Use StringBuilder to pad String with blank spaces or other characters

I'm a beginner to java and this is my first post on Stackoverflow. Although my initial code is similar to other posts here, my question relates to implementing StringBuilder and so the reason for this post.

I've created a method

**Initial code**
private static String padAgain(String result,int padNum)  
    {  
          String str = "";  
          for(int i = padNum; i >= 0; i--)  
        {  
            str = String.format("%" + padNum + "s", result).replace(" ", "*");  
        }  
          return str;  
    }  

to left pad the string with blank spaces (or special characters) using String.format method. I'm using the same in my program to right justify numbers. Numbers start with 2 (single-digit) and increase in multiples of 2 (containing n no. of digits) with the spaces being inversely proportional to the increase in digits.

Here is the output for illustrative purposes:

    Enter exponent value for base-2 - Enter 0 to exit: 
128
Square    1: **************************************2 items
Square    2: **************************************4 items
Square    3: **************************************8 items
Square    4: *************************************16 items
Square    5: *************************************32 items
Square    6: *************************************64 items
Square    7: ************************************128 items
Square    8: ************************************256 items
Square    9: ************************************512 items
Square   10: ***********************************1024 items
Square   11: ***********************************2048 items
Square   12: ***********************************4096 items
Square   13: ***********************************8192 items
Square   14: **********************************16384 items
Square   15: **********************************32768 items
Square   16: **********************************65536 items
Square   17: *********************