Simple way to repeat a string

Question

I\'m looking for a simple commons method or operator that allows me to repeat some string n times. I know I could write this using a for loop, but I wish to avoid f

Answer 1

using Dollar is simple as typing:

@Test
public void repeatString() {
    String string = "abc";
    assertThat($(string).repeat(3).toString(), is("abcabcabc"));
}

PS: repeat works also for array, List, Set, etc

Answer 2

Here's a way to do it using only standard String functions and no explicit loops:

// create a string made up of  n  copies of  s
repeated = String.format(String.format("%%%ds", n), " ").replace(" ",s);

Answer 3

I wanted a function to create a comma-delimited list of question marks for JDBC purposes, and found this post. So, I decided to take two variants and see which one performed better. After 1 million iterations, the garden-variety StringBuilder took 2 seconds (fun1), and the cryptic supposedly more optimal version (fun2) took 30 seconds. What's the point of being cryptic again?

private static String fun1(int size) {
    StringBuilder sb = new StringBuilder(size * 2);
    for (int i = 0; i < size; i++) {
        sb.append(",?");
    }
    return sb.substring(1);
}

private static String fun2(int size) {
    return new String(new char[size]).replaceAll("\0", ",?").substring(1);
}

Answer 4

Despite your desire not to use loops, I think you should use a loop.

String repeatString(String s, int repetitions)
{
    if(repetitions < 0) throw SomeException();

    else if(s == null) return null;

    StringBuilder stringBuilder = new StringBuilder(s.length() * repetitions);

    for(int i = 0; i < repetitions; i++)
        stringBuilder.append(s);

    return stringBuilder.toString();
}

Your reasons for not using a for loop are not good ones. In response to your criticisms:

1. Whatever solution you use will almost certainly be longer than this. Using a pre-built function only tucks it under more covers.
2. Someone reading your code will have to figure out what you're doing in that non-for-loop. Given that a for-loop is the idiomatic way to do this, it would be much easier to figure out if you did it with a for loop.
3. Yes someone might add something clever, but by avoiding a for loop you are doing something clever. That's like shooting yourself in the foot intentionally to avoid shooting yourself in the foot by accident.
4. Off-by-one errors are also mind-numbingly easy to catch with a single test. Given that you should be testing your code, an off-by-one error should be easy to fix and catch. And it's worth noting: the code above does not contain an off-by-one error. For loops are equally easy to get right.
5. So don't reuse variables. That's not the for-loop's fault.
6. Again, so does whatever solution you use. And as I noted before; a bug hunter will probably be expecting you to do this with a for loop, so they'll have an easier time finding it if you use a for loop.

Answer 5

Java 8's String.join provides a tidy way to do this in conjunction with Collections.nCopies:

// say hello 100 times
System.out.println(String.join("", Collections.nCopies(100, "hello")));

Answer 6

This contains less characters than your question

public static String repeat(String s, int n) {
    if(s == null) {
        return null;
    }
    final StringBuilder sb = new StringBuilder(s.length() * n);
    for(int i = 0; i < n; i++) {
        sb.append(s);
    }
    return sb.toString();
}