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 "";
}

这是一个高效的实用工具类，用于Java中的左填充，右填充，中心填充和零填充字符串。

package com.example;

/**
 * Utility class for left pad, right pad, center pad and zero fill.
 */
public final class StringPadding {

    public static String left(String string, int length, char fill) {

        if (string.length() < length) {

            char[] chars = string.toCharArray();
            char[] output = new char[length];

            int delta = length - chars.length;

            for (int i = 0; i < length; i++) {
                if (i < delta) {
                    output[i] = fill;
                } else {
                    output[i] = chars[i - delta];
                }
            }

            return new String(output);
        }

        return string;
    }

    public static String right(String string, int length, char fill) {

        if (string.length() < length) {

            char[] chars = string.toCharArray();
            char[] output = new char[length];

            for (int i = 0; i < length; i++) {
                if (i < chars.length) {
                    output[i] = chars[i];
                } else {
                    output[i] = fill;
                }
            }

            return new String(output);
        }

        return string;
    }

    public static String center(String string, int length, char fill) {

        if (string.length() < length) {

            char[] chars = string.toCharArray();

            int delta = length - chars.length;
            int a = (delta % 2 == 0) ? delta / 2 : delta / 2 + 1;
            int b = a + chars.length;

            char[] output = new char[length];
            for (int i = 0; i < length; i++) {
                if (i < a) {
                    output[i] = fill;
                } else if (i < b) {
                    output[i] = chars[i - a];
                } else {
                    output[i] = fill;
                }
            }

            return new String(output);
        }

        return string;
    }

    public static String zerofill(String string, int length) {
        return left(string, length, '0');
    }

    private StringPadding() {
    }

    /**
     * For tests!
     */
    public static void main(String[] args) {

        String string = "123";
        char blank = ' ';

        System.out.println("left pad:    [" + StringPadding.left(string, 10, blank) + "]");
        System.out.println("right pad:   [" + StringPadding.right(string, 10, blank) + "]");
        System.out.println("center pad:  [" + StringPadding.center(string, 10, blank) + "]");
        System.out.println("zero fill:   [" + StringPadding.zerofill(string, 10) + "]");
    }
}

输出如下:

left pad:    [       123]
right pad:   [123       ]
center pad:  [    123   ]
zero fill:   [0000000123]

在Dzone上找到的

用零填充:

String.format("|%020d|", 93); // prints: |00000000000000000093|

我花了一点时间才想明白。 真正的关键是阅读Formatter文档。

// Get your data from wherever.
final byte[] data = getData();
// Get the digest engine.
final MessageDigest md5= MessageDigest.getInstance("MD5");
// Send your data through it.
md5.update(data);
// Parse the data as a positive BigInteger.
final BigInteger digest = new BigInteger(1,md5.digest());
// Pad the digest with blanks, 32 wide.
String hex = String.format(
    // See: http://download.oracle.com/javase/1.5.0/docs/api/java/util/Formatter.html
    // Format: %[argument_index$][flags][width]conversion
    // Conversion: 'x', 'X'  integral    The result is formatted as a hexadecimal integer
    "%1$32x",
    digest
);
// Replace the blank padding with 0s.
hex = hex.replace(" ","0");
System.out.println(hex);

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);
}

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
    }
}