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 "";
}

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();
}

你可以使用内置的StringBuilder append()和insert()方法， 对于可变字符串长度的填充:

AbstractStringBuilder append(CharSequence s, int start, int end) ;

例如:

private static final String  MAX_STRING = "                    "; //20 spaces

    Set<StringBuilder> set= new HashSet<StringBuilder>();
    set.add(new StringBuilder("12345678"));
    set.add(new StringBuilder("123456789"));
    set.add(new StringBuilder("1234567811"));
    set.add(new StringBuilder("12345678123"));
    set.add(new StringBuilder("1234567812234"));
    set.add(new StringBuilder("1234567812222"));
    set.add(new StringBuilder("12345678122334"));

    for(StringBuilder padMe: set)
        padMe.append(MAX_STRING, padMe.length(), MAX_STRING.length());

概括一下Eko的答案(Java 11+):

public class StringUtils {
    public static String padLeft(String s, char fill, int padSize) {
        if (padSize < 0) {
            var err = "padSize must be >= 0 (was " + padSize + ")";
            throw new java.lang.IllegalArgumentException(err);
        }

        int repeats = Math.max(0, padSize - s.length());
        return Character.toString(fill).repeat(repeats) + s;
    }

    public static String padRight(String s, char fill, int padSize) {
        if (padSize < 0) {
            var err = "padSize must be >= 0 (was " + padSize + ")";
            throw new java.lang.IllegalArgumentException(err);
        }

        int repeats = Math.max(0, padSize - s.length());
        return s + Character.toString(fill).repeat(repeats);
    }

    public static void main(String[] args) {
        System.out.println(padLeft("", 'x', 5)); // => xxxxx
        System.out.println(padLeft("1", 'x', 5)); // => xxxx1
        System.out.println(padLeft("12", 'x', 5)); // => xxx12
        System.out.println(padLeft("123", 'x', 5)); // => xx123
        System.out.println(padLeft("1234", 'x', 5)); // => x1234
        System.out.println(padLeft("12345", 'x', 5)); // => 12345
        System.out.println(padLeft("123456", 'x', 5)); // => 123456

        System.out.println(padRight("", 'x', 5)); // => xxxxx
        System.out.println(padRight("1", 'x', 5)); // => 1xxxx
        System.out.println(padRight("12", 'x', 5)); // => 12xxx
        System.out.println(padRight("123", 'x', 5)); // => 123xx
        System.out.println(padRight("1234", 'x', 5)); // => 1234x
        System.out.println(padRight("12345", 'x', 5)); // => 12345
        System.out.println(padRight("123456", 'x', 5)); // => 123456

        System.out.println(padRight("1", 'x', -1)); // => throws
    }
}

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 "";
}

不管怎样，我一直在寻找一些可以填充的东西，然后我决定自己编写代码。它非常简洁，你可以很容易地从中推导出padLeft和padRight

    /**
     * Pads around a string, both left and right using pad as the template, aligning to the right or left as indicated.
     * @param a the string to pad on both left and right
     * @param pad the template to pad with, it can be of any size
     * @param width the fixed width to output
     * @param alignRight if true, when the input string is of odd length, adds an extra pad char to the left, so values are right aligned
     *                   otherwise add an extra pad char to the right. When the input is of even length no extra chars will be inserted
     * @return the input param a padded around.
     */
    public static String padAround(String a, String pad, int width, boolean alignRight) {
        if (pad.length() == 0)
            throw new IllegalArgumentException("Pad cannot be an empty string!");
        int delta = width - a.length();
        if (delta < 1)
            return a;
        int half = delta / 2;
        int remainder = delta % 2;
        String padding = pad.repeat(((half+remainder)/pad.length()+1)); // repeating the padding to occupy all possible space
        StringBuilder sb = new StringBuilder(width);
//        sb.append( padding.substring(0,half + (alignRight ? 0 : remainder)));
        sb.append(padding, 0, half + (alignRight ? 0 : remainder));
        sb.append(a);
//        sb.append( padding.substring(0,half + (alignRight ? remainder : 0)));
        sb.append(padding, 0, half + (alignRight ? remainder : 0));

        return sb.toString();
    }

虽然它应该是相当快的，它可能会受益于使用一些韵母在这里和那里。