Is there some easy way to pad Strings in Java?
Seems like something that should be in some StringUtil-like API, but I can't find anything that does this.
30 Answers
678
votes
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*
272
votes
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
*********
199
votes
Apache StringUtils has several methods: leftPad, rightPad, center and repeat.
But please note that — as others have mentioned and demonstrated in this answer — String.format() and the Formatter classes in the JDK are better options. Use them over the commons code.
88
votes
37
votes
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"
26
votes
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();
}
22
votes
16
votes
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...
14
votes
6
votes
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.
6
votes
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: 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);
5
votes
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-------";
5
votes
4
votes
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! ;-)
2
votes
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());
2
votes
2
votes
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 "";
}
2
votes
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 nullStringUtils.leftPad(myString, 1): if string is null, this statement would return null too- then use
defaultStringto give empty string to prevent concatenate null
2
votes
@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
1
votes
1
votes
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.
1
votes
1
votes
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();
}
1
votes
This utility class that is useful for left pad and zero fill of Strings. I hope that it helps someone!
package your.package.name;
/**
* Utility class for left pad and zero fill of Strings.
*
*/
public class LeftPad {
public static String lpad(String string, int length, char fill) {
return new String(lpad(string.toCharArray(), length, fill));
}
public static String zerofill(String string, int length) {
return new String(zerofill(string.toCharArray(), length));
}
private static char[] lpad(char[] chars, int length, char fill) {
int delta = 0;
int limit = 0;
if (length > chars.length) {
delta = length - chars.length;
limit = length;
} else {
delta = 0;
limit = chars.length;
}
char[] output = new char[chars.length + delta];
for (int i = 0; i < limit; i++) {
if (i < delta) {
output[i] = fill;
} else {
output[i] = chars[i - delta];
}
}
return output;
}
private static char[] zerofill(char[] chars, int length) {
return lpad(chars, length, '0');
}
/**
* For tests!
*/
public static void main(String[] args) {
String string = "123";
char pad = ' ';
System.out.println("left pad blank: " + string);
System.out.println("Result: [" + LeftPad.lpad(string, 10, pad) + "]");
System.out.println();
System.out.println("zero fill: " + string);
System.out.println("Result: [" + LeftPad.zerofill(string, 10) + "]");
}
}
This is the output:
left pad blank: 123
Result: [ 123]
zero fill: 123
Result: [0000000123]
0
votes
0
votes
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"
0
votes
How about using recursion? Solution given below is compatible with all JDK versions and no external libraries 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.
0
votes
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));
-1
votes
