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);

你可以通过保留填充数据来减少每次调用的开销，而不是每次都重新构建:

public class RightPadder {

    private int length;
    private String padding;

    public RightPadder(int length, String pad) {
        this.length = length;
        StringBuilder sb = new StringBuilder(pad);
        while (sb.length() < length) {
            sb.append(sb);
        }
        padding = sb.toString();
   }

    public String pad(String s) {
        return (s.length() < length ? s + padding : s).substring(0, length);
    }

}

作为一种替代方法，您可以将结果长度作为pad(…)方法的参数。在这种情况下，在该方法中而不是在构造函数中调整隐藏填充。

(提示:为了获得额外的学分，让它是线程安全的!: -)

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);

在番石榴中，这很简单:

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

这里有一个并行版本的你有很长的字符串:-)

int width = 100;
String s = "129018";

CharSequence padded = IntStream.range(0,width)
            .parallel()
            .map(i->i-(width-s.length()))
            .map(i->i<0 ? '0' :s.charAt(i))
            .collect(StringBuilder::new, (sb,c)-> sb.append((char)c), (sb1,sb2)->sb1.append(sb2));

填充到10个字符:

String.format("%10s", "foo").replace(' ', '*');
String.format("%-10s", "bar").replace(' ', '*');
String.format("%10s", "longer than 10 chars").replace(' ', '*');

输出:

  *******foo
  bar*******
  longer*than*10*chars

密码字符显示“*”:

String password = "secret123";
String padded = String.format("%"+password.length()+"s", "").replace(' ', '*');

输出与密码字符串长度相同:

  secret123
  *********