How Can I Pad a String in Java?

Since Java 1.5, String.format() can be used to left/right pad a given string.

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) + "*");
}

And the output is:

Howto               *
               Howto*

Padding to 10 characters:

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

output:

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

Display '*' for characters of password:

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

output has the same length as the password string:

  secret123
  *********

In Guava, this is easy:

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

Something simple:

The value should be a string. convert it to string, if it's not. Like "" + 123 or Integer.toString(123)

// let's assume value holds the String we want to pad
String value = "123";

Substring start from the value length char index until end length of padded:

String padded="00000000".substring(value.length()) + value;

// now padded is "00000123"

More precise

pad right:

String padded = value + ("ABCDEFGH".substring(value.length())); 

// now padded is "123DEFGH"

pad left:

String padString = "ABCDEFGH";
String padded = (padString.substring(0, padString.length() - value.length())) + value;

// now padded is "ABCDE123"

Have a look at org.apache.commons.lang.StringUtils#rightPad(String str, int size, char padChar).

But the algorithm is very simple (pad right up to size chars):

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

Besides Apache Commons, also see String.format which should be able to take care of simple padding (e.g. with spaces).

Since Java 11, String.repeat(int) can be used to left/right pad a given string.

System.out.println("*".repeat(5)+"apple");
System.out.println("apple"+"*".repeat(5));

Output:

*****apple
apple*****

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...

i know this thread is kind of old and the original question was for an easy solution but if it's supposed to be really fast, you should use a char array.

public static String pad(String str, int size, char padChar)
{
    if (str.length() < size)
    {
        char[] temp = new char[size];
        int i = 0;

        while (i < str.length())
        {
            temp[i] = str.charAt(i);
            i++;
        }

        while (i < size)
        {
            temp[i] = padChar;
            i++;
        }

        str = new String(temp);
    }

    return str;
}

the formatter solution is not optimal. just building the format string creates 2 new strings.

apache's solution can be improved by initializing the sb with the target size so replacing below

StringBuffer padded = new StringBuffer(str); 

with

StringBuffer padded = new StringBuffer(pad); 
padded.append(value);

would prevent the sb's internal buffer from growing.

This took me a little while to figure out. The real key is to read that Formatter documentation.

// 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: 
    // 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);

Found this on Dzone

Pad with zeros:

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

Here is another way to pad to the right:

// put the number of spaces, or any character you like, in your paddedString

String paddedString = "--------------------";

String myStringToBePadded = "I like donuts";

myStringToBePadded = myStringToBePadded + paddedString.substring(myStringToBePadded.length());

//result:
myStringToBePadded = "I like donuts-------";

You can reduce the per-call overhead by retaining the padding data, rather than rebuilding it every time:

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);
    }

}

As an alternative, you can make the result length a parameter to the pad(...) method. In that case do the adjustment of the hidden padding in that method instead of in the constructor.

(Hint: For extra credit, make it thread-safe! ;-)

java.util.Formatter will do left and right padding. No need for odd third party dependencies (would you want to add them for something so trivial).

[I've left out the details and made this post 'community wiki' as it is not something I have a need for.]

you can use the built in StringBuilder append() and insert() methods, for padding of variable string lengths:

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

For Example:

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());

This works:

"".format("%1$-" + 9 + "s", "XXX").replaceAll(" ", "0")

It will fill your String XXX up to 9 Chars with a whitespace. After that all Whitespaces will be replaced with a 0. You can change the whitespace and the 0 to whatever you want...

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 "";
}

Let's me leave an answer for some cases that you need to give left/right padding (or prefix/suffix string or spaces) before you concatenate to another string and you don't want to test length or any if condition.

The same to the selected answer, I would prefer the StringUtils of Apache Commons but using this way:

StringUtils.defaultString(StringUtils.leftPad(myString, 1))

Explain:

• myString: the string I input, can be null
• StringUtils.leftPad(myString, 1): if string is null, this statement would return null too
• then use defaultString to give empty string to prevent concatenate null

@ck's and @Marlon Tarak's answers are the only ones to use a char[], which for applications that have several calls to padding methods per second is the best approach. However, they don't take advantage of any array manipulation optimizations and are a little overwritten for my taste; this can be done with no loops at all.

public static String pad(String source, char fill, int length, boolean right){
    if(source.length() > length) return source;
    char[] out = new char[length];
    if(right){
        System.arraycopy(source.toCharArray(), 0, out, 0, source.length());
        Arrays.fill(out, source.length(), length, fill);
    }else{
        int sourceOffset = length - source.length();
        System.arraycopy(source.toCharArray(), 0, out, sourceOffset, source.length());
        Arrays.fill(out, 0, sourceOffset, fill);
    }
    return new String(out);
}

