在番石榴中，这很简单:

Strings.padStart("string", 10, ' ');
Strings.padEnd("string", 10, ' ');

public static String LPad(String str, Integer length, char car) {
  return (str + String.format("%" + length + "s", "").replace(" ", String.valueOf(car))).substring(0, length);
}

public static String RPad(String str, Integer length, char car) {
  return (String.format("%" + length + "s", "").replace(" ", String.valueOf(car)) + str).substring(str.length(), length + str.length());
}

LPad("Hi", 10, 'R') //gives "RRRRRRRRHi"
RPad("Hi", 10, 'R') //gives "HiRRRRRRRR"
RPad("Hi", 10, ' ') //gives "Hi        "
RPad("Hi", 1, ' ')  //gives "H"
//etc...

看看org.apache.commons.lang.StringUtils#rightPad(String str, int size, char padChar)。

但算法非常简单(填充到字符大小):

public String pad(String str, int size, char padChar)
{
  StringBuilder padded = new StringBuilder(str);
  while (padded.length() < size)
  {
    padded.append(padChar);
  }
  return padded.toString();
}

s =字符串填充 n =期望长度 c = Char到pad

private String padStart( String s, int n, char c ){
    if( n <= 0 ) return s;
    else return String.format("%" + n + "s", s).replace(' ', c);
}

public static String padLeft(String in, int size, char padChar) {                
    if (in.length() <= size) {
        char[] temp = new char[size];
        /* Llenado Array con el padChar*/
        for(int i =0;i<size;i++){
            temp[i]= padChar;
        }
        int posIniTemp = size-in.length();
        for(int i=0;i<in.length();i++){
            temp[posIniTemp]=in.charAt(i);
            posIniTemp++;
        }            
        return new String(temp);
    }
    return "";
}

从Java 1.5开始，string. format()可以用于左/右填充给定的字符串。

public static String padRight(String s, int n) {
     return String.format("%-" + n + "s", s);  
}

public static String padLeft(String s, int n) {
    return String.format("%" + n + "s", s);  
}

...

public static void main(String args[]) throws Exception {
 System.out.println(padRight("Howto", 20) + "*");
 System.out.println(padLeft("Howto", 20) + "*");
}

输出为:

Howto               *
               Howto*