What is the easiest way to generate a String of n repeated characters?

Question

Given a character c and a number n, how can I create a String that consists of n repetitions of c? Doing it manually is too cumbersome:

StringBuilder sb = new StringBuilder(n);
for (int i = 0; i < n; ++i)
{
    sb.append(c);
}
String result = sb.toString();

Surely there is some static library function that already does this for me?

Answer 1

int n = 10;
char[] chars = new char[n];
Arrays.fill(chars, 'c');
String result = new String(chars);

EDIT:

It's been 9 years since this answer was submitted but it still attracts some attention now and then. In the meantime Java 8 has been introduced with functional programming features. Given a char c and the desired number of repetitions count the following one-liner can do the same as above.

String result = IntStream.range(1, count).mapToObj(index -> "" + c).collect(Collectors.joining());

Do note however that it is slower than the array approach. It should hardly matter in any but the most demanding circumstances. Unless it's in some piece of code that will be executed thousands of times per second it won't make much difference. This can also be used with a String instead of a char to repeat it a number of times so it's a bit more flexible. No third-party libraries needed.

Answer 2

If you can, use StringUtils from Apache Commons Lang:

StringUtils.repeat("ab", 3);  //"ababab"

Answer 3

Google Guava Time!

Strings.repeat("a", 3)

http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Strings.html

Answer 4

To have an idea of the speed penalty, I have tested two versions, one with Array.fill and one with StringBuilder.

public static String repeat(char what, int howmany) {
    char[] chars = new char[howmany];
    Arrays.fill(chars, what);
    return new String(chars);
}

and

public static String repeatSB(char what, int howmany) {
    StringBuilder out = new StringBuilder(howmany);
    for (int i = 0; i < howmany; i++)
        out.append(what);
    return out.toString();
}

using

public static void main(String... args) {
    String res;
    long time;

    for (int j = 0; j < 1000; j++) {
        res = repeat(' ', 100000);
        res = repeatSB(' ', 100000);
    }
    time = System.nanoTime();
    res = repeat(' ', 100000);
    time = System.nanoTime() - time;
    System.out.println("elapsed repeat: " + time);

    time = System.nanoTime();
    res = repeatSB(' ', 100000);
    time = System.nanoTime() - time;
    System.out.println("elapsed repeatSB: " + time);
}

(note the loop in main function is to kick in JIT)

The results are as follows:

elapsed repeat  : 65899
elapsed repeatSB: 305171

It is a huge difference

Answer 5

Java SE 11

String#repeat​(int count), introduced as part of Java SE 11, makes it quite easy to do.

Demo:

public class Main {
    public static void main(String[] args) {
        char ch = 'c';
        int n = 20;
        String result = String.valueOf(ch).repeat(n);
        System.out.println(result);
    }
}

Output:

cccccccccccccccccccc

Answer 6

take look at this sample

Integer  n=10;
String s="a";
String repeated = new String(new char[n]).replace("\0", s);

answer taken form Simple way to repeat a String in java so vote up there

Answer 7

Here is an O(logN) method, based on the standard binary powering algorithm:

public static String repChar(char c, int reps) {
    String adder = Character.toString(c);
    String result = "";
    while (reps > 0) {
        if (reps % 2 == 1) {
            result += adder;
        }
        adder += adder;
        reps /= 2;
    }        
    return result;
}

Negative values for reps return the empty string.

Answer 8

I am adding another way to solve it:

public String buildString(Character character, int nTimes) {
    return String.format("%" + nTimes + "s", " ").replace(' ', character);
}

Answer 9

If you are not on Java 11+ (otherwise I would go with Arvind Kumar Avinash's answer), you can also combine String#join(CharSequence, Iterable<? extends CharSequence> and Collections#nCopies(int, T):

String result = String.join("", Collections.nCopies(c, n));

Answer 10

Just add it to your own...

public static String generateRepeatingString(char c, Integer n) {
    StringBuilder b = new StringBuilder();
    for (Integer x = 0; x < n; x++)
        b.append(c);
    return b.toString();
}

Or Apache commons has a utility class you can add.

Answer 11

A simple and clean solution would be to use String.format and then replace characters with desired character.

String.format("%5s", "").replace(' ', '#')
// Output : #####

If you are using Java 11

you can just use String.repeat()

"#".repeat(5)