));
        assertThat(StringUtils.center("India", 6, '-'), is("India-"));
        assertThat(StringUtils.center("Eclipse IDE", 21, '*'), is("*****Eclipse IDE*****"));
    }

    @Test
    public void worksWithFormat() {
        String format = "|%1$-10s|%2$-10s|%3$-20s|\n";
        assertThat(String.format(format, StringUtils.center("FirstName", 10), StringUtils.center("Init.", 10), StringUtils.center("LastName", 20)),
                is("|FirstName |  Init.   |      LastName      |\n"));
    }
}

class StringUtils {

    public static String center(String s, int size) {
        return center(s, size, ' ');
    }

    public static String center(String s, int size, char pad) {
        if (s == null || size <= s.length())
            return s;

        StringBuilder sb = new StringBuilder(size);
        for (int i = 0; i < (size - s.length()) / 2; i++) {
            sb.append(pad);
        }
        sb.append(s);
        while (sb.length() < size) {
            sb.append(pad);
        }
        return sb.toString();
    }
}

public static String center(String text, int len){
    String out = String.format("%"+len+"s%s%"+len+"s", "",text,"");
    float mid = (out.length()/2);
    float start = mid - (len/2);
    float end = start + len; 
    return out.substring((int)start, (int)end);
}

public static void main(String[] args) throws Exception{
    // Test
    String s = "abcdefghijklmnopqrstuvwxyz";
    for (int i = 1; i < 200;i++){
        for (int j = 1; j < s.length();j++){
            //center(s.substring(0, j),i);
            System.out.println(center(s.substring(0, j),i));
        }
    }
}

Converted the code found at https://www.leveluplunch.com/java/examples/center-justify-string/ into a handy, small one-line function:

public static String centerString (int width, String s) {
    return String.format("%-" + width  + "s", String.format("%" + (s.length() + (width - s.length()) / 2) + "s", s));
}

Usage:

public static void main(String[] args){
    String out = centerString(10, "afgb");
    System.out.println(out); //Prints "   afgb   "
}

I think it's a very neat solution that's worth mentioning.