How to Repeat a String in Java

The challenge

Write a function called repeat_str which repeats the given string src exactly count times.

repeatStr(6, "I") // "IIIIII"
repeatStr(5, "Hello") // "HelloHelloHelloHelloHello"

The solution in Java

Java comes with a built-in on the String class. Called repeat, which allows you to very easily repeat that string n number of times.

"Hello".repeat(2); // "HelloHello"

Using String::repeat()

Therefore, we could solve our above problem as follows:

class Solution {
  static String repeatStr(int repeat, String string) {
    return string.repeat(repeat);
  }
}

Obviously, we probably want to validate our input before we just do it, so maybe wrap it in a ternary:

public class Solution {
    public static String repeatStr(final int repeat, final String string) {
        return repeat >= 0 ? string.repeat(repeat) : "";
    }
}

Write all the code yourself?

Alternatively, you could always just write the method yourself, doing something like this:

public class Solution {
    public static String repeatStr(final int repeat, final String string) {
        StringBuilder sb = new StringBuilder();

        for (int i = 0; i < repeat; i++) {
            sb.append(string);
        }

        return sb.toString();
    }
}

Or maybe with less code, like this:

public class Solution {
    public static String repeatStr(final int repeat, final String string) {
        String s = "";
        for (int i = 0; i < repeat; i++) s += string;
        return s;
    }
}

Java 8 and above

If you’re feeling Java8 and above thoughts, you could always do this:

public class Solution {
  public static String repeatStr(final int repeat, final String string) {
    return java.util.stream.IntStream.range(0, repeat).mapToObj(i -> string)
        .collect(java.util.stream.Collectors.joining()).toString();
  }
}

Or maybe combine it with the Collections nCopies possible solution:

import java.util.Collections;

public class Solution {
  public static String repeatStr(final int repeat, final String string) {
    return repeat < 0 ? "" : String.join("", Collections.nCopies(repeat, string));
  }
}

Test cases to validate

It’s always important to have some tests which will help us validate our application code.

import org.junit.Test;
import static org.junit.Assert.assertEquals;
import java.util.Random;

public class SolutionTest {
    @Test public void test4a() {
        assertEquals("aaaa", Solution.repeatStr(4, "a"));
    }
    @Test public void test3Hello() {
        assertEquals("HelloHelloHello", Solution.repeatStr(3, "Hello"));
    }
    @Test public void test5empty() {
        assertEquals("", Solution.repeatStr(5, ""));
    }
    @Test public void test0a() {
        assertEquals("", Solution.repeatStr(0, "kata"));
    }

    private final char[] characters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQERSTUVWXYZ0123456789!@#$%^&*()-=_+[]{}|;:,.<>/?`~".toCharArray();

    @Test public void testRandom() {
        final Random rand = new Random();
        for (int testIteration = 0; testIteration < 10; ++testIteration) {
            final StringBuilder text = new StringBuilder(rand.nextInt(32));
            for (int i = 0; i < text.capacity(); ++i) {
                final int chi = rand.nextInt(characters.length);
                text.append(characters[chi]);
            }
            final String string = text.toString();

            final int timesToRepeat = rand.nextInt(32);
            final StringBuilder sb = new StringBuilder(timesToRepeat * string.length());
            for (int i = 0; i < timesToRepeat; ++i) {
                sb.append(text);
            }
            final String expected = sb.toString();

            assertEquals(expected, Solution.repeatStr(timesToRepeat, string));
        }
    }
}