Simple test method:

public static void main(String... args){
    System.out.println("012345678901234567890123456789");
    System.out.println(pad("cats", ' ', 30, true));
    System.out.println(pad("cats", ' ', 30, false));
    System.out.println(pad("cats", ' ', 20, false));
    System.out.println(pad("cats", '$', 30, true));
    System.out.println(pad("too long for your own good, buddy", '#', 30, true));
}

Outputs:

012345678901234567890123456789
cats                          
                          cats
                cats
cats$$$$$$$$$$$$$$$$$$$$$$$$$$
too long for your own good, buddy 

All string operation usually needs to be very efficient - especially if you are working with big sets of data. I wanted something that's fast and flexible, similar to what you will get in plsql pad command. Also, I don't want to include a huge lib for just one small thing. With these considerations none of these solutions were satisfactory. This is the solutions I came up with, that had the best bench-marking results, if anybody can improve on it, please add your comment.

public static char[] lpad(char[] pStringChar, int pTotalLength, char pPad) {
    if (pStringChar.length < pTotalLength) {
        char[] retChar = new char[pTotalLength];
        int padIdx = pTotalLength - pStringChar.length;
        Arrays.fill(retChar, 0, padIdx, pPad);
        System.arraycopy(pStringChar, 0, retChar, padIdx, pStringChar.length);
        return retChar;
    } else {
        return pStringChar;
    }
}

• note it is called with String.toCharArray() and the result can be converted to String with new String((char[])result). The reason for this is, if you applying multiple operations you can do them all on char[] and not keep on converting between formats - behind the scenes, String is stored as char[]. If these operations were included in the String class itself, it would have been twice as efficient - speed and memory wise.

Use this function.

private String leftPadding(String word, int length, char ch) {
   return (length > word.length()) ? leftPadding(ch + word, length, ch) : word;
}

how to use?

leftPadding(month, 2, '0');

output: 01 02 03 04 .. 11 12

A lot of people have some very interesting techniques but I like to keep it simple so I go with this :

public static String padRight(String s, int n, char padding){
    StringBuilder builder = new StringBuilder(s.length() + n);
    builder.append(s);
    for(int i = 0; i < n; i++){
        builder.append(padding);
    }
    return builder.toString();
}

public static String padLeft(String s, int n,  char padding) {
    StringBuilder builder = new StringBuilder(s.length() + n);
    for(int i = 0; i < n; i++){
        builder.append(Character.toString(padding));
    }
    return builder.append(s).toString();
}

public static String pad(String s, int n, char padding){
    StringBuilder pad = new StringBuilder(s.length() + n * 2);
    StringBuilder value = new StringBuilder(n);
    for(int i = 0; i < n; i++){
        pad.append(padding);
    }
    return value.append(pad).append(s).append(pad).toString();
}

This is an efficient utility class for left pad, right pad, center pad and zero fill of strings in Java.

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) + "]");
    }
}

This is the output:

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

Java oneliners, no fancy library.

// 6 characters padding example
String pad = "******";
// testcases for 0, 4, 8 characters
String input = "" | "abcd" | "abcdefgh"

Pad Left, don't limit

result = pad.substring(Math.min(input.length(),pad.length())) + input;
results: "******" | "**abcd" | "abcdefgh"

Pad Right, don't limit

result = input + pad.substring(Math.min(input.length(),pad.length()));
results: "******" | "abcd**" | "abcdefgh"

Pad Left, limit to pad length

result = (pad + input).substring(input.length(), input.length() + pad.length());
results: "******" | "**abcd" | "cdefgh"

Pad Right, limit to pad length

result = (input + pad).substring(0, pad.length());
results: "******" | "abcd**" | "abcdef"

Another solution utilizing recursion.

This is compatible with all JDK versions and no external libraries are required:

private static String addPadding(final String str, final int desiredLength, final String padBy) {
    String result = str;
    if (str.length() >= desiredLength) {
        return result;
    } else {
        result += padBy;
        return addPadding(result, desiredLength, padBy);
    }
}

NOTE: This solution will append the padding, with a little tweak you can prefix the pad value.

Here's a parallel version for those of you that have very long Strings :-)

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));

Generalizing Eko's answer (Java 11+) a bit:

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
    }
}

For what it's worth, I was looking for something that would pad around and then I decided to code it myself. It's fairly clean and you can easily derive padLeft and padRight from this

    /**
     * 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();
    }

While it should be fairly fast it could prolly benefit from using a few finals here and there.

A simple solution would be:

package nl;
public class Padder {
    public static void main(String[] args) {
        String s = "123" ;
        System.out.println("#"+("     " + s).substring(s.length())+"#");
    }
}