问格式化程序不能在字符串上使用吗？

EN

public class StringBuilderTester {

    public static void main(String[] args) {
        StringBuilder matrix = new StringBuilder();
        Random r = new Random();
        Formatter formatter = new Formatter();
        for (int j=0;j<10;j++) {
            String[] strings = new String[10];
            StringBuilder formatString = new StringBuilder();
            for (int i=0;i<10;i++) {
                formatString.append("%-10.5s");
                strings[i] = new Integer(r.nextInt(10)).toString();
            }
            formatString.append("\n");
            matrix.append(formatter.format(formatString.toString(), strings));  //gives me unexpected result: 1+2+3+..+10=55 lines of 10 cols of random ints
            //matrix.append(String.format(formatString.toString(), strings));  //gives me expected results: 10 lines of 10 cols of random ints
        }
        System.out.println(matrix.toString());
    }
}